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Tracing the Water Masses of the NorthAtlantic and the Mediterranean

Final report of the EU project TRACMASS 1 June 1998−30 May 2001

MAST contract: MAS3−CT97−0142

Meteorologiska institutionen Stockholms universitetKristofer DöösSouthampton Oceanography Centre Peter KillworthAndrew CowardRobert MarshMei−Man LeeLaboratoire de Physique des OcéanBruno BlankeSabrina SpeichMaria Valdivieso

Istituto Fisica dell’Atmosfera, Consiglio Nazionale delle RicercheRosalia Santoleri Daniele Iudicone Koninlijk Nederlands Meteorologisch InstituutSybren Drijfhout Pedro de VriesEnte per le Nouve technologie, l’Energia et l’AmbienteVincenzo ArtaleVolfango Rupolo Salvatore Marullo

The Conveyor Belt: blue trajectories represent the deep brach and consists mainly of North Atlantic Deep Water, thered trajectories represent the shallow branch and is the return flow to the North Atlantic of the warm and less saltysurface waters

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The Final Report of TRACMASS

1. Introduction2. The Ocean Circulation Models3. Theoretical studies of the trajectory methods 4. The Conveyor Belt5. The Mediterranean Water mass circulation6. Summary of the 25 TRACMASS articles7. Final management report for the entire period

The TRACMASS team at the first meeting on the Swedish island of Askö. From left to right: Mei−Man Lee, VolfangoRupolo, Sabrina Speich, Maria Valdivieso, Daniele Iudicone, Rosalia Santoleri, Bruno Blanke, Sybren Drijfhout,Andrew Coward, Kristofer Döös, Robert Marsh and Peter Killworth.

The TRACMASS team at the final meeting on Italian island of Ventotene. From left to right in the back: ConstanteLuttazzi, Rosalia Santoleri, Daniele Iudicone, Robert Marsh, Mei−Man Lee, Pedro de Vries, Bruno Blanke and infront: Sabrina Speich, Sybren Drijfhout, Kristofer Döös, Volfango Rupolo and Andrew Coward.

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Chapter 1. Introduction

TRACMASS has used and developed a new water masstracing method based on Lagrangian trajectories toinvestigate the North Atlantic and Mediterranean watermass circulation as they result from numerical simulationsof the global ocean.

The North Atlantic Deep Water, which is formed in theNorth Atlantic and then flows south and then convertedinto shallower warm water either by isopycnic ventilationin the Southern Ocean, which is then converted intowarmer water in the northward Ekman transport, or bytropical upwelling through the thermocline. This formsthe deep branch of the World Ocean thermohalineConveyor Belt and is represented by the blue trajectorieson the cover figure. The red trajectories in the samefigure represent the shallow branch and is the return flowto the North Atlantic of the warm and less salty surfacewaters. This world ocean overturning Cell is the mostimportant mechanism to distribut heat salt and mass. Thepupose of this project is to trace the path of the Conveyorbelt both quantitatively and qualititaively and toinvestigate the similar circulation in the Mediterraneanand its impact on the Conveyor Belt.

We have used numerical simulations made with fourdifferent general circulation models. Two high resolutionCox−Bryan type (OCCAM and MEDMOM), one C−gridmodel with depth coordinates (OPA) and one isopycnicmodel (GIM). They are all global except MEDMOM,which is only applied over the Mediterranean. We havefrom these numerical simulationsextracted the velocity,temperature and salinity fileds and traced the watermasses with Lagrangian trajectories.

Our Lagrangian tool has several advantages withrespect to a more traditional model for transport oftracers, for which passive quantities are advected anddiffused with sophisticated (and expensive) numericalschemes, and makes use of the fields (velocity,diffusivity) produced by an ocean GCM. Some of thestrong points of the approach are: speed and accuracy inthe computation of the trajectories, access to estimates ofdirectional transports (for water flowing from one givenoceanic section to another); and to backward integrations(for tracking water masses in the past). Various excitingstudies have recently illustrated the power of the approach(Döös 1995, Blanke and Raynaud 1997).

The TRACMASS project aimed at documenting thelarge scale ocean circulation simulated by several oceangeneral circulation models (OGCM) from a Lagrangianpoint of view. Such diagnostics are based on multipleindividual trajectory computations in the velocity fieldcomputed by the OGCM, with the joint use of anappropriate numerical advection scheme [Blanke andRaynaud, 1997] and an efficient description of a watermass in terms of the particles that compose it [Döös,1995]. They were to be applied to different classes ofOGCMs (one isopycnic model, three z−coordinatemodels), developed over distinct domains (theMediterranean Sea or the global ocean), with differentgrid resolutions. Thus, some necessary numerical coding

and theoretical developments were associated to theachievement of TRACMASS, in order to ensure robustand powerful Lagrangian analyses for the tracing of NorthAtlantic and Mediterranean water masses.

One major problem related to Lagrangian tracingtechniques of selected water masses is linked to oceantime scales for such large scale advection processes.Lagrangian trajectories should be integrated long enoughto document the full extent of basin scale (or global scale)water mass movements, i.e., several hundreds (or eventhousands) of years, whereas direct OGCM simulationsseldom exceed a few tens of years, because of CPUlimitations or inherent drifting problems.

Pathways and mechanisms of oceanic heat and freshwater transports are critical issues in the comprehensionof the present climate and its stability. Indeed, oceancirculation transfers heat and fresh water betweendifferent climate regimes and between different oceanbasins. This transport is coupled to convectiveoverturning, which links the full ocean volume to theclimate at decade−to−century time−scales. For example,surface water sinks into the deep layers of the northernNorth Atlantic and draws warm surface water from thesouth, warming the European climate. Unless the sinkingregions, principally the North Atlantic, are compensatedby upwelling within the confines of their local basin(which does not appear to be strong enough), inter−oceancirculation is implied. But what is its form, and how doesit feed back into convective processes and their associatedclimate phenomena?

Each of the oceans is exposed to different forcing, andtakes on distinct stratification in temperature andchemistry. In the North Pacific there is more precipitationthan evaporation, so the thermocline is low in salt, withlimited, shallow convection; in contrast, the Indian andAtlantic are evaporative oceans, so their upper layers aremore saline than those of the Pacific. In the NorthAtlantic the dense, salty surface water causes deepconvection at surface ocean temperatures well above thefreezing point of sea water. Even the Southern Ocean,which is less constrained by geometry, has considerablelongitudinal variations of near surface salinity andassociated bottom water formation rates and properties.

The chemical and thermal differences between themajor ocean basins would be much larger were it not forpassages permitting inter−ocean exchange. Thesepassages are constrictive links in the circulation’s abilityto equalise ocean properties between basins. Only byviewing the global nature of ocean circulation within themajor basins and its connecting passages can the ocean’srole in the climate system be fully appreciated. Indeed,the physical structure of the global North Atlantic DeepWater (NADW) driven thermohaline circulation (THC)and its efficiency in regulating the climate is substantiallyaffected by the nature and very existence of inter−oceanexchange of water masses. But the sources, pathways andphysical properties of these exchanges are not wellenough established to allow its influence on the climatesystem to be quantified.

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Figure: The lower branch of the Conveyor Belt simulated by OCCAM. As a function of time (upper panel), as afunction of depth (middle panel) and as a function of density (lower panel).

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Figure: The upper branch of the Conveyor Belt simulated by OCCAM. As a function of time (upper panel), as afunction of depth (middle panel) and as a function of density (lower panel).

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To study the Mediterranean water mass circulation andthe impact of the Mediterranean outflow in the NorthAtlantic circulation, several sensitivity studies wereperformed running the GCM’s in order to improve themodel performance. In the first phase of the projectsensitivity studies of both surface forcing and tracerdiffusivity parameterisation was performed usingMEDMOM in order to eliminate drift in the model tosuccessively apply the routines and methodologiesdefined during the TRACMASS project, integratingparticles using eulerian velocity field from a steadyperpetual year, representative of the climatologicalMediterranean circulation. Moreover, due to the relativelysmallness of the Mediterranean domain, MEDMOMsimulation was used also to define the optimal strategy oftime sampling of the eulerian velocity field for offline

lagrangian integration and to develop Lyapunov exponenttechniques for characterising Lagrangian two−particledispersion in non−asymptotic condition. Finally in the lastyear of the project we have studied the impact of theMediterranean outflow in the North Atlantic performingsensitivity studies using OPA global simulation.

The ocean circulation models that have been used tosimulate the velocity fields are presented in chapter 2.The trajectory model that we use to simulate ourtrajectories is presented in chapter 3. In chapter 4 wepresent a summary of the Conveyor Belt resultswhich areall realted to the North Atlantic Deep Wate. In chapter 5we will summarise the circulation in the Mediterraneanand its impact on the Conveyor Belt. In chapter 6, 7 and 8we will summarise the TRACMASS articles that havebeen published, are in press and are to be submitted soon

Figure: Trajectories in the mediterranean simulated by MEDMOM.

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Chapter 2. The Ocean Circulation Models

TRACMASS has access to output from four numericalocean models; 2 global, z−coordinate models, 1 globalisopycnic model and 1 regional (Mediterranean) z−coordinate model. The models vary in resolution, contentand forcing. Importantly, the type and frequency of

available model output also varies. The differences aresummarised in the tables below. An illustration of the thethree global models is given in Fig. 1 which shows theinstantaneous sea surface temperature during January foreach of the three global models.

Model: OPAContact partner: LPO

Manual/reference Delecluse et al., 1993; Madec et al., 1997

Discretisation: Finite differences on a staggered "C" grid [Arakawa, 1972]

Domain: Global, from the Southern Ocean at 78¡S to 90¡N. Curvilinear grid withpole singularity on land [Madec and Imbard, 1996]. The zonal resolution is2¡ within the whole southern hemisphere and is variable in the northernhemisphere. The meridional grid interval also varies from 0.5¡ at theequator increasing through 0.9¡ at 10¡N and 10¡S and never exceeding1.5¡.

Vertical resolution: Mixed Layer + 30 fixed width levels varying in thickness from 10meach in the top 150m to 340m each at depth.

Topography source: 5’x5’ ETOPO5Prognostic variables: Potential temperature, salinity, horizontal velocity compnents, turbulent

kinetic energy (Blanke and Delecluse [1993]), barotropic streamfunction.Surface forcing: Hellerman and Rosenstein’s [1983] monthly mean wind−stress

climatology and Esbensen and Kushnir’s [1981] monthly mean heat fluxclimatology. The fresh water budget is given by Oberhuber’s [1988]climatology, with a relaxation toward Levitus’ [1982] seasonal surfacesalinity

Interior forcing: "Robust−diagnostic". A restoring term towards Levitus’ [1982]climatology is added to the temperature and salinity equations. Therestoring coefficient is the inverse of a time scale and varies with depthand with the distances from the coast and the equator

Data available forTRACMASSinvestigations:

Monthly averages of the last year of a 10−year run. Each datasetcontains running means of the main state variables

Model specific physics: Rigid−lid. Vertical eddy viscosity and diffusivity coefficients arecomputed from a 1.5 turbulent closure scheme which permits an explicitformulation of the mixed layer as well as a minimum diffusion in thethermocline.

The solar radiation is allowed to penetrate in the top meters of theocean. Lateral mixing of heat and salt occurs only along neutral surfaces.

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Model: OCCAMContact partner: SOC

Manual/reference Webb, D.J. et al 1998Discretisation: Finite differences on a staggered "B" grid [Arakawa, 1972]

Domain: Global, from the Southern Ocean at 78.5¡S to 90¡N. Two−grid system toavoid pole singularity. A standard latitude−longitude grid covers theSouthern hemisphere and the Pacific and Indian oceans. A second grid,rotated through 90¡ so that its poles lies on the geographical equator,covers the North Atlantic and Arctic basins. The two grids communicateeach timestep at the Atlantic equator and Bering Strait. The zonal andmeridional resoultion is uniform at 0.25¡ in each direction on each modelgrid.

Vertical resolution: 36 fixed width levels varying in thickness from 10m at the surface to250m at depth

Topography source: 5’x5’ ETOPO5

Prognostic variables: Potential temperature, salinity, horizontal velocity components, seasurface height.

Surface forcing: 1 set of mean monthly winds derived from ECMWF wind stresses for1986−88. Surface relaxation to monthly Levitus [1994] SST (30 daytimescale). The fresh water flux is derived from the salinity differencebetween the model and monthly Levitus [1994] surface salinities. Thisdifference is converted to a freshwater flux which drives a volume changevia the sea surface height field. Additional runs have also been performedusing 6−hourly ECMWF wind−stresses from 1993 onwards

Interior forcing: "robust diagnostic" forcing for 4 model years; none subsequently

Data available forTRACMASSinvestigations:

Climatological run: Annual and seasonal averages derived from 4 years(model years 8 to 12) of 15 day−interval instantaneous snapshots of themodel state variables (98 datasets).

High frequency wind run: 2 years (1993,1994) of 5 day−interval runningmeans of the model state variables (146 datasets). 1995 and 1996 will beavailable soon.

Model specific physics: Free surface. Improved advection scheme (quadratic upstream).Normal Laplacian diffusion and momentum terms to represent

horizontal mixing. For diffusion of tracers the value of the horizontaldiffusion coefficient is 1*106 cm 2 /s. For the velocity field the horizontalviscosity is 2*106 cm2 /s.

After day 480 the model uses Pacanowski and Philander (1981) verticalmixing for the tracer fields.

The velocity fields use normal Laplacian mixing in the vertical with acoefficient of 1 cm2 /s.

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Model: GIM

Contact partner: SOC

Manual/reference Bleck, R. et al 1992, based on MICOM v.2.8

Discretisation: Finite differences on a staggered "C" grid [Arakawa, 1972]Domain: Quasi−global, from the Southern Ocean to 74¡N. A standard latitude−

longitude grid with uniform zonal and meridional resoultion of 1.25¡. Theland mask has a resolution of 2.5¡ latitude by 3.75¡ longitude.

Vertical resolution: 20 layers (Mixed layer + 19 isopycnic layers). Sigma−0 in the range24.70 to 28.13.

Topography source: 5’x5’ ETOPO5

Prognostic variables: Layer thickness, salinity, horizontal velocities. Layer potentialtemperature is also prognostic within the mixed layer

Surface forcing: Monthly mean climatological fluxes: wind stress [Hellerman andRosenstein (1983)], heat fluxes and evaporation rate [Esbensen andKushnir (1981)], precipitation rate [Jaeger (1976)] plus relaxation fluxes toensure that sea surface temperature and salinity remain close to monthlymean climatological values [Levitus (1982)] based on a fixed relaxationcoefficient of 35 W m−2 K−1; surface fluxes of heat and freshwater, and thewind mixing power (but not the wind stress) are set to zero at "ice−covered" model gridpoints, defined as such if either climatological SST ormodel mixed layer temperature falls below −1.8 ¡C

Interior forcing: Diapycnal mixing: entrainment/detrainment between mixed layer andinterior layers (according to available TKE, buoyancy forcing, baroclinicshear); diffusive mixing of interior layers, parameterized with theMcDougall and Dewar (1998) scheme, using diffusivity c/N, where c =0.575 x 10−3 cm−2 s−2, yielding typical vertical diffusivity 10−4 cm2 s−2 in thedeep ocean.

Lateral (isopycnic) layer thickness mixing: analagous to bolus transport,but literally "layer interface depth diffusion" in MICOM, is parameterizedwith isopycnal diffusivity ud*L where ud is a constant "thickness diffusionvelocity" of 1 cm s−1 and L is grid−cell width (typically 100 km). Thesevalues yield diffusivities of order 103 m2 s−1 .

Data available forTRACMASSinvestigations:

Monthly snapshots of: T, S, h, u, v for each layer; barotropic (u,v) andsea level height running means of: zonal, meridional and diapycnal massfluxes for each layer.

Model specific physics: Free surface, online integration of layer mass fluxes

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Model: MEDMOMContact partner: ENEA

Manual/reference Roussenov et al., 1995; Wu and Haines, 1996Discretisation: Finite differences on a staggered "B" grid [Arakawa, 1972]

Domain: Mediterranean basin + Atlantic Òbuffer zoneÓ to 13¡E. A standardlatitude−longitude grid with uniform zonal and meridional resoultion of0.25¡.

Vertical resolution: 19 fixed width levels

Topography source: 5’x5’ ETOPO5Prognostic variables: Potential temperature, salinity, horizontal velocity components, sea

surface height.Surface forcing: ECMWF monthly or daily wind stresses. Surface temperature is relaxed

to satellite monthly or daily values (Gacic et al., 1997) using a winddependent time scale ranging from 2 hours to 5 days. In practice the

stronger the wind, the shorter the relaxation time is. Surface salinity isrelaxed in all the domain toward MODB MED5 (Brankart and Brasseur ,1996) climatology using a constant 5 days time scale in all the domain.

Interior forcing: Temperature and salinity are relaxed on 5 days time scale on all levelsonly

in the atlantic buffer zone.Data available for

TRACMASSinvestigations:

Results from 6 experiments are available each using different diffusivityparameterizations and surface forcing. Daily forcing runs use fields from1988 applied repeatedly for O(400) model years. Some years havesnapshot and averaged fields stored every 10 days. In the currentconfiguration, 1 model year integration requires approx.4 hours of CPU ona DEC 4100 Alpha station. Higher frequency datasets can be produced onrequest.

Model specific physics: Rigid Lid. Constant eddy viscosity is assumed (nuH = 1.5x106 cm2/sec,nuV = 1.5 cm2/sec ) both in time and in space and is represented by alaplacian or biharmonic operator.

Vertical diffusivity is constant in space and time but with a fixedvertical profile ( high values (from 3 to 1 cm2/sec) in the first 80 meterrapidly decreasing to a very low value at depth). Tracer horizontaldiffusivity is computed by a time and space dependent parameterization(Babiano et al., 1987). This assumes diffusivity is proportional to eddykinetic energy and inversely proportional to the root square of theenstrophy, where both eddy kinetic energy and enstrophy are computedaveraging on a neighbourood of the point having the size of the eueriancorrelation length.

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Chapter 3. Theoretical studies of the trajectory methods

1. Introduction

The TRACMASS project aimed at documenting thelarge scale ocean circulation simulated by several oceangeneral circulation models (OGCM) from a Lagrangianpoint of view.

Such diagnostics are based on multiple individualtrajectory computations in the velocity field computed bythe OGCM, with the joint use of an appropriate numericaladvection scheme [Blanke and Raynaud, 1997] and anefficient description of a water mass in terms of theparticles that compose it [Döös, 1995].

They were to be applied to different classes of OGCMs(one isopycnic model, three z−coordinate models),developed over distinct domains (the Mediterranean Seaor the global ocean), with different grid resolutions. Thus,some necessary numerical coding and theoreticaldevelopments were associated to the achievement ofTRACMASS, in order to ensure robust and powerfulLagrangian analyses for the tracing of North Atlantic andMediterranean water masses.

One major problem related to Lagrangian tracingtechniques of selected water masses is linked to oceantime scales for such large scale advection processes.Lagrangian trajectories should be integrated long enoughto document the full extent of basin scale (or global scale)water mass movements, i.e., several hundreds (or eventhousands) of years, whereas direct OGCM simulationsseldom exceed a few tens of years, because of CPUlimitations or inherent drifting problems.

One way to by−pass this issue is the development ofoffline Lagrangian calculations, making a full use ofarchived velocity (and associated tracers) fields andrelying on multiple loops over the archived period. One isthen confronted with a triple question:− is there any way to correct poorly sampled velocities(excluding the mean effect of eddies or even the seasonalcycle) when calculating offline trajectories ?− alternatively, how frequent must be the archived thevelocity field of an OGCM for ensuring sufficientaccuracy of subsequent offline trajectory calculations(with respect to virtually exact online computations) ?− can one feel comfortable working with the limitedarchived velocity field (for instance over one single year)of a drifting OGCM simulation ?

The third issue concerns mostly both GIM andOCCAM simulations. The OPA model was indeed run ina robust diagnostic mode: the OGCM was forced by meanseasonal atmospheric fluxes and its density field wasstrongly constrained on an observational dataset oftemperature and salinity so that it soon asymptoted amean seasonal cycle. As for MEDMOM, theMediterranean circulation could be spun up until quasi−equilibrium, using appropriate climatological surfaceforcing and restoring conditions in the Atlantic Ocean(west of the Gulf of Cadiz). TRACMASS could deriveappropriate ways to diagnose the component of the

circulation due to drift in OCCAM, and provide options todeal with associate spurious diapycnal fluxes.

The second question was addressed in the frameworkof the no− (or only weakly−)drifting MEDMOM and OPAmodels, for which accurate archives of the velocity fieldcould be made available, either over the fullMediterranean Sea (MEDMOM) or a North Atlanticsubregion of the global domain (OPA). In the specificcase of the MEDMOM configuration, offline trajectoriescould even be tested against pseudo online calculations(i.e., offline trajectories calculated with a velocity fieldsampled at the exact period of the model time step).

The first issue is especially important when dealingwith expensive models as OCCAM, for which thesharpness of the horizontal grid (here, 1/4¡ on a globalscale) precludes a frequent output of the model, thatwould sample ideally the full spectrum of modelled oceanvariability. The use of the bolus velocity, diagnosed fromthe correlation between eddy velocity and isopycnic layerthickness, proved helpful in TRACMASS to recoverrather realistic Lagrangian calculations from a poorlysampled velocity field (averaged over four successivethree month periods).

Therefore, the three issues are at the root of thetheoretical activities planned in TRACMASS and areintroduced in the following sections before beingpresented more in detail in relevant companion papers.They deal mostly with:− specific model developments of the trajectory code andthe Lagrangian methodology;− the determination of the optimal sampling period of anOGCM for the analysis of subsequent time−dependentvelocity fields, and some developments about thepredictability of trajectories making use of Lyapunovexponent calculations;− the definition of an appropriate methodology fordealing with simulations not yet adjusted in time (i.e.,incorporating drift);− the investigation of the influence of the bolus velocityon the trajectories, and more specifically the diagnostic ofeddy−induced mass transports;− the comparison of results obtained from onlineLagrangian analyses with diagnostics based on passivetracers.

2. Some fundamental issues

The equations of motion of the OPA model arediscretised on a C−grid [Arakawa, 1972]. Thisdifferencing turns out ideal for the computation ofsuccessive analytical streamline segments [Blanke andRaynaud, 1997] for a given time sampled velocity field ofthe OGCM.

This mass−preserving trajectory scheme is especiallyrelevant for the tracing of ocean water masses. A watermass on a given geographic section is inseminated withtens or hundreds of thousands of particles [Döös, 1995;

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Blanke and Raynaud, 1997], each of which associatedwith an infinitesimal fraction of the incoming transport.For selected final destinations (another geographicsection, or the fulfilment of a hydrographical criterion)infinitesimal transports may be added, and directional(i.e., section−to−section) transports can be diagnosed.

GIM (isopycnic formulation) and MEDMOM andOCCAM (B−grid) models did not fit immediately theframework initiated with OCCAM and OPA (ARIANE).Therefore, some relevant code developments wereplanned and undertaken to allow the calculation ofequivalent Lagrangian diagnostics. This stage ofTRACMASS work is introduced in section 1b.

2.a Individual calculations

The analytical calculation on a C−grid of three−dimensional (3D) streamlines, for periods over which thevelocity field is assumed to be constant, defines aconvenient way to derive particle trajectories within anymodel gridcell.

Such a way to compute trajectories results:−fast (for a given gridcell, it only calculates entry and exitpositions);−accurate (it fully respects the 3D non−divergence of theflow, i.e., mass conservation);−flexible (it can be used for backward or forwardintegrations);−adaptable for non−stationary velocity fields (theintegration can be done over successive time steps,namely the available sampling time of the output, overwhich the velocity is maintained constant).

Assuming a linear variation of each velocitycomponent along its corresponding axis, basic trajectoryequations are written along any of the three coordinates.Integrating these equations in time links each coordinate(x, y or z) with time in any given gridcell. The transittime along each direction is diagnosed by imposing thecorresponding edge of the gridcell as a final position. Theminimum of the three estimates gives the effective transittime, and allows the calculation of the exact final positionover the appropriate edge of the gridcell.

2.b Detailed calculations [see Blanke and Raynaud,1997]

The three components of the velocity are known overthe six faces of each cell. The non divergence of thevelocity field then ensures continuous trajectories withinthis cell. We develop here some equations with thetensorial formalism used in the OPA model which allowsa more general approach than a simple Cartesian view,with for instance the description of a distorted grid overthe sphere. The divergence of the three−dimensionalvelocity field V = (U, V, W) is expressed as:

∆V = b −1 [∂i(e

2 e

3 U) + ∂

j(e

1 e

3 V) + ∂

k(e

1 e

2 W)], (1)

where, n = i, j or k refers to the grid index for thethree axes, ∂

nrefers to the corresponding finite difference,

e1, e

2and e

3are the scale factors (in the three directions)

computed at each velocity gridpoint, and b is the producte

1 e

2 e

3computed at the centre of a given cell

("temperature" gridpoint). For any choice of the grid, thenon divergence of the flow now simply writes as:

∂iF + ∂

jG + ∂

kH = 0, (2)

where F, G and H designate the transports in the threedirections, with F = e

2 e

3 U, etc.

We assume now that the three components of thetransport vary linearly between two opposite faces of oneindividual cell. This hypothesis respects the local three−dimensional non divergence of the flow. It means that,within a given cell, F depends linearly on i, G dependslinearly on j and H depends linearly on k, where i, j and kare considered as fractional within the cell (i.e., as noninteger). In the cell extending from i = 0 to i = 1, onecan write for instance for F:

F(r) = F0 + r ∆F, (3)

with r ∈ [0, 1] and F(0) = F0, and where ∆F stands

for F(1) ÐF(0). One can also write the equation linkingposition and velocity:

dr/ds = F, (4)

with s = (e1 e

2 e

3) −1 t and x = e

1 r. With some

adequate initial conditions, e.g., r = 0 for s = 0, one cancombine equations (3) and (4), and find the timedependency of r within the considered cell:

r = F0 ∆F −1 [exp(∆F s) Ð 1]. (5a)

If ∆F = 0, only the limit of (5a) for ∆F → 0 is to beconsidered:

r = F0 s. (5b)

Similar relationships are of course obtained along bothother directions. Since these expressions only apply in oneindividual cell, one also has to determine the time when agiven particle switches to another cell, or equivalently thetime when r is equal to its exit value (here r = 1). Timedependency is obtained from a different writing of (4):

ds = F −1 dr. (6)

Using (3), one obtains the following expression for ds:

ds = (F ∆F) −1 dF. (7)

A crossing time in the zonal direction can only beobtained if F(1) and F(0) have the same sign, and thisimplies F - 0 in the cell. If this condition is not verifiedfor F, the three−dimensional non divergence of thevelocity field ensures that at least one other directionsatisfies it. One can assume that this condition is checkedin the zonal direction. The pseudo time s is then related tothe transport F by:

S = ∆F −1 ln(F / F0). (8)

The crossing time in this direction corresponds to themoment when the transport reaches the exit face value,F(1):

∆s = ∆F −1 ln(F1 / F

0), (9a)

or, if ∆F = 0, its limit when ∆F → 0:

∆s = 1 / F0. (9b)

As previously mentioned, at least one of the threecrossing times is to be defined through such aformulation. The shortest one defines the travelling timein the considered cell. Thus, if the particle first attains thezonal extremity of the cell, its positions on the meridionaland vertical axes are deduced from the equations of thetrajectories using s = ∆s. Computations are done for the

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next cell, with a starting point equal to the exit point ofthe previous one, and the "age" of the particle is regularlyupdated summing the expressions (9) obtained for ∆s.

2.c Quantitative diagnostics

Quantitative results are obtained by increasingsignificantly the number of particles (up to severalmillions), following the methodology introduced by Döös[1995].

Due to water incompressibility, one given particle withan infinitesimal volume is to conserve its infinitesimalmass along its trajectory. As a current can be entirelydetermined by the particles that compose it, the transportof a given water mass can be calculated from its ownparticles and their associated infinitesimal transport.

Directional mass transfers are thus estimated bysumming the infinitesimal transports of all particlesachieving the same connection. Offline diagnostics allowbackward computations of trajectories and the joint use ofbackward and forward experiments gives access to theerror in computing directional transports, by determiningany of them with two independent calculations.

2.d Mass streamfunctions

Following a set of particles in the ocean and summingtheir algebraic transport at each velocity point of the gridthey run across, one obtains a 3D field that corresponds tothe flow of the water mass in study.

For a fraction of a given initial section (as, e.g., awater mass, with identified hydrographical properties, ona zonal transatlantic section), we define multiple initialpositions for particles, each one of them associated with afraction T

iof the total water mass transport. We compute

trajectories for all the particles, and we sum algebraicallythe T

i’s on each junction of two cells of the model, on the

velocity gridpoints of the staggered C−grid. Northwardand eastward movements are counted positive, whilesouthward or westward movements are counted negative.We obtain a three−dimensional transport field thatcorresponds to the flow of the water mass in study, withinthe domain of integration of the trajectories. As oneparticle entering one model gridcell through one of its sixfaces has to leave it (by another face), the transport fieldexactly satisfies:

∂iT

x + ∂

jT

y + ∂

kT

z = 0, (10)

where Tx, T

yand T

zdesignate the directional flows (in

sverdrups) in the three directions, and where n = i, j, or krefers to the grid index for the three axes. Integrating thisfield along a selected direction (either the vertical, or thezonal or meridional axis), we obtain a two−dimensionalnon−divergent field that we study by means of astreamfunction. For a vertically−integrated transport or azonally−integrated transport, we define ψ

hand ψ

yz

respectively with:

{∂iψ

h = Σ

k T

y ; ∂

jψ

h = − Σ

k T

x}

{∂iψ

yz = Σ

k T

z ; ∂

jψ

yz = − Σ

k T

y} (11)

and contours of ψh

or ψyz

provide an adequate view ofthe movement in projection onto the selected plane.

As contours cannot cross each other (by constructionof a streamfunction), the more accurately and selectively

we define the initial conditions the particles, the moresimilar to actual projections of trajectories the contoursresult. The choice of a horizontal projection usually turnsout judicious, but an additional plane of projection orplots of selected individual trajectories may helpfullyprecise the movement of the water mass.

Such streamfunctions are at the root of manyTRACMASS−related diagnostics, and can be derivedfrom any of the models involved in the project. Thismethodology was introduced in a JPO paper dedicated tothe circulation of the warm water masses within theequatorial Atlantic {Blanke et al., JPO 1999}.

3. Coding

3.a GIM

As the original trajectory numerical scheme wasdeveloped for a natural implementation in z−coordinatemodels, it needed crucial modifications and adjustmentsbefore being usable in GIM.

The development of the Lagrangian code for thisglobal isopycnic model was undertaken together with thetesting of the account for diffusive effects in thecomputation of a particle’s movement. and the GIMtrajectory code was soon fully working after severalmodifications applied to the initial approach of Döös[1995] and Blanke and Raynaud [1997].

The details of the calculation are listed and discussedin a paper recently submitted for publication {Marsh andMegann, 2001, submitted to Ocean Modelling}.

3.b MEDMOM

The MEDMOM model is based on the Cox model thatuses a B−grid while TRACMASS routines for off−linetrajectory integrations were initially developed for C−gridded velocity fields.

Simple interpolation routines have been written totransform the MEDMOM output into adequately griddedvariables, conserving mass fluxes through basin−widesections, but changing the geometry of the domain byextending the velocity field one grid point further into thecoast. The subsequent errors on the whole trajectories andassociated time scales can be considered negligible at firstorder.

Off−line Lagrangian trajectories based on a storage ofthe velocity field equal to the model time step were alsodeveloped, and took advantage of the local adaptation andmodification of a vectorised version of ARIANE(developed at LPO).

The Lagrangian diagnostic of the thermohalinecirculation of the Mediterranean Sea and the investigationof Lagrangian trajectories’ predictability could beaddressed via these developments, and are the objects oftwo papers in preparation {Iudicone et al., 2002ab, inpreparation}.

3.c OCCAM

The simulations of the OCCAM model analysed withinTRACMASS incorporate an active free surface (whereasthe MEDMOM and OPA simulations were run with arigid−lid assumption), as well as the prescription of a

14

non−zero vertical velocity at the sea surface (as the netevaporation minus precipitation fresh water flux).

The trajectory code developed for OCCAM accountsfor these surface features, and lets individual trajectoriesescape the ocean model when necessary. The lattertrajectories can be allowed to enter the surface layer ofthe model in regions with precipitation. This is doneemploying a probability distribution that incorporates thestrengths of the vertical transports signifying precipitationand the distance between the sites of exiting and re−entering the surface.

The above was applied in all papers dealing withOCCAM results {Nycander et al., Tellus 2001; De Vrieset al., International WOCE Newsletters 2000}.

3.d Time−dependent code

The original Lagrangian diagnostics derived witheither OPA or OCCAM were based on a discontinuoussuccessive update of the archived velocity field in thetrajectory calculations.

A time−dependent trajectory scheme could bedeveloped and proved especially useful when dealingwith seasonally model velocity outputs.

This scheme uses a linear interpolation both in spaceand time allowing analytical solutions for the trajectoryinside a model gridcell. Contrary to the case of stationaryfields, the transit time through each gridcell must now bedetermined numerically. A description of how to applyand implement the analytical results has already beenpublished {De Vries and Döös, JAOT 2001}.

3.e Best positioning of initial particles

The best positioning of the particles over initialsections may be defined as the one that gives the highestaccuracy on the computed directional transports, for areasonable number of initial positions [see Blanke andRaynaud, 1997].

The sensitivity of quantitative Lagrangian diagnosticsto the space distribution of the initial positions used tocalculate them was investigated. Some mathematical andstatistical arguments could be derived to attest therelevance of the strategy privileged until then [Blanke andRaynaud, 1997], with respect to more simpledistributions.

In all MEDMOM and OPA quantitative diagnosticsrun with ARIANE, the volume (one time and two spacedirections) of each individual gridcell (transport Ti) isdivided into Ni3 sub regions, with Ti/Ni

3 < T0, where T0 isa prescribed maximum transport for any given particle.The total number of particles is the sum of the Ni’s, and ahomogeneous distribution is adopted within each gridcell,as introduced for estimating interhemispheric massexchanges in {Blanke et al., JPO 1999} or fordocumenting the mass flux directed from the surfacemixed layer toward the interior ocean in {Blanke et al.,2001, submitted to JPO}.

3.f One final remark

All four Lagrangian codes (developed for GIM,MEDMOM, OCCAM and OPA) are freely available via alink from the main TRACMASS website.

4. Time sampling strategy

4.a Diagnostics with OPA in its non eddy−resolvingclimatological version

The time−sampling problem was first investigated bymeans of Lagrangian diagnostics applied to distinctvalues of the sampling period of the velocity fieldcomputed by the OPA model, in the North Atlantic, run inits robust−diagnostic mode (i.e., with monthly meanclimatological internal and surface constraints).

Low−order Lagrangian statistics were estimated fromdifferent sets of particle trajectories launched in the OPAmodel northern subtropical gyre and could show that forthese individual trajectories, the effects of varying thetime sampling within the range [1 month Ð 1 year] seemsnegligible, a result all at all consistent with a model high−frequency eddy field closely determined by the monthlyforcing and internal constraints {Valdivieso Da Costa andBlanke, 2001, submitted to JPO}.

4.b The time sampling problem studies in MEDMOM

Somewhat equivalent investigations were completedfor the MEDMOM eddy−permitting case, with additionalconsiderations related to the sensitivity of offlineLagrangian calculations to the frequency of theatmospheric forcing.

Recirculation was pictured as the component of theflow mostly sensitive to the sampling rate, with a strongsensitivity to the surface forcing frequency. These resultstend to assess the value of an optimal sampling periodclose to the frequency of the atmospheric forcing. Thethreshold for reliability was found to be 15 days for themonthly forcing and 3 days for the daily forcing, asexposed in detail in a paper currently under review{Iudicone et al., 2001, submitted to Ocean Modelling}.

4.c Diagnostics with OPA in the eddy−resolvingrealistic CLIPPER version

Trajectory experiments in an eddy−resolving primitiveequation model of the Atlantic (CLIPPER project,managed in Brest, Grenoble and Paris) were also used toinvestigate lateral dispersion processes in the Agulhasretroflection area, south of Africa, and to study itsdependence on the spatial and temporal resolution of theEulerian velocity dataset. A paper summarising the mainresults of the study is in preparation {Valdivieso Da Costaand Blanke, 2001, in preparation}.

4.d Lyapunov exponents techniques developed forMEDMOM

The MEDMOM framework was also used to developLyapunov exponent techniques for characterisingLagrangian two−particle dispersion in non−asymptoticcondition. This approach leads to a measurement ofdispersion rates at any scale, regardless of the assumptionof small−scale turbulence (unlike a model of tracerdispersion. It allowed the study of relative dispersion in abounded flow like the Mediterranean Sea and clarified theeffects of uncertainties in the velocity field, here due todifferent smoothing times, on Lagrangian predictability.

The regions most sensitive to the variability of the

15

velocity field could be identified, and the resultsemphasise the importance of chaotic advection, as aconsequence of local dynamical processes, in triggeringLagrangian non−stationarity in the most energetic regions{Iudicone et al., 2002b, in preparation}.

5. Drift

Averaging the data of an OGCM simulation that is stilldrifting in time may lead to the appearance of spuriousdiapycnal fluxes. These fluxes do not account for physicalprocesses that one truly wants to investigate or diagnosethoroughly, but represent basically the signature ofinflating or deflating density layers not yet adjusted to thesurface atmospheric forcing.

A method have been devised for the OCCAM datasetin order to correct these spurious fluxes. It consists ofdiagnosing and analysing both the local and mean drift ofeach density layer. From this a drift−compensatingvelocity field is computed that is then added to the modelvelocity field {De Vries and Drijfhout, 2002, inpreparation}.

6. Eddies and bolus transport

The importance of bolus velocity (i.e., the correlationbetween the eddy velocity and isopycnic layer thickness)in advecting water mass has already been demonstrated inOGCMs [Danabasoglu et al., 1994; Gent et al., 1995]where the bolus velocity is parameterised, and in anidealised eddy−resolving model [Lee et al., 1997] wherethe bolus velocity is diagnosed.

The effect of eddies can be accounted for, as can betransient behavior such as seasonal cycles. The effect ofvariability on trajectories is accounted for byincorporating eddy−associated transports (correlations ofdensity and velocity). However, when eddy transports aresmall/large the effect is small/large.

Therefore, as part of the TRACMASS investigation,the bolus velocity in an eddy−permitting global oceanmodel (OCCAM) was for the very first time computeddirectly from the model output. For this purpose, newannual−mean data sets were constructed from availablefive−day running mean calculations, including the effectsof the eddies and/or seasonal variability.

The use of these bolus fields was shown to reduce aspurious circulation cell, namely the Deacon Cell, in theSouthern Ocean that is present in the annual−meanvelocity field, and modify the upwelling behaviourdiagnosed in this region {Lee et al., 2002, in preparation}.

7. Tracers vs. floats

In order to have a better control on the analyses basedon individual Lagrangian trajectories, TRACMASSinvolved a comparison of the respective spreading of apassive tracer and of particles in the flow modelled in anisopycnic channel model.

The principal difference between trajectories ofparticles with the spreading of a passive tracer is inducedby mixing: the tracer will be mixed away with isopycnal

and diapycnal diffusion, which loses propertyinformation. In the trajectory method, the enhancedmixing of water mass properties is not describedexplicitly, but is given implicitly by the large along−trajectory changes of these properties.

Online tracer and particles diagnostics are consistent toshow that particles tend to move toward the regions oflarge layer thickness. These conclusions are obtained in anumerical set−up where mean and eddy flows are of thesame order of magnitude, but with an opposite sign. Theyevidence the effect of eddy diffusion on cross−frontspreading abilities, though on rather long time scales{Lee, 2001, accepted in JPO}.

8. Conclusions

The coding and testing of Lagrangian trajectory codesfor all the OGCMs involved in TRACMASS could beachieved in due time to allow a fast and profitableimplementation of the main physical diagnostics linked towater mass tracing for the Mediterranean Sea or the NorthAtlantic Deep Water.

The robustness of the results that will be introducedand discussed in the next chapters could be assessed byspecific sensitivity aiming at diagnosing the optimalsampling period of one given OGCM, or by gettingaround this point with the use of velocity fields takingaccount of a seasonally computed bolus component.Additional conclusions, comforting the use of Lagrangiantrajectories for water mass tracing, could be drawn in thecomparison of online trajectories with passive tracers inan idealised isopycnic model.

9. Cited references

Arakawa, A., 1972: Design of the UCLA generalcirculation model. Numerical simulation of weather andclimate. Tech. Rep. 7, Dept. of Meteorology, Universityof California, Los Angeles, 116 pp.

Blanke, B. and S. Raynaud, 1997: Kinematics of thePacific Equatorial Undercurrent: a Eulerian andLagrangian approach from GCM results.J. Phys. Oceanogr., 27, 1038−1053.

Danabasoglu, G., J. C. McWilliams and P. R. Gent,1994: The role of mesoscale tracer transports in the globalocean circulation. Science, 264, 1123−1126.

Döös, K., 1995: Interocean exchange of water masses.J. Geophys. Res., 100, 13499−13514.

Gent, P. R., J. Willebrand, T. J. McDougall and J. C.McWilliams. 1995. Parameterising eddy−induced tracertransports in ocean circulation models.J. Phys. Oceanogr., 25, 463−474.

Lee, M.−M., D. P. Marshall and R. G. Williams, 1997:On the eddy transfer of tracers: advective or diffusive ?J. Mar. Res., 55, 483−505.

10. Companion papers

Blanke, B., M. Arhan, G. Madec and S. Roche, 1999:Warm water paths in the equatorial Atlantic as diagnosed

16

with a general circulation model. J. Phys. Oceanogr., 29,2753−2768.

Blanke, B., S. Speich, G. Madec and R. Maugž, 2001.A global diagnostic of interior ocean ventilation.Submitted to Geophys. Res. Lett.

De Vries, P. and K. Döös, 2001: CalculatingLagrangian trajectories using time−dependent velocityfields. J. Atmos. Oceanic Tech., 18, 1092−1101.

De Vries, P., S. Drijfhout and A. Coward, 2000: Therole of the Southern Ocean in the upwelling of NorthAtlantic Deep Water. Int. WOCE Newslett., 39, 12−15.

De Vries, P. and S. Drijfhout, 2002, A new approachfor correcting drift in ocean models. In preparation.

Nycander, J, K. Döös, and A. Coward, 2001: Chaoticand regular trajectories in the Antarctic CircumpolarCurrent. In press in Tellus.

Iudicone, D., ..., 2001: ... Sensitivity to timesampling.... Submitted to Ocean Modelling.

Iudicone, D., ..., 2002: ... Thermohaline circulation ofthe Mediterranean Sea from Lagrangian calculations.... In

preparation.

Iudicone, D., ..., 2002: ... Predictability of Lagrangiantrajectories.... In preparation.

Lee, M.−M., 2001: Lagrangian motion of particles andtracers on isopycnals. Submitted in J. Phys. Oceanogr.

Lee, M.−M., A. Coward, P. De Vries, S. Drijfhout,2002: Eddy−induced transports in OCCAM. Inpreparation.

Marsh, B. and A. Megann, 2001: Tracing water masseswith particle trajectories in an isopycnic−coordinatemodel of the global ocean. In revision forOcean Modelling.

Valdivieso Da Costa, M. and B. Blanke, 2001:Sensitivity of trajectory calculations to the time resolutionof the velocity data from a non eddy−resolving OGCM.Submitted to J. Phys. Oceanogr.

Valdivieso Da Costa, M. and B. Blanke, 2002:Estimates of particle dispersion in an eddy−resolvingmodel of the Agulhas retroflection area, south of Africa.In preparation.

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Chapter 4. The Conveyor Belt or The origin, formation, fate andtransformation of the North Atlantic Deep Water

1. IntroductionPathways and mechanisms of oceanic heat and fresh

water transports are critical issues in the comprehensionof the present climate and its stability. Indeed, oceancirculation transfers heat and fresh water betweendifferent climate regimes and between different oceanbasins. This transport is coupled to convectiveoverturning, which links the full ocean volume to theclimate at decade−to−century time−scales. For example,surface water sinks into the deep layers of the northernNorth Atlantic and draws warm surface water from thesouth, warming the European climate. Unless the sinkingregions, principally the North Atlantic, are compensatedby upwelling within the confines of their local basin(which does not appear to be strong enough), inter−oceancirculation is implied. But what is its form, and how doesit feed back into convective processes and their associatedclimate phenomena?

Each of the oceans is exposed to different forcing, andtakes on distinct stratification in temperature andchemistry. In the North Pacific there is more precipitationthan evaporation, so the thermocline is low in salt, withlimited, shallow convection; in contrast, the Indian andAtlantic are evaporative oceans, so their upper layers aremore saline than those of the Pacific. In the NorthAtlantic the dense, salty surface water causes deepconvection at surface ocean temperatures well above thefreezing point of sea water. Even the Southern Ocean,which is less constrained by geometry, has considerablelongitudinal variations of near surface salinity andassociated bottom water formation rates and properties.

The chemical and thermal differences between themajor ocean basins would be much larger were it not forpassages permitting inter−ocean exchange. Thesepassages are constrictive links in the circulation’s abilityto equalise ocean properties between basins. Only byviewing the global nature of ocean circulation within themajor basins and its connecting passages can the ocean’srole in the climate system be fully appreciated. Indeed,the physical structure of the global North Atlantic DeepWater (NADW) driven thermohaline circulation (THC)and its efficiency in regulating the climate is substantiallyaffected by the nature and very existence of inter−oceanexchange of water masses. But the sources, pathways andphysical properties of these exchanges are not wellenough established to allow its influence on the climatesystem to be quantified.

1.a) State of the art on the NADW driven thermohalinecirculation

The first theory for THC emanates from Stommel[1957] for the Atlantic basin and Stommel and Aaron[1960] for an enlarged view spanning the global ocean.These two papers give a two−layer schematic of thecombined wind−driven and thermodynamically drivenocean general circulation, simulating NADW in the lowerlayer along with its compensation flow and the directlywind−forced response in the upper−layer. Upwelling fromthe lower to upper layers is indicated in the AntarcticCircumpolar Current (ACC) and in the interior of theocean. The sinking in the North Atlantic is presumed to

be due to deep convection forced by atmospheric coolingthere, and the upwelling shown in the interior is tobalance diffusive heat loss there across the boundarybetween the two layers. This circulation is also known asthe oceanic "conveyor belt" (CB) [Broecker, 1987; 1991].

Yet the nature and the very existence of a coherentglobal ocean THC pattern has received an increasedinterest in the past 15 years probably due to climateconsiderations [Gordon, 1986; Broecker, 1987; Rintoul,1991; Broecker 1991; Gordon et al. 1992; Schmitz, 1995;1996a and b; MacDonald and Wunsh 1996; Ganachaudand Wunsh, 2000].

The latest estimate from observations put the flux ofwater for THC to lie between 14 and 17 Sv (1 Sverdrup isequal to a million of tonnes a second of water).

Despite this flow estimate and direct observations ofNADW western boundary circulation in the AtlanticOcean, many aspects of the THC remain indeterminate, asfor example:� the NADW formation rate and characteristics � the spreading of NADW inside the Atlantic and

outside� the NADW upwelling/transformation � the structure and characteristics of the upper branch

or return flow of THC

1.b) THC Lower and Upper BranchDue to its simplicity the original Stommel [1957]

separation of the THC in a two layers cell has surviveduntil nowadays even though it has included some slightmodification since [e.g., Broecker, 1991, Schmitz, 1995].We adhere to this general simple scheme by defining theLower Branch of the conveyor belt the part of THC thatstarts by the NADW formation, continue with itsspreading and terminate where NADW upwells. Bycontrast, we define as Upper Branch the return flow thatcompensates for the NADW formation and spreading. TheUpper Branch is composed by different water masses thatspan from the surface to the lower thermocline (i.e.,intermediate waters). The connection of the two limbs ofthe THC is accomplished by the processes that forceNADW to upwell and transform.

1.c) TRACMASS approachUp to now, the various schemes proposed to depict the

global ocean thermohaline circulation have been derivedfrom combining various observations from hydrographic,current, and tracer data. Because observations arerestricted in space and time, they can only give partialinsight into the NADW−driven circulation. Generalcirculation models of the world ocean provide coherentthree−dimensional dynamical and thermodynamical fieldsvarying in time. Yet they only approximate reality andtheir results may be thought to depend too strongly on thepeculiarities their formalism. The aim of our project wasto elucidate the structure of the global ocean CB by takingadvantage of the completeness provided by numericalsimulations. To make our conclusions roobust and toprovide results that are not model dependent we usedthree different GCMs (GIM, OCCAM and OPA).

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The most natural approach to estimate origins andpathways of flow is to follow the movement of watermasses and their transformation. During the TRACMASSproject we have been successful in developing andapplying quantitative Lagrangian diagnostics to GCMsfields able to produce circulation schemes that materialisethe paths and transformations of water masses.

2. The Lower Branch

NADW is formed in the northern regions of the NorthAtlantic Ocean. The denomination of NADW representscomplex aggregate of several distinct dense water masses,formed and converted into different locations [Smethie,1993; Dickson and Brown, 1994; Fine, 1995].

2.a) Rate of NADW formation and characteristics Classically three origins are distinguished: Labrador

Sea Water (LSW), the least dense of the threecomponents, is formed by deep wintertime convection inthe central Labrador Sea; Denmark Strait Overflow(DSOW), the densest component of NADW, enters theWestern North Atlantic through the Denmark Strait,between Greenland and Iceland, and locally entrainssubpolar water (by overflow−enhanced mixing). The thirdclassical component, Island Scotland Overflow Water(ISOW), enters the North Atlantic mainly through theFaroe Bank Channel, between Island and Scotland. Itlocally entrains ambien subpolar water and eventuallyflows through the Charlie Gibbs fracture Zone (hence itsother designation, GFZW) around the Reykjanes Ridgetoward the western part of the basin, where it issandwiched between LSW and DSOW. Also a fourthwater source has been hypothesised by Pickart [1992], theShallow or Upper Labrador Sea Water (ULSW).

Trajectory experiments on the OPA fields were used tomeasure the North Atlantic origins to NADW [Blanke etal., 2001]. DSOW appears as the major contributionamong the overflows, while the greatest input is suppliedby the subpolar gyre. A paper summarising the mainresults is under revision in Journal of Phys. Oceanogr.[Blanke et al. 2001a].

The NADW in the North Atlantic of GIM was toounsteady for a meaningful study of the formation process,most trajectories converging on gridboxes of abyssal layerinflation in the subtropics [Marsh and Megann2001].OCCAM also suffers too much from drift tocomment on this issue.

2.b) NADW spreading The description of the circulation of NADW lies mostly

on extensive collections of accurate direct or indirectmeasurements. Various authors have produced circulationschemes [e.g., Schmitz and McCartney,1993; MacDonaldand Wunsh, 1996; Schmitz, 1996; Sloyan and Rintoul,2000; Smethie et al., 2000]. Here we propose resultsobtained by quantitative Lagrangian integrations on thethree TRACMASS global ocean GCMs dynamical fields.

OPA simulation give a full amplitude of the NorthAtlantic Deep Water overturning of 17.3 Sv. The deepcirculation in the North Atlantic consists of a deepwestern boundary current and a southward flow east ofthe Mid−Atlantic Ridge [Blanke et al., 2001a; Blanke etal., 2001b]. This last path (accounting of two−fifths of thetransport) confirms and quantifies some hypotheses

already formulated about the presence of a southwardflow of Labrador Sea Water in the eastern North Atlantic[Paillet et al., 1998].

The modelled Lagrangian view of the CB circulationappears as a puzzle of complex circuits involving manycircumnavigation of Antarctica, several recirculationwithin the tropical and subtropical gyres and a largespectrum of water transformation. [Pelou, 1999; Pelouand Speich, 2000] In OPA NADW spreads in the SouthAtlantic mostly as deep western boundary current.Separation from the continent occurs at three latitudes,the larger branch resulting the southernmost at around40¡−45¡S. NADW then slowly penetrates the ACC andupwells essentially in the Southern Ocean mostly aftermany circuits around Antartica.

NADW spreading in the Atlantic in OCCAM [Friocourtet al., ÒTracing water masses in the AtlanticÓ, KNMI−report in preparation; Balbous and Drijfhout, ÒTheConveyor Belt in OCCAM, KNMI−report TR−231, 2001]is completely dominated by the Deep Western BoundaryCurrent. The eastern basin of the South Atlantic is notquickly ventilated by a current at the eastern side of theMid Atlantic Ridge, but is slowly ventilated by diffusionand weak recirculations. Strong recirculations appear inthe North Atlantic (north and south of the Gulf Stream inthe subpolar and subtropical gyre), in the Indian Ocean(Subtropical gyre and Agulhas Current System) and in theAtlantic sector of the Southern Ocean, poleward of theAntarctic Circumpolar Current (ACC).

The role of eddies in the NADW spreading is mainly alarger focus of the spreading path in the core of the ACCand reducing the role of the recirculations in the Southernand Indian oceans. The role of drift is mainly to increasecross−isopycnal flow within the NADW outflow, withoutlarge qualitative changes in the spreading path. NADWspreading outside the North Atlantic in GIM was studiedby following southward deep flows across the equatorialAtlantic. These trajectories revealed very little northwardspreading into the Indian Ocean, predominant northwardspreading into the Pacific Ocean, and very little NADWreaching Drake Passage [Marsh and Megann 2001].

3. NADW upwelling and transformation It is unclear exactly how the NADW lightens and enters

the upper (return) branch of the thermohaline circulation.The classical view of a diapycnic mixing diffusing downheat in the Indian and Pacific Oceans [Stommel andAarons, 1960] contrasts with observations of turbulence[Gregg, 1989] and dye diffusion [Ledwell et al., 1998]that give too low values for ocean background diapycnaldiffusivity. (except for localized regions near roughtopography). A contrasting view is coming frommodelling studies who suggest a primordial role of theSouthern Ocean and the wind−driven Ekman transport[Toggweiler and Samuels, 1993; Döös and Coward, 1997;Rintoul and England, 2001].

Lagrangian calculations with OCCAM confirm theearlier Döös and Coward [1997] result with respect to thedominant role of the Southern Ocean in NADW−upwelling, also when is accounted for the eddy−induced(bolus) transports. About two−thirds of NADW seems toupwell in the Southern Ocean by Ekman pumping and issubsequently transformed by air/sea interaction. One−third is transformed by diapycnal mixing in the rest of the

19

World Ocean (De Vries and Drijfhout, 2001; De Vries etal., manuscript in preparation).

When trajectories are calculated from annual meanfields a large systematic shift occurs between first andlast upwelling of (transformed) NADW through thecritical density surface that divides the upper and lowerbranch of the THC. This shift can be explained by theDeacon Cell. Large−scale downwelling of previouslyupwelled NADW is suggested to occur between 30 and 40South. Including bolus velocities nearly cancels theDeacon Cell and the equatorward shift between first andlast upwelling is absent [De Vries and Drijfhout, 2001].When eddy transports are accounted for, any differencebetween first and last upwelling of (transformed) NADWis due to model deficiencies and drift, as large scaledownwelling of upwelled NADW has no physical basis inthe observations. The upwelling of NADW in theSouthern Ocean occurs simultaneously with circumpolarflow as NADW is advected within the ACC. Making aPoincare section in Drake Passage it can be seen thattrajectories gradually move upward and poleward eachtime they cross Drake Passage until they are advectedequatorward at the surface by the Ekman flow. Inbetween58 and 58.5 S and 200 and 1000m deep a core regionexists in which the water is nearly trapped in the ACC andtrajectories are regular [Nycander and Döös, 2001].

NADW upwelling in GIM varies according to whichmodel year mass fluxes (from a 50−year spinup) are usedto compute trajectories. Unsteadiness of the deep layers inGIM poses a limit on how much NADW actually upwells[Marsh and Megann 2001]. Relative to southward exportin the equatorial Atlantic, trajectories computed with year50 fluxes yield the highest proportion of NADWupwelling. Based these trajectories, a majority ofupwelling occurs in the Pacific Ocean northward of 30 S,while about 30% occurs in the Southern Ocean. Eddy−induced transports in GIM are parameterized by layerinterface diffusion and are implicit in the mass fluxesused to compute NADW trajectories for that model.Therefore, the NADW trajectories for GIM do not displaythe multiple upwellings and downwellings originallyfound (before including bolus velocities) with the NADWtrajectories for OCCAM. Following NADW aftertransformation to intermediate densities, GIM trajectoriestherefore reveal similar circumpolar behaviour to theÒmean+bolusÓ OCCAM trajectories.

4. The Upper BranchThe origins of the warm limb of the global conveyor

belt are the topics of permanent dabates amongoceanographers. According to a warm route hypothesis[Gordon, 1986], the Pacific and Indian Oceans are linkedto the upper Atlantic with an exchange of warm water viathe indonesian Throughflow and the Agulhas CurrentSystem south of Africa. In a cold route conjecture[Rintoul, 1991], the dominant contribution of water andheat into the Atlantic is obtained directly at the DrakePassage, south of America.

The global component of ocean circulation is usuallythought to be driven by temperature and salinitydifferences (and called thermohaline circulation), whilebasin−scale flow is considered to be wind−driven. But itis artificial to divide the circulation into thesecomponents, because nature satisfies all the forcingmechanisms with a single integrated circulation.

During TRACMASS we were able to show that the twoforcing are very intimately linked, especially inconditioning how, how much and what kind of upperlayers water return to the North Atlantic in order tocompensate the NADW overflow. In a first paper [Speichet al. 2001a] we emphasise that not only the warm andcold routes exist and that they are of the same order in thepresent state of climate, but we show evidence of a thirdroute, making Pacific water entering the Indian Oceansouth of Tasmania.

The three routes prove to be a robust structure of theCB as they are reproduced by the three TRACMASSglobal ocean GCMs despite the different modelconfigurations and forcing [Speich et al. 2001b, inpreparation]. Yet, the most striking feature for the CBupper branch we have evidenced so far is the robustnessand stability of the new route, the Tasman Leakage[Speich et al. 2001c]. The related water mass transportthat reaches the North Atlantic amounts to ~3 Sv in thethree GCMs and, most important, it represents the coldest,freshest, and densest water contribution at the equatorialAtlantic bsection. The Indian part of this route isconfirmed by an inverse model performed onhydrographic sets of WOCE data [Speich et al. 2001c].Indeed, the upper branch of the CB is subject to strongwater mass transformation on its route from the Pacific tothe North Atlantic Ocean. All waters are salinificated,mostly in−between the borders with the Pacific and theentry to the South Atlantic: Cape Agulhas. Least modifiedis the Tasmanian outflow as it flows deeper and is lessventilated than the other two routes originating at DrakePassage and the Indonesian Throughflow.

Eddies trap the water in strong currents and near thesurface. They enhance recirculations. Although the pathbecomes longer when eddy transports are included, thetravel time is shorter. Near the surface the flow pathbecomes more irregular and chaotic but the flow speedbecomes larger. Especially after the last upwelling in theSouthern Ocean eddies trap the water in the core of theACC and the amount of crossings through Drake Passageincreases with a factor of three by including the eddytransports (Drijfhout et al., manuscript in preparation).

5. Water masses ventilation and transformationWe applied the TRACMASS diagnostics to the OPA

global three−dimensional dynamical fields to estimate theventilation of the global ocean [Blanke et al, 2001c]. Forventilation we intend the process by which water istransferred from the surface mixed layer, a region directlysensitive to the air−sea coupling, to the interior ocean.Taking particle trapped under the mixing layer for morethan one year, we found 324 Sv of mixed layer watertravelling through the interior ocean with a leadingreplacement time of 125 years. The study highlights therole of ACC in the upwelling and trasformation of most ofglobal ocean water masses. AAIW results as the lessventilated water mass.

In OCCAM the effect of eddies on the water masstransformation and ventilation has been elucidated bycomparing trajectories deduced from the annual meantransports with trajectories calculated from annual meanplus bolus transports (Drijfhout et al., manuscript inpreparation). Eddies reduce the mass flux divergencebelow the mixed−layer, and so decrease the apparentwater mass transformation due to diapycnal mixing and

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up− and downwelling into different density classes in theinterior ocean. They enhance ventilation and subductionfrom the mixed layer in spring [Hazeleger and Drijfhout,2000], and therefore increase water mass transformationand up− and downwelling into different density classesdriven by air/sea interaction at the surface.

6. Cited references

Broecker, W. S., 1987; The biggest chill. Nat. Hist. Mag.,97, 74−82.

Broecker, W. S., 1991: The Great Ocean Conveyor.Oceanography, 4, 79−89.

Döös, K. and A. Coward, 1997: The Southern Ocean asthe major upwelling zone of North Atlantic DeepWater.Int. WOCE Newslett., 27, 3−4, 17.

Dickson, R. R., and J. Brown, 1994: The production ofNorth Atlantic Deep Water: Sources, rates, and pathways.J. Geophys. Res., 99, 12319−12341.

Fine, R. A., 1995: Tracers, time scales, and thethermohaline circulation: The lower lim in the NorthAtlantic. U.S. Nat. Rep. Int. Union of Geod. Geophys.1991−1994, Rev. Geophys., 33, 1353−1365.

Ganachaud, A. and C. Wunsch, 2000: Improved estimatesof global ocean circulation, heat transport and mixingfrom hydrographic data. Nature, 408, 453−457.

Gordon, A. L., 1986: Interocean exchange of thermoclinewater. J. Geophys. Res., 91, 5037−5046.

Gordon, A. L., R. F. Weiss, W. M. Smethie Jr. And M. J.Warner, 1992: Thermocline and intermediate watercommunication between the South Atlantic and Indianoceans. J. Geophys. Res, 97, 7223−7240.

Gregg, M. C., 1989. J. Geophys. Res., 94, 9686−9698.

Ledwell, J. R., A. J. Watson, and C. S. Law, 1998. J.Geophys. Res., 103, 21499−21529.

MacDonald, A. and C. Wunsch, 1996: An estimate of theglobal ocean circulation and heat fluxes. Nature, 382,436−439.

Paillet, J., M. Arhan and M. S. McCartney, 1998:Spreading of Labrador Sea Water in the eastern NorthAtlantic. J. Geophys. Res., 103, 10223−10239.

Pelou, K. Propagation de l’Eau Nord Atlantique Profondedans l’Atlantique Sud. Rapport de DEA, Universitž deBretagne Occidentale, Brest, France. 40 pp.

Pelou, K. and S. Speich, 2000:Devenir de l’EauAtlantique Profonde simulže par un modele de circulationgžnžrale. Laboratoire de Physique des Ocžans,, InternalReport. 45 pp.

Pickart, R. S., 1992: Water mass components of the NorthAtlantic deep western boundary current. Deep−Sea Res.,39, 1553−1572.

Rintoul, S. R., 1991: South Atlantic interbasin exchange.J. Geophys. Res., 96, 2675−2692.

Rintoul, S. R. and M. H. England, 2001: Ekman transportdominates local air−sea fluxes in driving variability ofSubantarctic Mode Water. J. Phys. Oceanogr. Accepted.

Schmitz, W. J. Jr., 1996a: On the World OceanCirculation; Volume I. Some global features/NorthAtlantic circulation. Woods Hole Oceanogr. Inst. Tech.Rept. WHOI−96−03.

Schmitz, W. J. Jr., 1996b: On the World OceanCirculation; Volume I. Some global features/Indian andPacific oceans. Woods Hole Oceanogr. Inst. Tech. Rept.WHOI−96−03.

Schmitz, W. J., 1995: On interbasin−scale thermohalinecirculation. Rev. Geophys., 33, 151−173.

Schmitz, W. J. Jr. And M. S. McCartney, 1993: On theNorth Atlantic circulation. Rev. Geophys., 100, 2441−2457.

Sloyan, B. M. and S. R. Rintoul, 2000: Estimates of areaaveraged dyapicnal fluxes from basin scale budgets. J.Phys. Oceanogr., 30, 2320−2341.

Smethie, W. M., Jr., 1993: Tracing the thermohalinecirculation in the western North Atlantic usingchlorofluorocarbons. Progr. Oceanogr., 31, 51−99.

Smethie, W. M., Jr., , R. A. Fine, A. Putzka and E. P.Jones, 2000: tracing the flow of North Atlantic deepWater using chlorof

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Chapter 5. The Mediterranean Water mass circulation andMediterranean outflow in the North Atlantic

1. Introduction.

To study the Mediterranean water mass circulation andthe impact of the Mediterranean outflow in the NorthAtlantic circulation, we perform several sensitivity studiesrunning the GCM’s in order to improve the modelperformance. In the first phase of the project sensitivitystudies of both surface forcing and tracer diffusivityparameterisation was performed using MEDMOM inorder to eliminate drift in the model to successively applythe routines and methodologies defined during theTRACMASS project, integrating particles using eulerianvelocity field from a steady perpetual year, representativeof the climatological Mediterranean circulation.Moreover, as pointed out in Chapter 3, due to therelatively smallness of the Mediterranean domain,MEDMOM simulation was used also to define theoptimal strategy of time sampling of the eulerian velocityfield for offline lagrangian integration and to developLyapunov exponent techniques for characterisingLagrangian two−particle dispersion in non−asymptoticcondition. Finally in the last year of the project we studythe impact of the Mediterranean outflow in the NorthAtlantic performing sensitivity studies using OPA globalsimulation. In the following we briefly resume the resultsobtained during the project referring to the enclosedextended abstract of the scientific paper.

2. Model development and sensitivity studies.

The circulation in the Mediterranean basin is stronglyinfluenced by important water masses formation eventand by the spreading of a relatively salty intermediatewater (Levantine Intermediate Water, LIW) that acts as asource of static instability in the heat forced winterconvection representing an important − if not decisive −element in the phenomenology of deep water formation(DWF) and consequently in its numerical representation.To properly represent the Mediterranean circulation wefocus our efforts in defining a better representation ofboth air sea interaction and eddy diffusivity phenomena.

The extreme meteorological events characterisingwater masses formation was token in to account forcingthe model with daily satellite SST and ECMWF windsdefining also a new − wind dependent− parameterisationof the surface relaxation temperature coefficient. In thesame time the sensitivity of the Mediterranean circulationto the variability of the horizontal mixing wasinvestigated, implementing in the MEDMOM aparameterization of mixing previously developed in thecontext of two−dimensional (2D). This parameterizationimproves the tracers transport due to large eddy dynamicsand, ensuring a more correct salt budget in the northwestern part of the basin,

contributes to maintain a realistic vertical stratificationand winter deep convection in long climatic integrations.The results obtained in the above experiments aredescribed in detail in [Artale et al., (2001), accepted in J.of Geoph. Res. and Rupolo et al., 2001, submitted to J. ofMar. Syst.]

3. The Mediterranean circulation traced byLagrangian diagnostics.

Here we will present the work performed during theTRACMASS project with regard to the thermohalinecirculation (THC, hereafter) of the Mediterranean Sea.More in particular we will report our results concerningthe circulation in the Mediterranean sea traced withLagrangian trajectories in MEDMOM (see Iudicone etal.(2001c) for details).

3.1 The main thermohaline cellWe integrated off−line particles using one year of the

Eulerian velocity field stored every 3 days. The integratedEulerian field can be considered representative of astationary state, it is obtained after having integrated themodel for the time necessary to reach a quasi stationarystate (we use the 100th and the 250th year of integration).

The main topic of the study was to investigate thecomposition of the Mediterranean outflow. At this aim,we have performed backward in time Lagrangianintegrations of about 100,000 particles with particleswere released at meridional section in the Gulf of Cadizand integrated back in time until they have reached againthis section. Particles informations like positions andtracer properties were stored, without stopping theintegration, at various sections in the basin.

In terms of transports, the return branch of THC wasfound to gain about 0.14 Sv in the western basin. Themixing in the Alboran Sea contributes about 0.1 Sv. Thegain from the Levantine to the Sicily Channel is only lessthan 0.1 Sv, i.e. the interaction of LIW with EMDW inthe eastern basin does not change the total amount of thiswater mass while significantly affects the tracerproperties.

In the Levantine, the return flow appears formed byfour main veins with presence of both LIW and EMDW.In the western Ionian, the eastward flow is separatedmeridionally into three cores, with most of the transportin the two veins flowing along the African (mostly LIW)and the Italian coast (LIW and EMDW)..

At the Strait of Sicily, transport has increased do tomixing with the surface eastward flow and the densitydistribution has broaden. Further west, at the SardiniaChannel the particle spatial distribution appears as broadvein situated in the middle of the Channel with themaximum of the transport at 400m and a maximum depthof 1400m. A significant contribution, 25%, is due towaters at a depth larger than 600m, i.e. it is due toTyrrhenian Deep Waters . Also surface waters contributesto the observed transport with a value somewhat higherthan 25% and this latter water mass could be ascribed to asurface contribution to the deep water formation in theNW Mediterranean. At the entrance of the Alboran Sea,the separation in the vertical disappears while the oneexisting in the density distribution is still visible. Themaximum of the transport is at 350m and it decreaseexponentially with depth down to 1500m, where it almostzero. As in the previous section, the transport due to waterdeeper than 600m is about the 25% of the total.

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3.2 Dense water masses formation As a first step, the sites of dense water masses

transformations in the MEDMOM has been defined byquantifying water masses transformation due to modelair−sea interactions. The role of main sub−basins in theformation of dense waters has then been investigated bycomputing the initial and final positions and tracerproperties of particles for selected sub−basin. A watermasses classification has been defined from an inspectionof the T/S diagrams of the incoming water. Among thegeneral results we found a complex mass transformationin the Aegean where the net incoming and out comingpercentages for the various classes are almost the same(e.g. dense waters are 15% of the total incoming andoutgoing flow) but a further analysis demonstrated that anintense water mass exchange between the various classesis occurring. For the case of the NWM the LIWconsumption by the deep water formation processes wasquantified to be the half of the incoming LIW flow. TheAdriatic Sea instead transforms almost all the availableLIW into fresher deep ADW.

3.2 Pathways and time scalesFurther attention has been devoted to the main

pathways and time scales of the incoming surface AtlanticWater (the upper branch of the THC) and for the returnTHC flow (lower branch of the main THC cell atintermediate depth). Main pathways between sub−basinwere identified and characterized in term of transport.Moreover, transit or exit−times between the sections werestored and histograms of time distributions werecomputed. In general all the distribution of these timescales are characterized by having a large skewness valuewith a very long tail toward large time scales. As anexample, we report here that the total surface transportfrom Gibraltar toward the Strait of Sicily is about 0.2 Svlarger than the transport associated to the direct path andthe mean arrival time is three times larger than the onerelated to the direct path, while the median does notchange significantly. Moreover, the characteristics timescales (both median and mean) of the return flow, that isrepresented by the Levantine to Gib path, are 7 to 8 timesthe surface pathway characteristics times of water massesfrom the Atlantic of the Levantine basin. The dense wateroverflows from the Adriatic and Aegean basin are insteadfound to be significative in the case of the Adriatic whilethe Aegean net contribution is very small. The Adriaticoverflow branches into two main paths, pathscharacterized by a same order transport; one it is direct tothe Strait of Sicily while the other recirculates in theEastern Basin as a deep flow.

Two other results of the Project were to estimate for thefirst time the Probability Density Funtion (PDF) of exittimes for several sub−basins and a new approach toLagrangian pathways decomposition, based on exit−timesPDF of the involved trajectories (see the TRACMASS IIyear report for details).

4. Dispersion properties of the surface circulation of theMediterranean Sea

As already pointed out in Chapter 3, MEDMOMsimulation was used to define the optimal strategy of thetime sampling of the eulerian velocity field and to studythe Lagrangian predictability and more in generalhorizontal mixing properties.

Two different papers were produced [Iudicone et al.(2001a) submitted to Oc. Modelling and Iudicone et al.(2001b) in preparation] . Here we will focus on the lattertopic.

In order to determine the dynamics of the mixing of theMediterranean Sea surface circulation, we analyzed largesets of experimental (satellite, in situ and Lagrangiandatasets) and numerical model data. The aim was to mapthe mixing efficiency and but above all to explain itsexpected geographical variability essentially byquantifying the spatial and temporal variability andrelating it to mixing.

Four different and independent approaches have beenused in the data analysis, conducted separately for 17basin sub−regions previously defined on the base of localdynamics.

4. Methods

Altimeter and AVHRR satellite dataFive years of TOPEX/POSEIDON data were used to

compute spatial and time spectra of Sea Surface Anomaly(SLA; five years of TOPEX/Poseidon (T/P) data) showedlarge variability from region to region. FollowingStammer (1998), maps of correlation time scales , EddyKinetic Energy and horizontal diffusivity were alsoproduced. An analysis of the same parameters alongsingle T/P tracks allowed to add several details to theinformation obtained by the mapping procedure.

4.1 Baroclinic growth ratesThe baroclinic time scale, whose inverse is a measure

of eddy growth rate, has been estimated by computing themean Richardson number for each of the 17 regions. Atthis aim the MODB5 climatological data set of in situtemperature and salinity was used to estimate verticalstratification and geostrophic vertical shear.

Te/Tl

The spatial distribution and seasonal variation of theratio between Lagrangian and Eulerian time scales (Tl andTe respectively) were obtained using the outputs of theMEDMOM and the TRACMASS ARIANE code for off−line diagnostics. Two large sets of particle were releaseduniformly at sub−surface depth.

Finite Size Lyapunov ExponentFinite Size Lyapunov Exponents (FSLEs) were

computed for both local clusters and uniformly distributednumerical particles, again computed using ARIANEadapted to MEDMOM. As regards to the characterizationof local mixing properties, we use the FSLE, at a fixedamplification factor r, as a Lagrangian indicator of thesensitivity to trajectory perturbations, and we mapped itonto the initial conditions. Anisotropy of mixingproperties was also investigated.

4.2 Main resultsIn the oceans, Lagrangian variability is due to both the

spatial dishomogeneities and the time variability of theEulerian velocity field even if in some particular regionone of the two limiting cases (or regimes) of frozenturbulence (prevalence of spatial dishomogeneities overtime variability of the Eulerian velocity field) and of fixeddrifter (prevalence of time variability over spatial

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dishomogeneities) can prevail. As main result, largemixing was found to occur in the Algerian basin, in theChannel of Sicily and southern Ionian Sea. The frozenturbulence regime was found to dominate in the first casewhile the and a more complex situation characterize thetwo other cases, with a prevalence of the fixed drifterregime. In this second case, the Eulerian diagnostics werefound to be of limited usefulness. The same occurred inthe case of intense semi−permanent or seasonally varyinggyres, like Iera−Petra, were Eulerian diagnostics (like T/Pand baroclinic time scales) overestimated the mixingrates. The FSLE approach was also compared withrecently developed Eulerian diagnostics of dispersionproperties but resolution limits did not allow anydefinitive conclusion.

4.3 Comparison with Lagrangian experimental dataTRACMASS was completely dedicated to numerical

Lagrangian computations and no real data analysis wasplanned at the beginning because no experimentalequivalent of numerical Lagrangian transportcomputations exists. The case of mixing properties issomehow different. In fact a direct comparison can bedone and it would indeed be useful in order to test thereliability and limits of the Lagrangian numerical results.Courtesy of E. Garcia (CSIC, Spain), the data of a clusterof 17 drifters released in 1996 in the Algerian currentwere available. In the previous analysis, this currentsystem resulted to be the most clearly characterized byshear dispersion. We have computed FSLE of this datasetand the main results were qualitatively confirmed. Sheardispersion as well as anisotropy are largely confirmed. Onthe contrary, absolute values of shears appeared to beunderestimated.

5. Mediterranean outflow.

The Mediterranean Sea is at the origin of the large saltyand warm anomaly that characterizes the intermediatewaters of the North Atlantic Sea. Given the manyfeedbacks existing in the thermohaline circulation and,more in general, in the climate dynamics, this anomalyhas been supposed to play a role, even if not well defined.In fact, the role of the variability of the Mediterraneanoutflow is still under debate and several differentscenarios have been proposed, ranging from almost zero−influence to the possibility of the causing the on−set of aglacial period.

Two different but related studies were performedduring the Project on the sensitivity and consequences ofthe Mediterranean outflow variability. First, the role ofseveral model settings in determining the outflowproperties in a GCM was investigated by using a regionalGCM of the Mediterranean Sea MEDMOM (see previoussections).

5.1 The sensitivity of the Mediterranean outflow todifferent MEDMOM settings

Dealing with the modelling of THC we had as firstpurpose to eliminate the model drift in order to have amodel able to represent a stationary and qualitativelycorrect Mediterranean circulation on long integrationexperiments without hydrological drift (see previoussections). We will briefly present the sensitivityexperiments performed using different surface forcing,discussing the results in term of MW outflow at Gibraltar.

All the experiments were run for 300 years of integration(about 13 months of CPU time on Alpha Digitalcomputer). The experiments can be subdivided in two subsets. The first subset is devoted to the study of theinfluence of the representation of horizontal mixing onthe Mediterranen circulation. In the second we study thesensitivity of the Mediterranean on the surface forcing.

T/S diagrams of the outgoing and MW flow in twomeridional section in the Alboran Sea and in the Gulf ofCadiz have been computed, obtained by averaging thehydrological properties of thousands of particles release inthe Alboran section and successively intercepted in theCadiz Gulf section. A negative linear relation wasobserved between the salinity (and temperature) in theAlboran Sea and the ratio between the water coming fromthe NW Mediterranean and the transport in the AlboranSea. Outside the Gibraltar outflow the hydrological valuesof the different experiments are more scattered. In generalexperiments in which diffusivity is represented by asecond order operator display warmer MWcharacteristics.

The time scales of the return flow, i.e. the branch of theTHC flowing from the Levantine basin to the Atlantic,were found linearly dependent (with negative slope) onthe transport at Gibraltar. Time scale values range from50 to 90 years in agreement with other estimates. Only theDAILY experiment shows longer time scales even incomparison with other runs with the same transport atGibraltar and then suggesting a larger role forrecirculations in the sub−basins. This linear behavior in adynamical situation characterized by large recirculations,implies that the whole thermohaline cell responds to theforcing at the main strait.

Finally it was possible to observe a rather clear directlinear dependence between the density of the MWoutflow and the mass transport at the strait. The samelinear relation is observed between the density of theoutflow (as it results from the inner basin dynamics andthe mixing at and west of the Strait of Gibraltar) and themean depth of this water mass in the Gulf of Cadiz. It isimportant to underline that these linear dependences aredue to both the contributions of salt and temperature. Thisconsideration is to be integrated by some final remarks onthe outflow characteristics Gibraltar and its from thewater properties. In the numerical simulations thetransport of mass at the Strait was found to follow apower law of the density difference between the twolayers in the strait, with an exponent close to ¼. If weconsider only the salinity difference at the strait as aproxy for the density, the exponent becomes close to .2.This result suggests that the basin response in term ofsalinity to changes in the surface freshwater fluxes is farfrom being linear.

5.2 The outflow of the Mediterranean Sea in the Atlantic The ORCA2 configuration of the OPA OGCM was

used to investigate the sensitivity of the characteristics ofthe Mediterranean Water (MW) in the Atlantic Sea and ofits variability.

This model includes all the most recentparameterizations of sub−grid or other unresolvedprocesses like the use of a free surface formulation, use ofisoneutral surfaces, the GM90 parameterization of eddytracer transport, a time and space variable isopycnaldiffusivity depending on a local baroclinic index, a

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bottom boundary layer parameterization, stratification−dependent vertical diffusivity for deep layers. At surface,heat fluxes and evaporation were parameterised by usingstandard bulk formulae. Climatological fields were usedfor precipitation. A restoring to Levitus98 surfacesalinities was also added.

One of the main problems in global circulation studieswith OGCM is to correctly reproduce the dense watermasses formation. In fact, this is a local process thatinvolves surface forcings, general sub−basin circulation,vertical and horizontal mixing and, in most cases, ice−ocean interactions.

To improve this fundamental component of the THC,the ORCA2 model was used in a recently developedconfiguration that includes the LLN CLIO ice model,having the same horizontal resolution.

The free−surface verion of ORCA2+CLIO was neverused before and several runs were performed in order toobtain a global THC description comparable withexperimental findings.

The results of this subsections are described in moredetails in Iudicone et al. (2001d)

5.2.1 Sensitivity experiments Our intention was to study the sensitivity to variation of

the Mediterranean outflow and then, at the contrary ofmost model with similar resolution, we wanted to keep inthe model a fully functioning Strait of Gibraltar. The firstproblem to solve was then the outflow itself. It now wellknown that the entrainment of Mediterranean water by theintermediate layer of the Atlantic Seain the Gulf of Cadizgenerates a large scale zonally elongated cycloniccirculation (with transport up to 10−12Sv). The OPA welldescribe this circulation but has not enough resolution toresolve the cyclonic circulation in Gulf of Cadiz. Warmsub−surface Atlantic water was then artificiallydownwelled and entrained by the Mediterranean outflow.The result was a Med anomaly too warm and too shallow.Several sensitivity runs leaded to a new topography forthe Gulf of Cadiz and to a different use of lateralboundary conditions in the area. As a consequence, amodel set−up was obtained with a Mediterranean restoredto climatological values but with the Alboran Sea anGibraltar free evolving.

The vertical distribution of the MW in the centralNorth Atlantic Sea was also wrong. Two main changeswere then introduced. First, the ECMWF wind climatolgywas substituted with one derived from 1993−1998 ofERS scatterometer data, coupled at high latitudes withNCEP winds. This change, while dramatically improvingAntartic intermediate and deep water formation as well assub−tropical and equatorial circulation, did not affected

the intermediate salty layer. On the contrary, theintroduction of a parameterization of double−diffusiveprocesses improved the MW description essentiallybecause altered the north−south transport of theMediterranean water, enhancing the amount of this waterat high latitudes.

It is worth to note that, a part of the direct scientificinterest of these simulations, the work done duringTRACMASS has allowed to set−up a global circulationmodel in a state−of the−art configuration that will allowthe European scientific community to produce in nextyears top−level scientific results.

5.2.2 The Mediterranean outflow and the globalthermohaline circulation

The role of the Mediterranean outflow in the THC wasthen investigated by performing a long run (550 years),with the Mediterranean Sea included, in order to reach asteady state for surface and intermediate layerscharacteristics (needing at least 100−200 years tostabilize). Another run of 200 years was performedstarting from year 350 of the previous simulation butwithout the Mediterranean. The fist result of the twoparallel simulation was to determine the time scale of theMediterranean water renewal that was found to be around20 years, in agreement with previous estimate.

In order to keep most of the Eulerian variability, at theend of both simulations, the model outputs of year 550was stored at 5 days resolution and off−line Lagrangiandiagnostics (the ARIANE code developed by B. Blankealso in the framework of TRACMASS) were used toquantitatively evaluate: a) the origin and fate of theMediterranean water masses; b) the differences existingbetween the two runs in terms of transports for the variousbranches of the THC.

The main result of (a) was that the Mediterranean wateris not directly reaching the Greenland basin but entersbefore in the Gulf Stream where is slowly lifted to upperlayers and is then able to enter the high latitude sub−basin. Regarding (b) no large change in the meridionaloverturning was found. Actually, while no difference areobserved at the Equator, the maximum of the Eulerianmeridional overturning was found to be slightly higher(less than 2. Sv) in the no−Mediterranean case. Using theLagrangian diagnostics, a rationale for this variation wasfound. The Mediterranean water reaches North AtlanticCurrent at mid−depths and there it upwells to sub−surfacelayers, then coming back toward mid−latitudes (and to theGulf of Cadiz).

It is to note that the set−up of a new simulation isundergoing in which the restoring to LEVITUS98 surfacesalinity is eliminated.

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Chapter 6. Summary of TRACMASS articles

1. A Global Diagnostic of Interocean Mass Transfer. Blanke, Speich, Madec and Döös. Journal of PhysicalOceanography: Vol. 31, No. 6, pp. 1623−1632.(2001)

2. Calculating Lagrangian trajectories using time−dependent velocity field. de Vries and Döös. ournal of Atmosphericand Oceanic Technology, Vol. 18, 1092−1101 (2001).

3. Chaotic and regular trajectories in the Antarctic Circumpolar Current. Nycander, Döös and Coward. Tellus. −Series A, Vol. 54, issue 1, p.99−106.

4. Do tropical cells ventilate the Indo−Pacific equatorial thermocline? Hazeleger, Vries, and van Oldenborgh. Geoph.Res. Letters, Vol. 28, 1763−1766 (2001).

5. Numerical simulation to resolve the issue of downstream migration of the Japanese eel. Kimura, Döös andCoward. Mar. Ecol. Prog. 1999

6. Warm water paths in the equatorial Atlantic as diagnosed with a general circulation model. Blanke, Arhan, Madec,Roche, J. Phys. Oceanogr., 29, 2753−2768 (1999)

7. Diagnosing and picturing the North Atlantic segment of the global conveyor belt by means of an ocean generalcirculation model, Blanke, Arhan, Speich, Pailler. Submitted to JPO

8. A global diagnostic of interior ocean ventilation. Bruno Blanke, Sabrina Speich, Gurvan Madec, Maugé, inpreparation

9. Tracing water masses with particle trajectories in an isopycnic−coordinate model of the global ocean. Marsh andMegann, In revision for Ocean Modelling (2001)

10. A new approach for correcting drift in ocean models. Pedro de Vries and Sybren Drijfhout, in preparation

11. Sensitivity of trajectory calculations to the time resolution of the velocity data from a non eddy−resolving OGCM.Valdivieso Da Costa, Blanke, Submitted to JPO, 2001

12. Estimates of particle dispersion in an eddy−resolving modelof the Agulhas retroflection area, south of Africa.Valdivieso Da Costa, Bruno Blanke, In preparation

13. Spurious diapycnal mixing of the deep waters in an eddy−resolving global ocean model. Lee, Coward and Nurser,Submitted to JPO

14. Tasman leakage: a new route in the global ocean conveyor belt. Speich, Blanke, Ganachaud, de Vries, Drijfhout,Döös, Marsh, Geophysical Research Letters. In press.

15. Warm and cold water routes of an O.G.C.M. thermohaline Conveyor Belt. Sabrina Speich, Bruno Blanke, LPO,Brest, Gurvan Madec, LODYC, Paris, Geophys. Res. Lett., 28, 311−314 (2001)

16. Lagrangian motion of particles and tracers on isopycnals. Lee, Submitted to Journal of Physical Oceanography

17. North Atlantic Deep Water spreading in the world ocean. Sabrina Speich, Bruno Blanke, Pedro de Vries andSybren Drijfhout, Robert Marsh, In preparation

18. The Spiralling North Atlantic Deep Water around Antarctica. Döös, Nycander and Coward, In preparation.

19. The role of surface fluxes in OGCM using satellite SST. Validation of and sensitivity to forcing frequency of theMediterranean thermohaline circulation. Artale, Iudicone, Santoleri, Rupolo, Marullo, D’Ortenzio, J. of Geoph.Res.

20. Horizontal space−time dependent tracer diffusivity field for a OGCM. A sensitivity study in the Mediterranean Seacase, V. Rupolo A. Babiano V. Artale D. Iudicone , J. of Mar. Syst.

21. Sensitivity of numerical tracer trajectories to uncertainities in OGCM velocity fields. Iudicone , Lacorata, Rupolo,Santoleri and Vulpiani. Submitted to Ocean Modeling

22. Finite−scale dispersion of OGCM Lagrangian trajectories for the Mediterranean Sea. Iudicone, G. Lacorata, V.Rupolo, R. Santoleri, A. Vulpiani. In preparation

23. The Eastern transient studied with Lagrangian diagnostics applied to a Mediterranean GCM forced by satelliteSST and ECMWF for the years 1988−1993. V. Rupolo, S. Marullo and D. Iudicone. Submitted to J. of Geoph. Res.

24. Tracing the circulation in the Mediterranean with Lagrangian trajectories in MEDMOM. Daniele Iudicone,Volfango Rupolo, Rosalia Santoleri, Vincenzo Artale

25. The Mediterranean water in an intermediate resolution OGCM. Daniele Iudicone, Gurvan Madec, Bruno Blanke

26. Impact of eddy−induced transport of the Lagrangian structure of the upper branch of the thermohaline circulation.Drijfhout S., P. De Vries, K. Döös, A. Coward 2002: Geophysical Research Letters. In press.

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1. A Global Diagnostic of Interocean Mass Transfers

BrunoBlanke and Sabrina Speich, Laboratoire de Physique des OcéansGurvan Madec, Laboratoire d’Océanographie Dynamique et de ClimatologieKristofer Döös, Meteorologiska institutionen, Stockholms universitetJournal of Physical Oceanography: Vol. 31, No. 6, pp. 1623−1632.(2001)

The global ocean circulation transfers mass, heat andsalinity between the various ocean sub−basins on timescales that are likely to interact with the evolution ofclimate regimes. As an effort to consolidate ourknowledge of the nowadays, global ocean climatologicalstate, we analyzed the output of a complex and realisticocean model with TRACMASS−related Lagrangiantechniques quantify all interocean mass transfers, andpicture associated mean pathways.

This global picture is obtained by means of an oceangeneral circulation model constrained on observedclimatologies for temperature and salinity, and acting as adynamical interpolator. This constraint forces the modelto diagnose a large scale circulation (mostly geostrophicin the ocean interior) in agreement with the flow thatcould have been calculated directly from theclimatologies.

The world ocean is divided into nine sub−basins,limited by model velocity gridpoints close to convenientgeographical limits: both tropics (approximated bylatitudes ±24° on the model mesh) and the Arctic polarcircle (approximated by latitude 66°) to distinguish theArctic, northern, tropical and southern basins; the 23°E(south of Africa), 145°E (Tasmania), 65°W (south ofAmerica) longitudes and the Indonesian throughflow (at125°E) to distinguish the Atlantic, Indian and PacificOceans.

Besides different known connections, our results (seeFigure) add new possible links between sub−basins andevaluate precisely and objectively their intensity andgeometry, distinguishing intra−basin recirculations (assubtropical gyres) and interbasin connections.

The warm colors emphasize the so−called upper limbof the Global Conveyor Belt (GCB). They correspondmostly to a northward transfer of mass throughout theAtlantic Ocean, with water supplied by either the PacificOcean through the Drake Passage or the Indian Ocean.The Indonesian throughflow and the southern AtlanticOcean are both seen to contribute to the warm route,whose intensity results much larger than the cold route.Some Antarctic Circumpolar Current (ACC) water is seentransmitted to the tropical Indian Ocean where it mergeswith Pacific origin waters. Part of it eventuallycontributes to the warm route.

The cold colors refer to the deep limb of the sameGCB. Again, the Atlantic Ocean is the main place wherethese transfers occur, with the formation of dense watersin the Arctic Ocean. The full amplitude of the NADWoverturning is to be read in the tropical Atlantic. The deepcirculation in the North Atlantic consists of a deepwestern boundary current and a southward flow east ofthe Mid Atlantic Ridge. This last path (accounting for twofifths of the transport) confirms and quantifies somehypotheses already formulated about the presence of asouthward flow of Labrador Sea Water in the easternAtlantic.

Figure: A global interbasin circulation scheme. Pathways are defined as the median half of each mass transportstreamfunction calculated for any pair of given sub−basin limits, and are overlapped and colored according to theirmean depth, defined as the average of the mean immersions calculated over the initial and final sections. Labelsexpress transports in sverdrups. The direction of the hatches gives the direction of the flow in the vicinity of the initialand final sections. Arrows document the main orientation of the flow.

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2. Calculating Lagrangian trajectories using time−dependent velocity fields

Pedro de Vries, KNMI (Koninlijk Nederlands Meteorologisch Instituut)Kristofer Döös, Meteorologiska institutionen, Stockholms universitetJournal of Atmospheric and Oceanic Technology, Vol. 18, 1092−1101 (2001).

A method to calculate Lagrangian trajectories from asampled set of time−varying velocity fields is presented.By simultaneously interpolating linearly in space andtime it is possible to derive analytical solutions for thetrajectory inside a grid box. Contrary to the case ofstationary fields the solutions are such that the transit timethrough a single grid box must now be determinednumerically. A description of how to apply andimplement the analytical results is given.

The accuracy and efficiency of our method isdemonstrated by applying it to a time−dependent two−dimensional model gyre. We also consider somecomparisons with Runge−Kutta integration schemes. Bycomparing analytical and numerical trajectories, weinvestigate the magnitude and behaviour of the errorsintroduced by using the approximation of interpolatingvelocities linearly in space and time. The main differencebetween analytical and numerically computed trajectories

can be expressed in terms of phase differences, which aredominantly determined by errors due to the linearinterpolation in space. Trajectories with a large curvatureexhibit larger phase

differences. For more general flow fields the time−interpolation errors will also contribute significantly tothese phase deviations. Compared to the case of astationary gyre, the total computational effort increasedby a factor of three. Applying our approach to the morecomplex case of a three−dimensional seasonal data setfrom the OCCAM project resulted in an 80% increase inCPU time.

We have thus shown that accurate and efficientalgorithms can be devised for calculating Lagrangiantrajectories that may prove useful for the off−lineanalyses of large data sets derived from generalcirculation models.

Figure: Meridional position y as a function of time t for = 3.4 and various amplitude values. Drawn (dotted) lines arethe analytical (computed) results. The slow oscillation is described by the period Ta. From top to bottom at t = 184 yr;a/x = 9.5, 19.5, and 39.5, respectively

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3. Chaotic and regular trajectories in the Antarctic Circumpolar Current

Jonas Nycander and Kristofer Döös, Meteorologiska Institutionen, Stockholms UniversitetAndrew C. Coward, Southampton Oceanography CentreTellus (2001)

Methods from chaos theory are applied to the analysisof the circulation in the Southern Ocean, using velocityfields produced by a realistic global ocean model. Weplot the intersections of individual trajectoriesencirclingAntarctica with a vertical plane in the Drakepassage. This so−called Poincaré section shows a drasticdifference between regular trajectories in a core region ofthe Antarctic Circumpolar Current (ACC), and chaotic,mixing trajectories in the surrounding region. It alsoshows that there is a region with overturning circulationof approximately 3.5 Sv in the ACC, with downwellingon the northern side and upwelling on the southern side,which may be related to the Deacon cell.

By plotting particle trajectories in the ACC on aPoincaré section wehave revealed features of the 3Dstructure of the ocean circulationthat cannot be seen onthe traditional zonally integrated meridional stream

function. In particular, we have seen that the trajectoriesin a core region of the ACC are regular and almosttrapped there, while the trajectories in the surroundingregion are chaotic and mixing. This structure is similar towhat has previously been seen in simple model flows,although the flow studied here is vastly more complex.Perhaps there is a connection to the fact that theconcentration of dissolved oxygen in hydrographicsections has a minimum in the ACC, indicating that thiswater is old.

We have also revealed the existence of a region withoverturning circulation of approximately 3.5 Sv in theACC, with downwelling on the northern side andupwelling on the southern side. This overturning occurs inthe region where the trajectories encircle Antarctica manytimes. There maywell exist additional overturning in otherparts of the Southern Ocean.

Poincaré section in the Drake passage. Successive intersections of thesame trajectory have here been connected byarrows. The starting points are regularly spaced along a horizontal line at 138 m depth, and the trajectories have beenfollowed forward and backward until they enter the strongly irregular region or leave the Southern Ocean.

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4. Do tropical cells ventilate the Indo−Pacific equatorial thermocline?

Wilco Hazeleger, Pedro de Vries and Geert Jan van Oldenborgh. KNMI (Koninlijk Nederlands Meteorologisch Instituut)Geoph. Res. Letters, Vol. 28, 1763−1766 (2001).

Source waters of the Indo−Pacific equatorialthermocline are studied with the high−resolution oceanmodel OCCAM. In the annual mean fields, tropical andsubtropical overturningcells are found that upwell at theequator and downwell at 5 degrees and 20 degreespoleward of the equator, respectively. Tropical cells arecommon in ocean models, but their role in ventilating theequatorial thermocline is obscure because the downwelledwater is too warm to match the subsurface equatorialwaters. In addition, there is not much observationalevidence for these cells.

Eulerian streamfunctions are mostly determined byusing long−term mean transports in Cartesian coordinates.There are two caveats when this procedure is followed.Firstly, the subsurface motion in the tropical Pacific isessentially three−dimensional and mainly followsisopycnal surfaces. In the case of slanted isopycnals,spurious upwelling and downwelling may then be seenwhen the transports are integrated zonally at constant zlevels. We therefore compared streamfunctions in levelcoordinates and in density coordinates.The tropical cellsare indeed seen to be much weaker when the overturningis considered in density coordinates.

Secondly, high−frequency eddy motions induce a masstransport in addition to the transport by the Eulerian mean

flow due to correlations between velocity and isopycnalthickness variations. This eddy−induced circulation has tobe taken into account when determining total transport bymeridional cells. This can be done by time averaging themass transports on isopycnals instead of at constant zlevels. When high−frequency mass fluxes are includedtropical cells are compensated by the eddy−inducedoverturning. Seasonal variations and tropical instabilitywaves are responsible for the compensation. In theNorthern Hemisphere eddies act almost entirely onintraseasonal time scales. On the Southern Hemispherethe seasonal variations are mostly responsible forcompensating the tropical cell. We conclude that high−frequency, small scale variability associated with tropicalinstability waves is important to the large−scale meancirculation in the tropics.

Summarising, the picture of the ventilation of theequatorial thermocline becomes remarkably simple. Thesubtropical cells transfer mass, heat, and salt from thesurface layers in the subtropics to the equatorialthermocline. This is true for both the Indo−Pacific Oceansand for the Atlantic. In the Pacific the largest transportscome from the south in accordance to observations.Strong tropical cells are shown to be an artefact.

Fig. Meridional streamfunctions in the Indo−Pacific Ocean (in Sv). Left: Eulerian mean data. Tropical overturningcells are observed. Right: Eulerian mean + eddy−induced circulation. Tropical cells are absent.

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5. Numerical simulation to resolve the issue of downstream migration of the Japanese eel

Shingo Kimura*, Kristofer Döös,# and Andrew Coward+ * Ocean Research Institute, University of Tokyo, Nakano, Tokyo, 164−8639, Japan # Meteorologiska Institutionen, Stockholms Universitet, S−10691, Stockholm, Sweden +Southampton Oceanography Centre, Empress dock, Southampton, SO14 3ZH, UKMar. Ecol. Prog. Ser. 186: 303−306, 1999

The expedition of the research vessel Hakuho−maruof the Ocean Research Institute, University of Tokyo in1991 discovered that the spawning ground of the Japaneseeel (Anguilla japonica) is located in the North EquatorialCurrent (NEC), west of the Mariana Islands at a salinityfront near 15_N 140_E (Tsukamoto, 1992; Kimura et al.,1994). Since then several studies have attempted toexplain how the leaf−like, tiny eel larvae (leptocephali)migrate 3000 km from the spawning ground in the NECto their growth habitat in east Asia (e.g. Kimura et al.1994). However, the observational and modelling basesfor these studies were limited and some questions abouttheir distributions downstream of the NEC remainedunanswered. Three such questions were: 1. Why the spawning ground had been previously

estimated to be south of Okinawa Islands ?2. Why the larvae are caught mainly off the west coast of

Taiwan , despite the fact that the main stream of theKuroshio flows to the east of Taiwan?

3. Why the abundance of the Japanese eel in Koreancoastal waters is considerably smaller than that inJapanese coastal waters ?Answers to these questions can be postulated but are

difficult to confirm using existing observations. In orderto help resolve these questions and to confirm physicaleffects on the larval distribution of the Japanese eel in theEast Asia, we conducted numerical simulations of thetransport of the larvae from their spawning ground. Bytreating the larvae as inorganic particles, we were able toapply the TRACMASS technique to high resolutionoutput from the OCCAM global model in order toinvestigate patterns of larval migration. For the purposesof this work, 93 particles were deployed at 140_E, 15_N,initially distributed evenly from the surface to 100 mdepth, and tracked for five years.

Figure shows larval trajectories released at the spawning ground (15_N, 140_E) with time and depth changes,respectively. Initially, the majority of the larvae were transported westward in the NEC. However, those inhabiting thesurface layer were transported northwestward by the Ekman transport and finally entrained into a subgyre locatedsouth of the Okinawa Islands. Many of these particles are trapped within the subgyre for more than one year. Thisresult helps to explain why considerably larger size leptocephali have been collected in and near the subgyres andillustrates how the existence of the subgyres have previously led to the mis−classification of the spawning ground.

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6. Warm water paths in the equatorial Atlantic as diagnosed with a general circulation model

Bruno Blanke, Michel Arhan, LPO, Gurvan Madec, LODYC, Sophie Roche, LPOJ. Phys. Oceanogr., 29, 2753−2768 (1999)

We used a general circulation model to study thethree−dimensional circulation of the equatorial AtlanticOcean. We addressed more specifically the northwardtransfer of mass, since it is a key feature of the globalthermohaline circulation and proves sensitive to smallertropical horizontal scale movements related to thesubtropical subduction cells and to the equatorialrecirculation cell.

Our diagnostics are based on the Lagrangian analysis ofmonthly mean velocity fields within the upper 1200m ofthe tropical Atlantic (10°S−10°N) and evidence the roleof the equator as a barrier for meridional movements anda zonal deflector for the warm flow. Our qualitativedescription in terms of equatorial pathways of the warmcomponent of the global conveyor belt is found coherentwith the most recent circulation schemes inferred fromdirect measurements.

A total amount of 37.3 Sv of warm water is seenapproaching the equator form the south at 10°S, withdominant contributions of the thermocline water (TW,12.8 Sv) and subthermocline water (subTW, 13.8 Sv). Ofthis amount, 19,9 Sv recirculate southward across thislatitude, while 17.4 Sv proceed to 10°N and contribute to

the interhemispheric exchanges. Considering the weaksouthward transfer of warm water from 10°N to 10°S (0.7Sv), the net amplitude of the meridional overturning cellseen by the model is 16.7 Sv, a value compatible withother estimates deduced from the observations.

The water flowing northward at 10°N is mainlycomposed of surface water (SW, 8 Sv) and subTW (5.8Sv), with only a weak contribution of TW (1.1 Sv). Thechange in the vertical breakdown at the crossing of theequator results from water mass conversion, particularlythe transmission of 4.8 Sv of incoming TW intooutflowing SW.

As 4.3 Sv of TW are also converted into southwardrecirculating SW, the total amount of TW upwelled intoSW between 10°S and 10°N is 9.1 Sv. This upwelling,along with the geostrophic convergence in the surfacelayer, feeds an equatorial Ekman divergence, amountingto about 25 Sv. The water conversion occurs along thewestern boundary and along a basinwide clockwisecirculation cell contributed to by the EquatorialUndercurrent, as a northern limb, and a part of the SouthEquatorial Current, for the westward return of theupwelled water.

Figure: Schematic view of the northward−transmitted (from 10°S to 10°N) and southward−recirculating (from 10°N to10°S) fluxes, sorted by water masses (Surface Water, Thermocline Water, subthermocline Water and AntarcticIntermediate Water). All transports are in Sv.

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7. Diagnosing and picturing the North Atlantic segment of the global conveyor belt by meansof an ocean general circulation model

Bruno Blanke, Michel Arhan, Sabrina Speich, Karine Pailler, LPO, FranceIn revision for J. Phys. Oceanogr. (2001)

NADW (North Atlantic Deep Water) is one of themajor deep-water masses in the World Ocean. Formed inthe North Atlantic at high latitudes, it plays a crucial rolein the meridional heat budget as it flows southward atdeep immersions (1000-4000 m), being necessarilybalanced in volume by an equivalent northward flow ofsurface (0-1000 m) warm water. The intensity,distribution, and variability of this warm limb of theglobal conveyor belt are known to influence the Europeanand, more generally, the global climate.

We used the monthly mean velocity, salinity andtemperature fields of a numerical simulation of the worldocean climatological circulation to study the intensity andpathways associated with the meridional overturning inthe North Atlantic.

Lagrangian diagnostics based on the computation ofseveral hundred of thousands of individualthree-dimensional trajectories are combined with anappropriate study of water mass potential densities inorder to describe the warm and cold limbs of the so-called‘‘conveyor belt’’.

We associate the cold limb of the North Atlanticoverturning with the particles flowing southward in thetropical Atlantic (at 5.3°N) with a potential densityincluded between 45.90 (reference: 4000 db) and 32.20(reference: 1000 db), and originating initially in anorthward warm flow at the same latitude with a potentialdensity less than 32.20 (reference: 1000 db), namely thewarm limb of the same overturning.

The description of this conversion by means ofindividual trajectories of numerical particles allows us tolink in a dynamic way both flows and therefore to definea warm limb and a cold limb explained by the very sameset of particles. Careful analysis and comparison of thehydrographical properties at the entrance and exit of theNorth Atlantic led us to an accurate estimate of watermass conversions occurring north of 5.3°N. TheLagrangian analysis ensured an adequate distinctionbetween meanders of the large-scale circulation orconversions occurring within the same limb of theAtlantic overturning, and true water mass modificationsrelated to journeys in the Mediterranean Sea (orimmediate neighborhood) or in the Arctic Ocean.

We especially focus on the particles explaining theselatter modifications and buid circulation schemes, first forthe warm limb of the NACB from 5.3°N to the Gulf ofCadiz (9°W) and 53°N and beyond, then for the cold limbfrom the Arctic straits to 53°N and then from 53°N or9°W to 5.3°N.

Our results evidence the relative importance of severalpathways with respect to others, depending on theconsidered water mass. For the warm limb, we cancontrast in the subequatorial region a purely westernboundary route and a more eastern inflow, and, in thesubtropical Atlantic, a Caribbean Sea route and a moredirect path north of the West Indies. For the cold limb, wediagnose the origins of the eastern route (east of theMiddle Atlantic Ridge) for the southward flowing upper

deep water and relate them to Labrador Sea Waterflowing from the western Atlantic and to a contributionfrom the Norwegian Sea through the Iceland-Scotlandpassage. The relative contributions of the Denmark Straitand the Iceland-Scotland Passage to the direct formationof the densest NADW layers without further recirculationin the subpolar gyre were investigated too. They amountto 2.8 Sv and 1.3 Sv, respectively. These circulationpatterns do not contradict some known schemes for theNorth Atlantic, and may help the interpretation of theresults from other models or from direct or indirect tracermeasurements.

Figure: Horizontal mass streamfunction related to thevertically integrated transport of the cold limb of theAtlantic overturning, for the cold water masses reaching5.3°N and originating in 53°N or in the Gulf of Cadiz(9°W) The names of the water masses are related to theirdefinition and state at 5.3°N: (a) UDW (upper deepwater) with a 0.5 Sv contour interval, (b) MDW (middledeep water)with a 0.5 Sv contour interval, (c) LDW(lower deep water) with a 0.25 Sv contour interval. Thevalue of the streamfunction is arbitrarily set to 0 overAfrica.

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8. A global diagnostic of interior ocean ventilation

Bruno Blanke, Sabrina Speich, LPO, Brest, FranceGurvan Madec, LODYC, Paris, FranceRudy Maugé, LPO, Brest, FranceIn preparation (2001)

Ventilation (the process by which water is transferredfrom the surface mixed layer, a region directly sensitiveto the air−sea coupling, to the interior ocean) usuallyoccurs over large scale domains as subduction or throughmuch more localized convection areas Ventilationanomalies as the result of climate variability may thusimpact the atmosphere in remote regions where the flowreturns to the mixed layer, as time scales usually proposedfor ocean advection do match the decadal period ofvarious ocean−atmosphere feedbacks.

We studied this process by means of the Lagrangiananalysis of monthly−mean ocean fields of a numericalmodel constrained by observed climatologies.

We documented with particles the mass flux across asurface of control defined as the envelope of the surfacemixed layer. Each particle explains an infinitesimalfraction of the total flux directed toward the interiorocean. As this surface varies in time, we needed first todocument the advective mass flux transmitted from themixed layer to the ocean interior, for each month of theclimatological year, and then we described the mass flux

transferred to the ocean interior due to monthlyshallowings of the surface mixed layer. To concentrate ongenuine interior ocean ventilation, we eliminated fromfurther diagnostics particles with journeys shorter thanone climatological year, since they only explain massinjection within the seasonal thermocline.

We found that 324 Sv of mixed layer water travelthroughout the interior ocean for periods longer than 12months, leading to an average volume replacement timeof roughly 125 years, with high latitude waters associatedwith the longest journeys in the interior ocean. We alsoevaluated the connections established on a global scale,with an appropriate mapping of the ventilation andcorresponding obduction regions, and highlighted the roleof the Antarctic Circumpolar Current as a main receptacleof the water masses formed throughout the world ocean(more than 35%).

Our results emphasize the connections achievedthroughout the interior ocean by ventilation, stressing onquantitative estimates of transports and renewal timescales.

Figure: Net ventilation rate for the global ocean contoured with a 50 m/yr contour interval, as diagnosed from theinitial and final positions of the trajectories documenting the ventilation process. Dotted areas refer to movements fromthe interior ocean to the surface.

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9. Tracing water masses with particle trajectories in an isopycnic−coordinate model of theglobal ocean

Robert Marsh and Alex Megann, SOC, UKIn revision for Ocean Modelling (2001)

Offline particle trajectories are obtained for a quasi−global isopycnic−coordinate OGCM using an analyticalmethod, adapted for use with online time−integratedisopycnal and diapycnal mass fluxes. The method ishighly efficient, allowing the calculation of largeensembles of such trajectories. These ensembles can beused to establish pathways and transformations associatedwith the global circulation of water masses on timescaleswhich are well in excess of any feasible model integrationlength.

The method is used here to investigate the important,yet poorly observed, transformation of North AtlanticDeep Water (NADW) through slow spreading, upwellingand diapycnal mixing (defined when and where densitydecreases below a threshold value). A fundamentalproblem arises through unsteadiness in the thickness ofNADW layers (due to various model flaws and/orintrinsic variability). Particles converge on gridboxeswhere layers inflate during the online time−integrationperiod. Depending on the degree of layer inflation, only afraction of NADW particles can be diagnosed astransformed at some point along their respectivetrajectories. However, the unsteadiness of layer thicknessdecreases during a 50−year spin−up, implying fewer

converged trajectories and an increased fraction oftransformed NADW.

Using trajectories to trace NADW southward across theequatorial Atlantic, with mass fluxes from successivedecades of model spin−up, the transformed percentage (ofNADW exported from the North Atlantic) increases from17−18% (with fluxes from years 10 and 30) to 41% (usingyear 50 fluxes). In the latter case, about a third of theNADW upwells south of 30 deg S after 500−1000 years.Most of the remaining two thirds upwells in the South andNorth Pacific after 1000−2500 years (see figure).

The convergence of some NADW trajectories exposesa problem in no way unique to isopycnic models. Incomputing similar trajectories for z−coordinate models,vertical velocity is inferred from continuity and includes acomponent associated with the imbalance of watermasses. A degree of non−physical water masstransformation (inferred upwelling across isopycnals) isthus permitted. Tracing NADW with offline trajectoriesfor an isopycnic−coordinate model has drawn furtherattention the generic problem of water mass unsteadinessin OGCMs.

Figure: Fields of (a) upwelling rate and (b) mean upwelling age, of NADW, obtained by following around 1.3 millionparticles southward across the Equator in the Atlantic sector (using annual−mean mass fluxes for model year 50)..

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10. A new approach for correcting drift in ocean models

Pedro de Vries and Sybren Drijfhout, KNMI (Koninlijk Nederlands Meteorologisch Instituut)In preparation (2001)

We introduce a new approach for correcting drift inlarge ocean global circulation models that have not yetachieved a steady state. For a set of density layers, thedivergent part of the horizontal velocities is correctedusing the long−term transformation rates of the layerthickness. This may greatly improve the analyses ofmodel outputs of OGCMs.

In nonsteady models density layers may inflate ordeflate linearly as a function of time (in the case of steadyunsteadiness). In analysing time−averaged data fromthese models, this drift component may appear as

a spurious diapycnal flux. In Fig. 1a the meridionaloverturning streamfunction is presented for the Atlanticbasin using the OCCAM model. The circulation shownincludes the eddy−induced contribution. Through thesigma_2 = 36.8 isopycnal approximately 10 Sverdrupsdownwell between 30 degrees south and 50 degrees north.Only 8 Sverdrups of North Atlantic Deep Water (NADW)is produced in the North Atlantic. The southward−flowingNADW becomes increasingly heavier. Thesecharacteristics are clear signal that the model is drifting.

Lagrange−trajectory calculations will evidently displaysimilar features.

We developed a method to remove in the masstransports the spurious diapycnal contributions due todrift. In its simplest form, the divergence of the correctingvelocity field equals the difference of the localtransformation rate of the thickness and the layer−averaged one. In this way the mean drift of a density layeris effectively removed. By making more judicious choiceshow to correct the local time−averaged diapycnaltransports the method can be improved.

The above has been applied to the OCCAM modeldata using a 3 degrees by 3 degrees horizontal resolutionand 36 density layers. In Fig. 1b the drift−correctedmeridional overturning streamfunction is shown. Clearly,the NADW flowing southward retains its density muchbetter now. Southward of 50 degrees north about 3Sverdrups downwell through the sigma_2 = 36.8isopycnal in the Atlantic basin. Clearly, the amount ofdiapycnal transports has been considerably reduced.

Figure Meridional overturning streamfunctions of the OCCAM model for the Atlantic basin. The circulations includethe eddy−induced flows. The dotted line (\igma_2 = 36.8) distinguishes between the upper limb and lower limb of theConveyor Belt. (a) The effects of drift in the model are evident in the large spurious diapycnal transports. (b) Acorrection for drift is included in the velocity fields. Spurious effects due to drift are reduced considerably.

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11. Sensitivity of trajectory calculations to the time resolution of the velocity data from a noneddy−resolving OGCM.

Maria Valdivieso Da Costa, Bruno Blanke, LPO, Brest, FranceSubmitted to J. Phys. Oceanogr. (2001)

A non eddy−resolving primitive equation model wasused to simulate the climatological annual cycle of theNorth Atlantic Ocean circulation. A series of Lagrangiansensitivity experiments using an off−line trajectory modelwere run over a 10−year period to determine thesensitivity of three−dimensional trajectories to changes inspatial and temporal resolutions of the input velocityfield. The reference experiment, against which all othertest experiments were compared, involved trajectoriescomputed with model velocity data averaged every 15hours (9 model time steps) but with the same spatialresolution as the model simulation (roughly 200 km). Thetrajectories of the test experiments were calculated withmodel velocity data averaged over several periods rangingfrom 2.5 to 360 days, or with spatially filtered velocitydata that mimic a coarser grid resolution.

Trajectory errors, as determined by the ensemble−averaged position deviations from the reference case,revealed the expected degradation of trajectory accuracyfor a decreasing temporal resolution of the input velocityfield. Mean errors were found to be typically 2.5 to 8% ofthe travel distance after 10 years, when a 30−dayaveraging interval or less was used to compute testtrajectories. Substantially larger errors, of the order of16% of the travel distance, seemed typical of trajectoriescomputed from spatially filtered velocity fields. For these

cases, the low spatial resolution was the dominant causeof trajectory deviations as error statistics showed littlesensitivity to changes in the temporal resolution of thevelocity field. As a general rule, three−dimensionaltrajectories are recommendable in order to limit trajectoryerrors. Neglecting the vertical velocity component yieldedresults with unacceptably large errors, of the order of 40%or more of the travel distance.

The growth of the trajectory position errors was alsoexamined as a function of travel time. In general, anexponential growth rate was identified for short times,provided that the standard deviations remained smallerthan, or of the order of, the Lagrangian integral lengthscale in the model subtropical gyre (~ 140 km).Thereafter, trajectory errors increased approximatelylinearly with travel time, until they became saturated afterabout 5 years of advection. This upper bound to trajectoryposition errors was attributed primarily to the low energycontent of the model flow for time scales greater thanroughly 500 days. Finally, it was also found thatanisotropy existed in the horizontal errors. The fact thatthe zonal component of position deviations wasconsistently larger (by a factor of about 2) than themeridional component could be attributed to the effects ofa meridional shear in the mean zonal model velocity (~ 1cm/s per degree of latitude).

Figure: Ensemble−averaged absolute and relative errors in trajectory position components for Experiments 2 to 7against travel time. Absolute errors (Plates a, b, c) are given in kilometers in both horizontal directions and in metersin the vertical direction. Relative errors (Plates d, e, f) are given in percentage of the average length of the referencetrajectories. The curves plotted for each mean statistic are labeled according to the temporal resolution of the velocityused in the calculation of the test trajectories.

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12. Estimates of particle dispersion in an eddy−resolving modelof the Agulhas retroflectionarea, south of Africa

Maria Valdivieso Da Costa, Bruno Blanke, LPO, Brest, FranceIn preparation

Trajectory experiments in an eddy−resolving primitiveequation model of the Atlantic (Clipper project, managedin Brest, Grenoble and Paris) are used to investigatelateral dispersion processes in the Agulhas retroflectionarea, south of Africa.

Experiments involve the calculation of 441simultaneous trajectories, initiated at a level close to thesurface, and in a region where the eddy energy shows thehighest values (37−40N, 18−22E) of the area. Two−dimensional trajectories are generated by advecting theparticles using the archived model horizontal velocityfield at different horizontal (1/6 or 1/3 degree) andtemporal (18 mn to 6 h) resolutions. Vertical motion isignored for simplicity.

The figure shows, in a "spaghetti diagram" form, afraction of the trajectories of the highest resolution case,for a 40−day period, superimposed on the r.m.s. speed ofthe eddy field (in cgs units). The particles launched in theretroflection zone were generally involved in somerotational, eddylike motions, with a typical diameter of

100−300 km. It is worth noting that particle trajectoriescover two separate regions connected along the axis of thesouthwestward jet (dashed line).

Using a variety of usual and robust ensemble statistics,including one− and two−particle dispersion statistics, westudied the dispersion process and its dependence on thespatial and temporal resolution of the Eulerian velocitydataset.

There is a significant tendency for the low−resolutionexperiments, in which the trajectory position errors arethe largest, to exhibit particles that move apart morerapidly than in the highest resolution case. This isquantified by direct estimates of ensemble−averagedparticle separation and diffusivity, as functions of thetravel time and the resolution of the ocean model.

Additional work is now in progress to explore theprecise degree of resolution sensitivity that exists in thesetwo−particle dispersion statistics, for various classes ofparticle separation.

Figure: "spaghetti diagram" form, a fraction of the trajectories of the highest resolution case, for a 40−day period,superimposed on the r.m.s. speed of the eddy field (in cgs units).

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13. Spurious diapycnal mixing of the deep waters in an eddy−resolving global ocean model

Mei−Man Lee, Andrew C. Coward and A.J. George Nurser, Southampton Oceanography CentreSubmitted to Journal of Physical Oceanography

Recent idealized studies have shown that the advectionscheme in high resolution z−coordinate models may driveunrealistically high rates of diapycnal mixing. Our aimhere is to see whether the diapycnal mixing associatedwith the advection scheme in a global eddy−resolving(1/4 by 1/4 degree) z−level model is sufficiently strong tocorrupt the thermohaline circulation. We diagnose thediapycnal fluxes by using the ideas of water masstransformation.

In the Southern Ocean, the model deep and bottomwaters drift rapidly away from the Levitus climatology,with dense isopycnals moving downwards at rates of up to35 m/year. The strong upward flux (up to 50 Sv) throughthe dense isopycnals cannot be explained by the incorrectsurface forcing (as a result of poor surface fluxes and noice model) as most of the anomalous diapycnal fluxes are

occurring in the deep ocean far from surface forcing.Hence, the excessive diapycnal flux is driven by diffusionin the model, both explicit and implicit.

The ‘effective’ diapycnic diffusivity driven by thenumerical diffusion (associated with the horizontaladvection scheme) is found to be the same order, 1−−10cm^2/s, as that driven by the explicit horizontal diffusion.For strong vertical velocities (~20 m/day) as in modelsforced by high frequency winds, the vertical advectionscheme also gives similar effective diffusivities. Theseeffective diffusivities are considerably greater thansuggested by observations. To alleviate these problems,we suggest that eddy−resolving z−level climate modelswill require (1) less diffusive horizontal advectionschemes and (2) better vertical resolution throughoutmuch of the water column.

Figure: The rate of change of the height of isopycnal surfaces (referenced at 2km), dz/dt (in m/year). Three runs of themodel are considered: MON is forced by climatological monthly−average winds and 6HW is forced by 6−hourlyECMWF wind fields. Both runs have explicit horizontal diffusivity. ISO is again forced by climatological winds, but,instead of explicit horizontal diffusivity, employs isoneutral diffusion. The contour intervals are 10 m/year. The solidlines indicate isopycnals moving upward and broken lines indicate isopycnals moving downward. The light shades arefor values less than −15 m/year and the dark shades are for values greater than 5 m/year.

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14. Tasman leakage: a new route in the global ocean conveyor belt

Sabrina Speich, BrunoBlanke, and Alexandre Ganachaud, LPO, Brest, FrancePedro de Vries and Sybren Drijfhout, KNMI, Netherlands,Kristofer Döös, Meteorologiska Institutionen, StockholmsUniversitet, Robert Marsh, Submitted to Nature (2001)

The existence of a new route that draws waters fromthe Pacific Ocean to the North Atlantic is presented.These waters constitute a sizeable component of the upperbranch of the thermohaline circulation that compensatesthe outflow of North Atlantic Deep Water (NADW) andextend the prevailing views that hitherto emphasised theroutes via Drake Passage and the IndonesianThroughflow.

We show that the new route materialises withcomparable magnitude and characteristics in threeindependent numerical realisations of the global oceancirculation. Lagrangian trajectory analyses indicate thatPacific waters flowing westward in the Tasman outflow,just south of Tasmania and north of the AntarcticCircumpolar Current (ACC), are able to cross the IndianOcean, and eventually reach the Atlantic via the AgulhasCurrent System (ACS). In addition, hydrographic datainterpolated by an inverse model9 are shown to provide

consistent evidence of the Tasmanian−Indian part of thisnew Pacific to Atlantic connection.

The models show that "Tasman leakage" water is notthe easternmost appendix of the Indian subtropical gyrebut is drawn from the Pacific Ocean. Once trapped in theTasman outflow this water largely comprises SubantarcticMode and Antarctic Intermediate waters (SAMW andAAIW). The "Tasman leakage" provides to the NorthAtlantic the densest, coldest and freshest water comparedto those coming from DRAKE and the IndonesianThroughflow. This occurs as the Tasman leakage is muchless exposed to air/sea interaction than the other waters. Itis the only route in which the majority of the water neverintercepts the ocean’s mixed layer. As a result, its originalwater mass salinity and temperature remain relativelyunaffected in the models, while transported to the NorthAtlantic.

Figure: Mixing layer origins of TAS water forthe three TRACMASS GCMs a) GIM, b) OPA, c) OCCAM

40

15.Warm and cold water routes of an O.G.C.M. thermohaline Conveyor Belt.

Sabrina Speich, Bruno Blanke, LPO, Brest, Gurvan Madec, LODYC, Paris, Geophys. Res. Lett., 28, 311−314 (2001)We used the monthly three−dimensional circulationcomputed by an OGCM, in equilibrium with the observedclimatological hydrology in the ocean interior, toinvestigate the upper branch of the global Conveyor Belt(CB). The thermohaline circulation was evaluated bymeans of a quantitative Lagrangian analysis. We diagnosefour distinct origins for the water masses reaching theNorth Atlantic Ocean at 20¡N, for a total transport of 17.8Sv. Two of them, DRAKE and I−TFL, have almostequivalent contributions (6.5 and 5.3 Sv respectively),and correspond to the classical cold and warm routes. Theother two are more original and correspond to a remoteflow from the passage between Australia and Antarctica(3.1 Sv) and an internal contribution of the Indo−Atlanticsector (2.9 Sv).

Only 2.3 Sv of the 6.5 from DRAKE flow directly toNATL while the rest first enters the Indian Ocean beforecoming back to the Atlantic basin with the ACS.Therefore, the modelled ACS appears as the major

provider of water for NATL (13.9 Sv: 4.2 Sv fromDRAKE, 5.3 Sv from I−TFL, 3.1 Sv from S−AUS and 1.3Sv from NATL itself). Our results are consistent with theearly findings of Gordon [1986] in terms of significantcontribution to the CB of water coming from the IndianBasin, and they emphasize the role of the Indo−Atlantic‘‘connection’’ hypothesized by Gordon et al. [1992].

The inferred water paths strongly support the key roleof the Southern Ocean in permitting interbasin exchangesof water masses. Our results reexamine the classicalwarm versus cold route representation. Of course, as theyarise from the integration of low resolution model anddata fields, they are intented only as a qualitativeindication of potential pathways. The structure of theresulting routes points out the substantial role played bylarge−scale recirculation cells in the upper CB branchand,therefore, in water mass transformations anddistribution.

Figure: Horizontal streamfunction related to the vertically−integrated transport of the northward−transmitted warmwaters to the North Atlantic (0−1200m) with origins a) in the Drake Passage b) in the Indonesian Throughflow c)South of Australia.

41

16. Lagrangian motion of particles and tracers on isopycnals

Mei−Man LeeSubmitted to Journal of Physical Oceanography

This study looks at the behaviors of fluid particles thatmove along isopycnic surfaces. The motion of fluidparticles is affected by advection, diffusivity andisopycnic layer thickness. The aim of the study is todemonstrate the effect of layer thickness on the behaviorof fluid particles.

Consider a particular case of no net advection andconstant diffusivity, the fluid particles are likely to movetoward the region where isopycnic layer thickness isgreater. This is illustrated using a wind−driven eddy−

resolving isopycnic layer model in a zonal channelconfiguration.

Particles and tracers in the experiment demonstrate thatthe motion of the center of mass is up the layer thicknessgradient. In particular, a field of uniformly distributedparticles is seen to move toward the region of large layerthickness, hence the distribution of particles becomesasymmetric. Applying to the ocean, it can be speculatedthat if the conditions are right then isopycnic floatsreleased in the upper thermocline should have the similarasymmetric behavior.

Figure:

42

17. North Atlantic Deep Water spreading in the world ocean

Sabrina Speich, Bruno Blanke, LPO, Pedro de Vries and Sybren Drijfhout, KNMI, Robert Marsh, SOC, In preparation

The three TRACMASS global ocean generalcirculation models analysed with the LagrangianTRACMASS methodology are used to describe andquantify the paths, transports, and characetristics of NorthAtlantic Deep Water (NADW) that constitutes the lowerbranch of the Conveyor Belt (CB). Models shows thatNADW spreads in the South Atlantic mostly as DeepWestern Boundary Current. NADW experiences a strongupwelling in the near the western boundary of theequatorial Atlantic. NADW manly separates from theSouth American continent between 40¡S and 45¡S.Nevertheless, branches of minor importance are evidentsouth of the equator and near 25¡S. They transport watereastward but they do not cross the Atlantic ridge, except

for the uppermost layer of NADW. Also evident arevarious deep recirculation structures.

About 16 Sv of NADW are transmitted South of Africa,in the Indian sector of the Southern Ocean. Once in theSouthern Ocean most of NADW experiences a slowsouthward drift around Antartica and slowly it looses itscharacteristics experiencing a gradual upwelling. Soon orlate particles reach the mixing layer of the SouthernOcean, where they experiences a strong northward Ekmandrift, especially into the South Pacific.

The modelled Lagrangian view of the CB circulationappears as a puzzle of complex circuits involving manycircumnavigation of Antarctica, several recirculationwithin the tropical and subtropical gyres and a largespectrum of water transformation.

Figure: Spreading of NADW in the Atlantic basin using the OCCAM model. The flow is mainly a Deep WesternBoundary Current. The flow incorporates the eddy−induced velocity field.

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18. The Spiralling North Atlantic Deep Water around Antarctica

Kristofer Döös and Jonas Nycander, Meteorologiska Institutionen, Stockholms UniversitetAndrew C. Coward, Southampton Oceanography CentreIn preparation

When the North Atlantic Deep Water (NADW) entersthe Southern Ocean it will or ventilate at the surface orflow north in the deep ocean into the Pacific and Indianwhere it will eventually ventilate. The part that ventilateat the surface is believed to do so along the isopycnals sothat it progresses south in a spiral towards the surface.This is however difficult to see in a horizontal projectionof the NADW in Figure 1. which might be due to itschaotic behaviour. In Figure 2 we have therefore plotteda Poincaré section in the Drake Passage of the NorthAtlantic Deep Water where it is possible to see how manyorbits around Antarctica the water has done since it hasentered the Southern Ocean from the North Atlantic. TheNADW will here be in the deep ocean in the first circuitaround Antarctica and then progressively becomeshallower after every orbit. The water will in this wayslowly approach the surface but not from the north to thesouth that one might believe, on the contrary at least inthe Drake Passage where it will tend to converge in azone 58°S in the shallowest 1000 meters. On the otherhand if the NADW is plotted in a poincaré section as afunction of how many circuits there are left beforeventilating and exiting the Southern Ocean it clearly

shows how it approaches the surface in a spiral towardsAntarctica.

Figure 1. Trajectories of the North Atlantic Deep Wateras a function of depth after entering the Southern Oceanalong the South American Coast.

Figure 2: Poincaré section in the Drake Passage of the North Atlantic Deep Water. The colour indicates how manyorbits around Antarctica the water has done since it has entered the Southern Ocean from the North Atlantic.

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19.The role of surface fluxes in OGCM using satellite SST. Validation of and sensitivity toforcing frequency of the Mediterranean thermohaline circulation.

Artale V., D. Iudicone, R. Santoleri, V. Rupolo, S. Marullo, F. D’Ortenzio

The aim of this work is to correctly represent air seainteractions in order to reproduce in long integrationexperiments (O(400 years)) of the order of the renewaltimes of deep waters a steady and realistic simulation ofthe Mediterranean circulation. The effect of highfrequency surface momentum and heat fluxes on somekey ocean processes of the Mediterranean thermohalinecirculation is investigated comparing results fromexperiment forced by monthly and daily wind andsatellite SST. To retain the high frequency event weselected a particular year (the 1988) representative of themean climatology and to avoid the lack of synoptic andreliable heat and freshwater flux datasets, the restoringapproach has been used and a new parameterization forthe heat component has been proposed. Thisparameterezation assumes that the relaxation coefficientdepends on wind intensity and regime and requests theuse of simultaneous satellite daily SST estimates asrestoring field. The consistency of the proposedparameterization and of its numerical implementationwith the previous oceanic boundary layer studies has been

verified trough the analysis of the Saunders’proportionality constant. The proposed parameterization,coupling the surface heat fluxes and wind trough theskin−bulk temperature difference, recovers the highvariability of air−sea exchanges and the extreme events inthe air−sea interaction characterized, in the Mediterraneansea, by short time scales and a limited area extensions,and seems to be crucial in the more realistic description ofthe Mediterranean THC. This result emerged in particularfrom a comparative analysis on specific processes with amonthly−forced control run and with differentobservational datasets. Moreover, this comparison showsthe relevance of high frequency in the description of thesome dynamical processes relative to the intermediatewater mass transformation and the significant role ofhorizontal advection in deepwater formation in the north−western Mediterranean Sea. In particular it is shown thatin the experiment with high frequency forcing a steadystate of the model is obtained in which both hydrologicalproperties and dynamics of the different water masses iswell represented.

Figure: Caption 1: /θ S Diagram of the core of Levantine intermediate water for the MODB climatology (red points)and the experiments forced with monthly and daily surface forcing (green and black points, respectively). Note thatthe bias of about 0.5 °C observed between climatology and results from the monthly experiment (green points) isdefinitely reduced when daily forcing are used. Model data are relative to the 100th year of integration.

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20. Horizontal space−time dependent tracer diffusivity field for a OGCM. A sensitivity studyin the Mediterranean Sea case

V. Rupolo, A. Babiano, V. Artale and D. Iudicone, J. of Mar. Syst.J. of Mar. Syst.

The sensitivity of the Mediterranean circulation to thevariability of the horizontal mixing is investigated, usingthe MEDMOM, a Bryan−Cox type general circulationmodel (GCM). Attention is focused on a parameterizationof mixing previously developed in the context of two−dimensional (2D) turbulence, that is for the first timeimplemented in a GCM. This parameterization is suitablefor velocity fields characterized by the presence ofgeostrophic coherent structures, and it is a directapplication of the well known Taylor’s dispersionrelation. Theoretical and experimental justifications of theparameterization are discussed and results from fournumerical experiments, with different tracer mixing

schemes, are presented. It is shown that the proposeddiffusivity parameterization improves the tracers transportdue to large eddy dynamics and, ensuring a more correctsalt budget in the north western part of the basin,contributes to maintain a realistic vertical stratificationand winter deep convection in long climatic integrationsO(400 years). In particular, using the proposedparameterisation, the westward flow of LIW bifurcates inthe western in two branches. The northward branch ofLIW fill the North Western Basin of an intermediatelayer of relatively salty water contributing to enhance heatforced deep convection in the Gulf of Lyon.

Figure: Position of particles initially released at a intermediate depth in a section joining Sardinia to Africa (blackline) at different time delay (200, 300, 400, 800 and 2800 days for rows top to bottom) for the different experiment.Note that in the experiment with the proposed parameterisation (EXP2), LIW path bifurcates filling the North westernbasin before recirculating toward Gibraltar at greater depth. Colors represent depth particles. Lines joining particles10 days apart are drawn, giving an indication of particle speed. Particles position are obtained using TRACMASSroutines.

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21. Sensitivity of numerical tracer trajectories to uncertainties in OGCM velocity fields

D. Iudicone, G. Lacorata, V. Rupolo, R. Santoleri and A.Vulpiani

The aim of this study was to evaluate the sensitivity of theLagrangian predictability, in terms of to time−sampling.GCM off−line trajectories computations are often used toovercome limits in computer power as well as to performin depth analysis of same model output. Limits in datastorage in most cases does not allow to store modeloutputs for each time step and a coarser time resolution isthen needed. When dealing with such issues, one of theproblems is the choice of the time sampling for the modeloutput because trajectories computation is consequentlysubject to errors due to the coarse time resolution of theEulerian velocity field. In particular, the loss ofinformation can severely affect estimates of dispersionproperties, like mixing efficiency or trajectoriespredictability, that in some cases are directly related tothe Eulerian variability.

Three different data sets of numerical drifters have beenobtained degrading the time sampling (1 day, 1 monthand 1 year) of the velocity field computed from a

Mediterranean general circulation model.

The Finite−Scale Lyapunov Exponent (FSLE) techniqueis used to characterize, for each of the three data sets,Lagrangian sensitivity to the time resolution of the field.

The relevance of this technique is that it can be applied inthose cases where asymptotic conditions do not exist andclassical diffusion arguments are not valid. Our dataanalysis shows that relative dispersion has two regimes:exponential spreading due to chaotic advection at smallscales (mesoscale) and super−diffusion at larger scales(up to sub−basin scales). It is shown that the uncertaintyin the knowledge of trajectories generated by degradingthe time sampling of the field is restricted to small spatialscales and converges to the intrinsic dispersion rate forscales larger than 100 Km. Finally we show that theFinite−Scale Lyapunov Exponent technique can beemployed to visualize the geographical regionscharacterized by high Lagrangian unpredictability.

Figure: the II kind FSLE of the daily versus monthly sampled case. Units are days−1.

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22. Finite−scale dispersion of OGCM Lagrangian trajectories for the Mediterranean Sea

Daniele Iudicone, Guglielmo Lacorata, Volfango Rupolo, Rosalia Santoleri

Lagrangian statistics has been widely used to estimatedispersion properties, using homogeneous turbulent flowhypothesis, relating the diffusivity coefficient to theLagrangian velocity autocorrelation function. In theoceans, Lagrangian variability is due to both the spatialdishomogeneities and the time variability of the Eulerianvelocity field even if in some particular region one of thetwo limiting cases (or regimes) of frozen turbulence(prevalence of spatial dishomogeneities over timevariability of the Eulerian velocity field) and of fixeddrifter (prevalence of time variability over spatialdishomogeneities) can prevail. In order to determine thedynamics of the mixing of the Mediterranean Sea surfacecirculation, we analyzed experimental and numericalmodel data with the aim of mapping the mixing efficiencyand then defining if and where one of the two regimeswas prevailing. Four different and independentapproaches have been used in the data analysis, conductedseparately for 17 basin sub−regions, defined on the baseof local dynamics. . The first was to characterizedispersion properties of the surface circulation field bymeans altimeter and SST satellite data. Spatial and timespectra of Sea Surface Anomaly (SLA; five years ofTOPEX/Poseidon (T/P) data) showed large variabilityfrom region to region. Following Stammer (1998),correlation time scales , Eddy Kinetic Energy andhorizontal diffusivity were estimated. Before mappingthese parameters, an along−track analysis was performed,chosing the most representative T/P ground tracks and inorder to avoid errors induced by extrapolation andsmoothing due to mapping. Secondly, the baroclinic timescale has been estimated by computing the meanRichardson number for each of the 17 regions. At this aimthe MODB5 climatological data set of in situ temperatureand salinity was used to estimate vertical stratificationand geostrophic vertical shear. As third approach, anestimate of the ratio between Lagrangian and Euleriantime scales (Tl and Te, respectively) were obtained usingthe outputs of a CGM of the Mediterranean Sea. Fourth,the Finite Size Lyapunov Exponents (FSLEs) werecomputed for both local clusters and uniformly distributednumerical particles. As regards to the characterization oflocal mixing properties, we use the FSLE, at a fixedamplification factor r, as a Lagrangian indicator of thesensitivity to trajectory perturbations, and we mapped itonto the initial conditions. As main result, large mixing was found to occur in theAlgerian basin, in the Channel of Sicily and southernIonian Sea. The frozen turbulence regime was found todominate in the first case while the and a more complexsituation characterize the two other cases, with aprevalence of the fixed drifter regime. In this second case,the Eulerian diagnostics were found to be of limitedusefulness. The same occurred in the case of intensesemi−permanent or seasonally varying gyres, like Iera−Petra, were Eulerian diagnostics (like T/P and baroclinictime scales) overestimated the mixing rates.

In order to test the reliability and limits of thenumerical results, FSLE of a cluster of experimentalLagrangian data were computed. The main results werequalitatively confirmed. The FSLE approach was also

compared with recently developed Eulerian diagnostics ofdispersion properties.

Figure: Figure: Tl, Te and Tl/Te in the MEDMOM forparticles released during winter. Units in the first twoplots are days.

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23. The Eastern transient studied with Lagrangian diagnostics applied to a MediterraneanGCM forced by satellite SST and ECMWF for the years 1988−1993

Volfango Rupolo, Salvatore Marullo and Daniele Iudicone

We study the heat contribution to the Eastern transient byforcing a Mediterranean GCM with ECMWF winds andrelaxing surface temperature to daily satellite SST for theyears 1988−1993.

Even if with strong bias in salinity values, the generalmechanism of the transient is reproduced, with afreshening of intermediate layers due to the lifting of theold Adriatic water. Lagrangian diagnostics applied to themodel results leads both to a direct visualisation of thedeepening of the Aegean overflow. In the cold years1992 and 1993 the Antikithera flow of deep water reach1.5 Sv and display a turbulent spreading in the deepIonian basin with the development energetic cyclonic and

anticyclonic structures. Quantitative Lagrangiandiagnostics allow to directly the contribution of the Straitin the deep water renewal, and it results that in oursimulation about 1.1×1014 m3 coming from the AegeanSea through the Antikithera Strait sinks deeper than 600meters in the Eastern basin. This estimate is roughly thehalf of experimental estimate obtained from basin wideobservations. This discrepancies could be associated tothe effect of the freshwater flux that it is not considered inthis work. Finally we estimated quantitatively theupwelling of the old deep water in the Ionian basin thatresulting to be of the same order of the Aegean inflow ofdeep water, it is mainly located near the coasts.

Figure: Schematic view of the Adriatic and Aegean overflow for 1993. Light blue lines are superposed to a plot ofpositions of particles initially released at a intermediate depth (300−600 m.) with an uniform distribution in all theEastern Mediterranean. The plot indicates particles position after 254 and 326 (panels C and D) days of integration.Colours indicated depth, from blue (300 m) to white (> 1500 m.). Note that Aegean deep water sinks deeper thanwater Adriatic deep water. Deepest particles from Aegean sea reach 2700 meters of depth. In the climatic situation(not shown here) the overflow of Adriatic water closely follow the eastern coast of Calabria and sinks deeper than theAegean water.

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24. Tracing the circulation in the Mediterranean with Lagrangian trajectories in MEDMOM

Daniele Iudicone, Volfango Rupolo, Rosalia Santoleri, Vincenzo Artale

The aim of the study here presented was to quantify themain properties of the Mediterranean thermohalinecirculation (THC) by using Lagrangian off−linediagnostics applied to outputs of a GCM of theMediterranean basin circulation. The Mediterranean Seacirculation can be simplified with a three layer watermasses scheme. A first layer of Modified Atlantic Water(MAW, 0−200 m.), an intermediate layer of basicallyLevantine Intermediate Water (LIW) and deep waters,Western Mediterranean and Eastern MediterrenaneanDeep Waters (WMDW and EMDW, respectively).Despite this simplified pattern, the THC in theMediterranean Sea is rather complex; most of thebathymetrically distinct Mediterranean sub−basins areevaporative them self and are characterized by water massformation processes; moreover the existence of sills andstraits strongly influence the spreading of the intermediateand deep waters. Deep water is formed in the easternbasin in the Adriatic and in the Aegean sea, and in theNorth Western Mediterranean.

We integrated off−line particles using one year of theEulerian velocity field stored every 3 days. The integratedEulerian field can be considered representative of astationary state, it is obtained after having integrated themodel for the time necessary to reach a quasi stationarystate (we use the 100th and the 250th year of integration

The main topic of the study was to investigate thecomposition of the Mediterranean outflow. At this aim,we have performed backward in time Lagrangianintegrations of about 100,000 particles with particleswere released at meridional section in the Gulf of Cadizand integrated back in time until they have reached againthis section. Particles informations like positions andtracer properties were stored, without stopping theintegration, at various sections, located at: Gibraltar

Strait, within and at eastern entrance of the Alboran Sea,across the North Western Mediterranean (NWM) fromSpain to Sardinia at 40°N, in the Sardinia Channel, at theStrait of Sicily (one for each entrance i.e. at the westernand eastern sills), in the western Ionian and finally ameridional one is in the Levantine, going fromapproximately the Rhodes Island to the African coast andcrossing the Island of Crete.

In terms of transports, the return branch of THC wasfound to gain about 0.14 Sv in the western basin. Themixing in the Alboran Sea contributes about 0.1 Sv. If weassume that the net contribution of these two latter sub−basins is due to the WMDW, we have a value that inpercentage is much less that the 40% found from the T/Sanalysis. Moreover the gain from the Levantine to theSicily Channel is only less than 0.1 Sv. The interaction ofLIW with EMDW in the eastern basin does not changethe total amount of this water mass while significantlyaffects the tracer properties.

In the Levantine, the return flow appears formed byfour main veins, two at intermediate depths and two atdepths higher than 1000m. The presence of both LIW andEMDW in this eastern section can be traced in the densityhistogram by the presence of a broad peak around 28.92and extending from 28.8 up to 29.1 and a narrow peakcentered at 29.14.

In the western Ionian, the eastward flow is separatedmeridionally into three cores, with most of the transportin the two veins flowing along the African (mostly LIW)and the Italian coast (LIW and EMDW). In spite of asmall increase in the total flux, transformations occurredin the Adriatic basin and in the Aegean Basin changedsignificantly the mean depth and the density distributionof the return flow.

Figure: A branch of pathway of the outflow of dense water from the Aegean Sea toward the Channel of Sicily isshowed. Particle were chosen has having the shortest transit times (first 5% of the PDF). This branch is part of thesouthern vein of the return branch of the THC in the Ionian Sea, briefly discussed in the above text.

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25. The Mediterranean water in an intermediate resolution OGCM

Daniele Iudicone, Gurvan Madec, Bruno Blande

The ORCA2 configuration of the OPA OGCM wasused to investigate the sensitivity of the characteristics ofthe Mediterranean salinity anomaly in the Atlantic Seaand of its variability.

This model includes all the most recentparameterizations of sub−grid or other unresolvedprocesses like the use of a free surface formulation, use ofisoneutral surfaces, the GM90 parameterization of eddytracer transport, a time and space variable isopycnaldiffusivity depending on a local baroclinic index, abottom boundary layer parameterization, stratification−dependent vertical diffusivity for deep layers. At surface,heat fluxes and evaporation were parameterised by usingstandard bulk formulae. Climatological fields were usedfor precipitation. A restoring to Levitus98 surfacesalinities was also added. To improve the modelling of afundamental component of the THC, the dense watermasses formation, the ORCA2 model was used in arecently developed configuration that includes the LLNCLIO ice model, having the same horizontal resolution.

Several model improvements were performed during apreliminary part of the work. In particular, even if themodel resolution is not well resolving regional seas, wesuccessfully obtained a realistic Mediterranean outflowwhile keeping a Mediterranean basin, even if withtemperature and salinity almost everywhere restored toclimatological values. Also, the model parameters werechanged in order to obtain a global THC close toexperimental observations.

The sensitivity of the pattern and intensity of theMediterranean Water anomaly to wind forcings anddouble diffusivity was investigated. In the first case, theECMWF wind climatolgy was substituted with onederived from 1993−1998 of ERS scatterometer data,coupled at high latitudes with NCEP winds. This change,while dramatically improving Antartic intermediate anddeep water formation as well as sub−tropical andequatorial circulation, did not affected the intermediatesalty layer. On the contrary, the introduction of aparameterization of double−diffusive processes altered

the north−south transport of the Mediterranean water,enhancing the amount of this water at high latitudes.

The role of the Mediterranean outflow in the THC wasthen investigated by performing a long run (550 years),with the Mediterranean Sea included, in order to reach asteady state for surface and intermediate layerscharacteristics (needing at least 100−200 years tostabilize). Another run of 200 years was performedstarting from year 350 of the previous simulation butwithout the Mediterranean. The fist result of the twoparallel simulation was to determine the time scale of theMediterranean water renewal.

At the end of both simulations, the model outputs ofyear 550 was stored at 5 days resolution and off−lineLagrangian diagnostics were used to quantitativelyevaluate: a) the origin and fate of the Mediterranean watermasses; b) the differences existing between the two runsin terms of transports for the various branches of theTHC. The main result of (a) was that the Mediterraneanwater is not directly reaching the Greenland basin butenters before in the Gulf Stream where is slowly lifted toupper layers and is then able to enter the high latitudesub−basin. Regarding (b) no large change in themeridional overturning was found. Actually, while nodifference are observed at the Equator, the maximum ofthe Eulerian meridional overturning was found to beslightly higher (less than 2. Sv) in the no−Mediterraneancase. Using the Lagrangian diagnostics, a rationale forthis variation was found. The Mediterranean waterreaches North Atlantic Current at mid−depths and there itupwells to sub−surface layers, then coming back towardmid−latitudes (and to the Gulf of Cadiz). This creates anoverturning cell of about 2 Sv that has the opposite signof the main THC one. Eliminating the Mediterranean thencorresponds to eliminate this cell and then have, locally, ahigher meridional overturning.

The paper will be probably divided into a part I on thesensitivity studies and the description of the fate of MW,together with a comparison with OCCAM results (withKNMI), and a part II on the MED/no−Med experiments.

Figure: The pathway of the branch of the Mediterranean outflow that reaches the Greenland Sea. Units are Sv.

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Chapter 7. Final management report for the entire period

1) Statement from the Co−ordinator

This is the final TRACMASS management report for theentire period between 1 June, 1998 − 31 May 2001. Allthe tasks have been fulfilled succesfully. The project hasbeen almost all the time in advance on the time tablewhich has enabled us to focus on the science. We haveall been very enthusiastic in our work and aware that hisco−operation would not have been possible without thisEU−MAST funding. We have produced 25 scientificarticles with a budget of one million euros. This mightnot be the right place for us to say this but we very muchquestion that the big EU projects of today will be able tobe as productive. But hopefully we are completely wrongon this specuation. We have unsuccessfully tried to find away for us to continue to cooperate after TRACMASS.Hopefully we shall have enough momentum for the nextyear to finish all the ideas that have come out of theproject. The third year TRACMASS meeting was held onthe Italian island of Ventotene the 16 to 19 May 2001.

2) Communications and publications

There is a full publication list and summaries of all thepapers in the scientific report.

On the TRACMASS home page there are links to all theTRACMASS reports, used codes and all other relevantdocumentations.

www.misu.su.se/~doos/tracmass

3) Detailed description of the individual tasks thatwere planned and executed

A.1 Calculate Lagrangian trajectories in OCCAM thatillustrate the origin and formation of NADW.

Responsible partner: MISU, other partners: SOC

Start: 1 June 1998, End: 30 November 1998, Duration:6 months

Specific objective: . Calculate a first set of Lagrangiantrajectories in OCCAM that illustrate the origin andformation of NADW.

Methodology: Use available trajectory program andapply it on the averaged OCCAM data.

Deliverables: A first set of trajectories

Input from: −, Output to: A.5, B.1, C.1, Feedback with:A.2, A.3, D.4

A.2 Calculate Lagrangian trajectories in OPA thatillustrate the origin and formation of NADW.

Responsible partner: LPO, other partners: −

Start: 1 June 1998, End: 30 November 1998, Duration:6 months

Specific objective: . Calculate a first set of Lagrangian

trajectories in OPA that illustrate the origin and formationof NADWMethodology: Use available trajectory program andapply it on the averaged OPA data.

Deliverables: A first set of trajectoriesInput from: −, Output to: A.5, B.2, C.1, Feedback with:A.1, A.3

A.3 Calculate Lagrangian trajectories in GIM thatillustrate the origin and formation of NADW

Responsible partner: SOC, other partners: −Start: 1 December 1998, End: 31 May 1999, Duration: 6months

Specific objective: . Calculate a first set of Lagrangiantrajectories in GIM that illustrate the origin and formationof NADW

Methodology: Use available trajectory program andapply it on the averaged GIM data.

Deliverables: A first set of trajectories

Input from: D.1, Output to: A.5, B.3, C.1, Feedbackwith: A.1, A.2

A.4 Simulate time dependent trajectories that illustratethe origin and formation of NADW.

Responsible partner: LPO, other partners: MISU,KNMI, SOC

Start: 1 June 1999, End: 30 November 1999, Duration:6 months

Specific objective: . Calculate a first set of time dependentLagrangian trajectories that illustrate the origin andformation of the NADW

Methodology: Use available trajectory programs andapply it on the model simulations.

Deliverables: A first set of trajectories

Input from: A.1, A.2, A.3, D.5, Output to:A.5, B.4,Feedback with: −

A.5 Analysis of the trajectories that illustrate the originand formation of NADW.

Responsible partner: KNMI, other partners: MISU, LPO,SOC

Start: 1 November 1999, End: 31 May 2001, Duration:18 months

Specific objective: .To study with all the experienceaccumulated during the TRACMASS the origin andformation of the NADW.

Methodology: Use all the knowledge and the besttrajectory programs for the final

Deliverables: write final report and prepare papers forjournals

Input from: A.1, A.2, A.3, A.4, Output to:−, Feedbackwith: B

B.1 Calculate Lagrangian trajectories in OCCAM thatillustrate the fate and transformation of NADW.

52

Responsible partner: MISU, other partners: SOC

Start: 1 December 1998, End: 31 May 1999, Duration: 6months

Specific objective: . Calculate a first set of Lagrangiantrajectories in OCCAM that illustrate the fate andtransformation.

Methodology: Use available trajectory program andapply it on the averaged OCCAM data.

Deliverables: A first set of trajectories

Input from: A.1, Output to: B.5, Feedback with: B.2,B.3

B.2 Calculate Lagrangian trajectories in OPA thatillustrate the fate and transformation of NADW.

Responsible partner: LPO , other partners: −

Start: 1 November 1998, End: 31 May 1999, Duration:6 months

Specific objective: . Calculate a first set of Lagrangiantrajectories in OPA that illustrate the fate andtransformation.Methodology: Use available trajectory program andapply it on the averaged OPA data.

Deliverables: A first set of trajectoriesInput from: A.2, Output to: B.5, Feedback with: B.1,B.3

B.3 Calculate Lagrangian trajectories in GIM thatillustrate the fate and transformation of NADW.Responsible partner: SOC, other partners: −

Start: 1 November 1999, End: 31 May 2000, Duration:6 months Specific objective: . Calculate a first set of Lagrangiantrajectories in GIM that illustrate the fate andtransformation.

Methodology: Use available trajectory program andapply it on the averaged GIM data.

Deliverables: A first set of trajectories

Input from: A.3, D.1, Output to: B.5, Feedback with:B.1, B.2

B.4 Simulate time dependent trajectories that illustrate thefate and transformation of NADW.

Responsible partner: MISU, other partners: LPO, SOC,KNMI

Start: 1 June 2000, End: 30 November 2000, Duration:6 months

Specific objective: . Calculate a first set of timedependent Lagrangian trajectories that illustrate the originand formation of the NADW

Methodology: Use available trajectory programs andapply it on the model simulations.

Deliverables: A first set of trajectories

Input from: A.4, B.1, B.2, B.3, D.5, Output to:B.5,Feedback with: −

B.5 Analysis of the trajectories that illustrate the fate andtransformation of NADW.

Responsible partner: MISU, other partners: LPO, SOC,KNMI

Start: 1 December 1999, End: 31 May 2001, Duration:18 months

Specific objective: .To study with all the experienceaccumulated during the TRACMASS the fate andtransformation of the NADW.

Methodology: Use all the knowledge and the besttrajectory programs for the finalDeliverables: write final report and prepare papers forjournals

Input from: B1, B.2, B.3, B.4, Output to:−, Feedbackwith: A

C.1 Trace the MW in the North Atlantic.Responsible partner: IFA, other partners: LPO, KNMI,SOC, MISU

Start: 1 December 2000, End: 31 May 2001, Duration: 6months

Specific objective: . This subproject will use the pre−existing numerical simulation to follow the MW path onthe North Atlantic basin.

Methodology: Lagrangian diagnostics will be carried outon the existing OCCAM data to trace the MW in theNorth Atlantic.

Deliverables: A detailed map of the MW path in theNorth Atlantic basin will be produced. Write final report.

Input from: A.1, A.2, A.3, C.4, D.4, Output to: C.5,Feedback with: A.1

C.2 Calculate Lagrangian trajectories in MEDMOM.

Responsible partner: IFA, other partners: ENEA

Start: 1 December 1998, End: 31 May 1999, Duration: 6months

Specific objective: Calculate a first set of Lagrangiantrajectories using MEDMOM simulations.

Methodology: Use the trajectory code developed in D.4

Deliverables: A first set of trajectories

Input from: D.4, Output to:C.3, Feedback with: −

C.3 Trace the circulation in the Mediterranean with theLagrangian trajectories in MEDMOM

Responsible partner: IFA, other partners: −

Start: 1 June 1999, End: 30 November 1999, Duration:6 months

Specific objective: This subproject will trace thecirculation in the Mediterranean Sea with Lagrangiantrajectories in MEDMOM under different atmosphericforcings.

Methodology: Analysis of the model outputs will becarried out using Lagrangian diagnostics to trace thethermohaline circulation in the different subbasins of theMediterranean.

Deliverables: Study of the variability of the thermohalinecirculation with different surface boundary condition willbe made.

Input from: D.4, C.2−, Output to: C.4, Feedback with:D.5

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C.4 Evaluate the Gibraltar outflow in differentMEDMOM simulations obtained in the sensitivity studies.

Responsible partner: IFA, other partners: −

Start: 1 December 1999, End: 31 May 2000, Duration: 6months

Specific objective: Evaluation of the Gibraltar outflowcharacteristic of different Mediterranean environmentalconditions.

Methodology: Analyse the MEDMOM outputs, obtainedunder different environmental conditions, to evaluate andtrace the corresponding Gibraltar outflows by usingLagrangian diagnostics.

Deliverables: A report on links between surface fluxes onthe Mediterranean sea and Gibraltar outflow will beproduced. Data on the Gibraltar outflow to be used asboundary condition on global model.

Input from: C.3, D.4, Output to: C.1, C.5, Feedbackwith: D.5

C.5 Evaluate the impact of the MW on the NADWformation by using different Gibraltar outflows in OPAglobal model.

Responsible partner: LPO, other partners: IFA

Start: 1 June 2000, End: 31 May 2001, Duration: 12months

Specific objective: .Study the MW influence on theformation of NADW.

Methodology: The model will be forced with differentGibraltar outflows in order to evaluate the different rateof NADW formation by using Lagrangian diagnostics.

Deliverables: After 6 months : A first set of trajectoriesobtained integrating OPA global

model. After 12 months: Comparison of MW path in thenorth Atlantic by using different

Gibraltar output in OPA global model. Write final reportand prepare papers for journals

Input from: C.4−, Output to: −, Feedback with: A.5

D.1 Write Lagrangian trajectory code for GIM.

Responsible partner: SOC, other partners: MISU

a) Start: 1 June 1998, End: 30 November 1998,Duration: 6 months

b) Start: 1 January 1999, End: 30 June 1999, Duration:6 months

Specific objective: .Develop a trajectory code for GIM

Methodology: The isopycnic coordinate model GIMrequires a complete rewriting of the trajectory code. a)develop code, b) include diffusive effects, etc.

Deliverables: A trajectory code for GIM

Input from: −, Output to: A.3, B.3, Feedback with: :A.1

D.2 Investigate the influence of the bolus velocity on thetrajectories.

Responsible partner: SOC, other partners: MISU

Start: 1 June 1998, End: 31 May 1999, Duration: 12months

Specific objective: . The behaviour of passive tracers andparticles is identical only if there is no time variability. Ifthe flow varies in time, then the movement of tracers canbe defined in terms of a ’bolus’ velocity as discussedabove. Computation of the bolus velocity is time−consuming (Lee et al., 1996; Treguier, privatecommunication) but straightforward, save in regions ofstatic instability, where it is not well defined. Mixedlayers, for example, may present a problem, as they alsodo to parameterisation schemes.

Methodology: Comparisons will be made of thedifferences between Lagrangian trajectories using onlythe time mean velocity, and those using a modifiedvelocity which includes the bolus velocity. This will becomputed for the calculations carried out in thesubprojects A, B and C.

Deliverables: In addition, possession of estimates of bolusvelocity for three different realisations of the physics ofocean basins will permit a detailed examination of eddyparameterisation schemes and, hopefully, the creation ofnew parameterisation methods. This does not form partof the workplan for the project, since the outcome of suchinvestigations is inherently uncertain.

Input from: −, Output to: A.1, B.1, C.1, Feedbackwith:−

Special comment: D.2 and D.3 are not task−based, butare investigations. Unlike much of the rest of the project,we cannot say a priori what will be found or whichapproach will prove valid, and decisions will be taken asthe work proceeds. Accordingly, a timetable cannot beproduced. Deliverables remain, of course, 6−monthlyprogress reports, and results which impinge on the rest ofthe project will be communicated immediately.

D.3 On−line Lagrangian trajectories compared to passivetracers in an idealised isopycnic model.

Responsible partner: SOC, other partners:

Start: 1 June 1998, End: 31 May 2001, Duration: 36months

Specific objective: . In principle, Lagrangian particles cancapture both the time−mean and time−varying velocity.In practice, it is difficult to achieve this, since one wouldencounter numerical accuracy and sampling strategyproblems, especially if one is interested in long timescales. However, with much observational float dataavailable and more to be released, it is important that wehave a full understanding of what float data really tell us.

Methodology: A study will therefore be conducted of thecomparison of passive tracer and Lagrangian particles onthe decadal time−scale using an idealised eddy−resolvingMICOM model.

Deliverables: report and paper if conclusive

Input from: −, Output to: A.5, B.5, Feedback with: :A,B, C

Special comment: D.2 and D.3 are not task−based, butare investigations. Unlike much of the rest of the project,we cannot say a priori what will be found or whichapproach will prove valid, and decisions will be taken asthe work proceeds. Accordingly, a timetable cannot beproduced. Deliverables remain, of course, 6−monthlyprogress reports, and results which impinge on the rest ofthe project will be communicated immediately.

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D.4 Implementation of trajectory code in MEDMOM.

Responsible partner:IFA, other partners: ENEA, MISU

Start: 1 June 1998, End: 30 November 1998, Duration:6 months

Specific objective: . Develop a trajectory code forMEDMOM

Methodology: This task will consist in taking thetrajectory code from OCCAM and implement it inMEDMOM.

Deliverables: A trajectory code for MEDMOM.

Input from: −, Output to: C.1, C.2, C.3, C.4, Feedbackwith: :A.1

D.5 Determine optimal sampling period for time−dependent velocity fields

Responsible partner: LPO, other partners: SOC, MISU

Start: 1 June 1999, End: 31 May 2000, Duration: 12months

Specific objective: . Most objectives of our Lagrangianmethodology rest on the best possible description of awater mass in terms of particles that compose it. Thepartitioning we currently use favours initial locationswhere the transport is the highest: each individual modelgrid cell (with a given transport T) is divided into N2

subregions (i.e., N2 particles), with N satisfying T/N2 ² Towhere To is a prescribed maximum transport to beassociated to one particle. A homogeneous distribution isadopted within each grid cell, and the total number ofparticles in use is the sum of the N’s. Adaptation to timedependent configurations, and developments ofappropriate measurements of the resulting accuracy of thecomputed transports are needed.

Methodology: Our Lagrangian tool is diagnostic, andoperates after a simulation with a general circulationmodel is made available to us. The temporal sampling ofthe GCM outputs has to be carefully chosen (andsufficiently fine) to make the computed Lagrangiantrajectories as reliable as possible. Undersampledtemporal variability leads to aliased trajectories, but onthe other hand the most accurate the sampling, the moreburdensome the Lagrangian integrations.

Deliverables: The project will help us refining thetemporal dimension of the Lagrangian approach, first bystudying the velocity frequency spectrum of Lagrangianparticles in the GCM, then by comparing such ’on−line’behaviours (for which trajectories are integrated oversuccessive time steps of the GCM, typically of the orderof a few hours) to more usual ’off−line’ diagnostics,constrained by the time sampling adopted for the outputs.

Input from: −, Output to: :A.4, B.4, C.5, D.6 Feedbackwith: C.3, C.4

D.6 Best positioning of particles over initial sections.

Responsible partner: LPO, other partners: −

Start: 1 June 2000, End: 31 May 2001, Duration: 12months

Specific objective: . We currently use a partitioning thatgave excellent results in the computation of the transportsassociated with the flow of an annual−mean PacificEquatorial Undercurrent. In this case, one measurement ofthe accuracy of the method consists in the evaluation of"section to section" transports (i.e., from 150°W to120°W) with both forward (from 150°W) and backward(from 120°W) integrations. The partitioning we adoptfavours initial locations where the transport is the highest:each individual model grid cell (with a given transport T)

is divided into N2 subregions (i.e., N2 particles), with Nsatisfying T/N2 = To where To is a prescribed maximum

transport to be associated to one particle. A homogeneousdistribution is adopted within each grid cell, and the totalnumber of particles in use is the sum of the N’s. Thisdistribution of particles was shown to provide a muchbetter accuracy than ’constant number of particles pergrid cell’ or ’spatially homogeneous’ distributions,because backward and forward estimates (for a similargiven number of initial particles) differed least.

Methodology: Our plans aim at developing partioningsindependent on the mesh of the ocean GCM. Ourdevelopments will be achieved in close collaboration withSOC to test and adapt them in the configuration ofdensity−coordinates. For simulations with seasonalvariability included, the distribution with time of initialpositions will be optimised in order to reduce redundancyand favour smooth samplings in both time and spacedirections. Current validation methods consist inindependent evaluations of "section to section" transports,for both forward and backward (reverse in time)integrations. Extended measurements of the accuracy ofthe Lagrangian transports will be developed, and adaptedfor on−line trajectories for which backward estimates areimpossible to carry on.

Deliverables:

• initial partitioning able to fit whatever modelconfiguration (vertical coordinate, and time sampling)

improved knowledge of the accuracy in transportmeasurements

Input from: D.5, Output to: A, B, C, Feedback with:C.2, C.3

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4) Personnel

Table of personnel working in the project during the three years

Institution Name Position Period employed Time spent on the project(% of a full time position)

TRACMASSfunded

ENEA Vincenzo Artale Dr. Scientist 01.06.98−31.05.01 15% no

Volfango Rupolo Scientist − Dr. student 01.06.98−31.05.01 75% no

IFA−CNR Rosalia Santoleri Dr. Scientist 01.06.98−31.05.01 15% no

Daniele Iudicone Scientist − Dr. student 01.06.98−31.05.01 100% yes

KNMI Sybren Drijfhout Dr. Scientist 01.06.98−31.05.01 20% yesPedro de Vries Dr. Scientist 01.06.98−31.05.01 100% yes

LPO Bruno Blanke Dr. Scientist 01.06.98−31.05.01 75% no

Sabrina Speich Dr. Scientist 01.06.98−31.05.01 75% no

Maria Valdivieso Dr. Scientist 01.06.98−31.05.01 100% yes

Karine Pailler Dr. student 01.09.98−30.11.98 100% no

Francois Laurent Technician 01.11.98−30.11.98 100% yes

MISU Kristofer Döös Dr. Scientist 01.06.98−31.05.01 100% Yes

Donatella Faggioli Dr. Scientist 01.06.99−31.05.00 100% yes

SOC Peter Killworth Dr. Scientist 01.06.98−31.05.01 15% yes

Andrew Coward Dr. Scientist 01.06.98−31.05.01 25% yes

Robert Marsh Scientist − Dr. student 01.06.98−31.05.01 25% yes

Mei−Man Lee Dr. Scientist 01.06.98−31.05.01 50% yes