climate stability and instability: transition from flywheel to driver? jochem marotzke school of...

Climate Stability and Instability: Transition from Flywheel to

Driver?

Jochem Marotzke School of Ocean and Earth ScienceSouthampton Oceanography Centre

Southampton, SO14 3ZHUnited Kingdom

NOAA Global SST Analysis, 4 - 9 November 2002

• North Atlantic warmer than North Pacific NADW formation not a simple forced response to stronger cooling by atmosphere: If it were, NA should be colder than NP.

• Ocean circulation active in setting fundamental properties

• High North Atlantic sea surface salinity (SSS) crucial for NADW formation

• Ocean circulation can, in principle, maintain NA SSS greater than NP SSS without bias in forcing such as Atlantic-to-Pacific atmospheric water vapour transport (Marotzke & Willebrand, 1991).

• True in reality? - “without bias in forcing”? Coupled GCMs give equivocal answers (e.g., Manabe & Stouffer, 1999).

• Is there another circulation mode that the MOC could attain?

• Is there another circulation mode that the MOC could attain?

• Could transitions to another mode be abrupt?

• Discuss intricacies using the example of ocean mixing

• Conceptual, mostly steady-state; illustrated w/ simple GCMs

Flywheel or Driver?

• Is there another circulation mode that the MOC could attain?

• Confirmation requires continuous MOC observations

• How can this be done?

• Could transitions to another mode be abrupt?

• Would an MOC transition be a passive response to external forcing, or be self-driven, possibly following a trigger?

Mixing in Stratified Waters (I):

• Sandström (1908, 1916; see Colin de Verdière 1993): Heating below cooling is required so that fluid can act as a heat engine (buoyancy-driven flow exists)

• Jeffreys (1926): Expansion below contraction is crucial, which is possible in presence of mixing even if heating & cooling occur at the same pressure

• Munk (1966): Mixing heats upwelling deepwater

• Weyl (1968): Mixing converts turbulent kinetic energy into potential energy, which is needed to drive flow

• Munk and Wunsch (1998): Energy for mixing derives significantly both from tides and from wind

Mixing in Stratified Waters (II):

• GCMs with fixed diffusivity: MOC increases with density gradient (e.g., Scott, thesis 2000)

• With fixed amount of energy available for mixing, MOC might decrease with density gradient (Walin 1990, Lyle 1997, Huang 1998, Nilsson & Walin 2001, Oliver, thesis in prep.)

• Series of GCM experiments: Nilsson & Walin (submitted):

Mixing and MOC:

Flywheel or Driver - Meaningless question?

•Expect mixing to matter mainly over very long timescales

•Time-dependent situations?

•Kevin Oliver (UEA, thesis in prep.): Considers transient behaviour in isopycnic box model with energy-dependent mixing (Nilsson & Walin, 2001)

Oliver (Thesis, UEA, in prep.)

Oliver (Thesis, UEA, in prep.)

FF increased from 0.3 to 0.4 Sv

FF decreased from 0.4 to 0.3 Sv

• Wang et al. 1999, idealised global model: “NADW” collapses under doubling of FW forcing within 1000 years

• NB: Collapse timescale unpredictable within factor 2

BUT:

• Steady-state: NADW increases with FW forcing

• NADW consistent with Rooth (1982) box model

• Total nearly constant

Convective mixing & sinking are different processes:

• Mauritzen (1996): DSOW derives from gradually sinking Atlantic Water, not convection in central Greenland Sea gyre

• Marotzke & Scott (1999): Sinking possible without convective mixing; sinking expected near boundaries

• Spall & Pickart (2001): Convective mixing & sinking co-located near sloping topography

If convective mixing is unimportant, why do we pay so much attention to its

fate in the North Atlantic?

If high-latitude salinity is so important in the North Atlantic, why is the freshwater

part of the surface buoyancy flux so small?

Schmitt et al., 1989

Large & Nurser, 2001

Blue: Ocean heat loss

Red: Ocean water gain

Red: Ocean density gain

• Pole-to-equator (and top-to-bottom) density contrast is dominated by temperature: The pycnocline is a thermocline

• Pole-to-equator (and top-to-bottom) density contrast is dominated by temperature: The pycnocline is a thermocline

• Water is dense because it is cold (from high latitudes)

• Which high latitudes ventilate deep ocean depends on SSS

• Density contrasts between high latitudes (competing DW formation sites) much smaller than between pole & equator

• Cross-equatorial coupling between high latitudes crucial

• Cooling dominates buoyancy flux in DW formation region

• Interhemispheric (& interocean?) dynamics central

Tziperman 1997

Wang et al. 1999

Klinger & Marotzke 1999

• Convective mixing determines dominant high latitudes but not global deepwater formation rate

• Interhemispheric (& interocean?) dynamics central

• Diapycnal mixing works on overall density contrast

• Controls global rate of upwelling deepwater

• Efficiency of convective mixing unimportant for global rate

• Distribution over competing high latitudes depends on surface density, hence SSS

• High latitudes with deepest convective mixing dominate (Needs to be qualified: Topography, overflows etc.)

• Convective mixing determines dominant high latitudes but not global deepwater formation rate

• Cooling dominates buoyancy flux in DW formation region

• Interhemispheric (& interocean?) dynamics central

Summary Part I:

Mixing and MOC:

Flywheel or Driver - Meaningless question?

• Timescales critical in dependence on mixing and FW forcing

Oceanic and atmospheric processes linked

inextricably

• Confirmation (of hypotheses of what controls MOC and its variability) requires continuous MOC observations as a starting point

• How can this be done?

26.5°N MOC Monitoring Proposal

• PIs: Jochem Marotzke, Stuart Cunningham, Harry Bryden (SOC)

• Submitted to NERC RAPID Programme (which is funded with £20M over 6 years)

• Requested: £4.7M over 5 years

• Would support 2 Post-docs, 1 Research Assistant, 1 Ph.D. Student

• Funding decision expected 25/26 November

Why 26.5°N?

• Near Atlantic heat transport maximum - captures total heat transport convergence into North Atlantic

• South of area of intense heat loss ocean atmosphere over Gulf Stream extension

• MOC dominates heat transport at 26.5°N

• Heat transport variability dominated by velocity fluctuations (Jayne & Marotzke, 2001)

• Florida Strait transport monitored for >20 years (now: Johns, Baringer & Beal, Miami, collaborators)

• 4 modern hydrographic occupations

Approach: Integrated

thermal wind(geostrophy)

• Ekman contribution to MOC included

• Surface layer Ekman transport assumed to return independent of depth

Model-based experiment design:

• Funded through NERC prior to conception of RAPID

• Joël Hirschi (post-doc), Johanna Baehr (M.Sc. student)

• “Deploy” antenna in high-resolution models, OCCAM (1/4°; SOC, Webb et al.; Hirschi), FLAME (1/3°; IfM Kiel, Böning et al.; Baehr )

• See Hirschi et al. poster

Blue:

Covered

Red:

MOC

Blue:

Recon-struction

Red: MOC Blue: Reconstruction

Black: OCCAM Heat Transport Green: Reconstruction

OCCAM FLAME

Red: MOC Blue: Reconstruction Cyan: 300 realisations with random error (1 Sv Florida Strait; 0.01 kgm-3)

OCCAM

Blue: Reconstruction Cyan: Thermal Wind Green: Ekman

FLAME

OCCAM

Transition from Flywheel to Driver:

• Importance of mixing in MOC dynamics

• Nature and location of mixing matter but are unknown (interior & boundary mixing; base of SO mixed layer; energetics)

1. What have we learned during the WOCE period?

• MOC could reorganise

• Dynamics of convection

Transition from Flywheel to Driver:

• DBE visualised inhomogeneity of mixing

• Deep Indian Ocean MOC: Well studied in WOCE projects (despite lack of WOCE 32S section); considerable deep mixing required to balance inflow.

2. What specifically was the WOCE contribution?

• Hydrographic sections gave accurate global estimate of MOC

Transition from Flywheel to Driver:

• Continuous observations of MOC drivers (heat & FW budgets of convection areas)

• Estimates of global distribution of mixing

3. What is required in the future (I)?

• Continuous observations of the MOC at selected latitudes

Transition from Flywheel to Driver:

3. What is required in the future (II)?

• Model-based experiment design for climate time series: Rational resource allocation

• Ocean (and coupled) models that represent coupled nature of mixing

• Improved (or development of) conceptual understanding of interaction between high latitudes (within and across oceans)