dispersal on weekly to yearly time scales in the abyssal ... and...cm velocities are influenced by...
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Dispersal on Weekly to YearlyTime Scales in the Abyssal
Pacific Nodule ProvinceA.M. Thurnherr
Lamont-Doherty Earth Observatory
Thurnherr: Abyssal Dispersal – p.1/24
Nodule Area
Thurnherr: Abyssal Dispersal – p.2/24
Fundamental Questions
1. (easy) What is range in mean and maximum flow velocities likelyto be 1 m (or 10 m) above the seabed in the abyssal Pacificnodule province over time scales of 1 month to 1 year?
2. (hard) What is the maximum distance in any direction a neutrallybuoyant particle within 500 m of the bottom is likely to travel overtime scales of 1 week, 1 month and 1 year?
Thurnherr: Abyssal Dispersal – p.3/24
Abyssal Circulation
abyssal Pacific is filled with AABW; pathways are topographicallyconstrained
two published circulation schemes are largely inconsistent with eachother ⇒ circulation unknown (common in abyssal ocean away fromtopography)
important question: on what time & space scales should thesecirculation schemes be relevant?
Thurnherr: Abyssal Dispersal – p.4/24
Circulation vs. CM DataDemidova (1999)
poor agreement between both AABW circulation schemes and 1–2year averaged current-meter velocities
possible reasons:CM velocities are influenced by local bottom topography (Demidov,1999), i.e. spatial scale of mean circulation is ≫ topographic scalemean-circulation time scale ≫ CM record lengthscirculation schemes are incorrect (↔ Brazil Basin before floats)
Thurnherr: Abyssal Dispersal – p.5/24
Abyssal Eulerian Mean Velocities
Weekly Averaged Velocities Monthly Averaged Velocities Yearly Averaged Velocities
10 cm/s
5 cm/s
CM data can be used to answer 1st question (What is range in meanand maximum flow velocities likely to be 1 m (or 10 m) above theseabed in the abyssal Pacific nodule province over time scales of 1month to 1 year?)
this question is important e.g. for sediment resuspension but not fordispersal on time scales of months or longer (because of spatialvariability → more later)
Thurnherr: Abyssal Dispersal – p.6/24
Abyssal Eulerian Mean Velocities
Weekly Averaged Velocities Monthly Averaged Velocities Yearly Averaged Velocities
10 cm/s
5 cm/s
on time scales of weeks to months, Eulerian mean velocities in abyssalocean can typically be in any direction (with non-uniform distributions)
weekly averaged speeds are dominated by mesoscale eddies(Demidov: synoptic scale) ⇒ several cm·s−1 typical
longer averages are characterized by smaller speeds; yearly-averagedvelocities are often <1 cm·s−1
Thurnherr: Abyssal Dispersal – p.7/24
Abyssal Eulerian Mean Velocities
Weekly Averaged Velocities Monthly Averaged Velocities Yearly Averaged Velocities
10 cm/s
5 cm/s
superficially, record-mean CM averages for deployments lasting morethan a few months (1–2 years typical) are similar to “mean” circulation:1. single direction
2. speeds of a few mm·s−1
⇒ 1-year or longer record averages are often implicitly assumed torepresent mean circulation (e.g. Demidov used CM data to constrainAABW transports)
Thurnherr: Abyssal Dispersal – p.8/24
Statistical Significance
-1
-0.5
0
0.5
1
0 10 20 30 40 50
Nor
mal
ized
Aut
ocor
rela
tion
Time Lag
uv
example record µ & (tidally dominated) σ: u = −0.4 ± 5.6 cm·s−1
v = −0.6 ± 4.3 cm·s−1
standard errors of the means can be estimated from stddev & integraltime scales of the records (Flierl & McWilliams, 1978):u = −0.4 ± 0.6 cm·s−1 v = −0.6 ± 0.9 cm·s−1
⇒ record mean is not significantly different from zero ⇒ 1-year-longrecord is not representative for circulation on longer time scales
this is typical for deep ocean; at one site in the North Atlantic 9-yearrecord mean was still not significantly different from zero (Müller &Siedler, 1992)
Thurnherr: Abyssal Dispersal – p.9/24
Abyssal Dispersal in Practice
0.5cm/s
Ledwell (2000)
CM data indicate northward flow of 0.5 ± 1.1 cm·s−1 (i.e.indistinguishable from zero) on the western ridge flank
coincident tracer measurements indicate that there was a mean flow ofa few mm·s−1 to the SW (NB: consistent with CM measurements!)
Thurnherr: Abyssal Dispersal – p.10/24
Abyssal Dispersal in Practice
0.5cm/s
Ledwell (2000)
note, however, that dispersal of the western tracer patch consists of1. SW-ward drift2. spreading (i.e. some tracer spreads against the mean flow!)
⇒ dispersal cannot be characterized by a single vectorThurnherr: Abyssal Dispersal – p.11/24
Advection vs. Eddy Diffusion
dispersal is combination of two effects:advection by low-frequency (mean) flow: dispersal ∝ time
eddy diffusion (random walk): dispersal ∝√
time
in typical deep-ocean settings, dispersal on time scales ofmonths to a year or two is often diffusion dominated (⇔ indispersal studies diffusion is often ignored):
2500m /s
1300m /s2
2mm/s
5mm/s
1cm/s
300
400
500
600
0 365 730 1095 1460 1825
Dis
tanc
e [k
m]
Time [days]
Eddy DiffusionMean−Flow Advection
0
200
100
Thurnherr: Abyssal Dispersal – p.12/24
Eddy Diffusive Dispersal
eddy diffusion is caused by combined spatial & temporalvariability ⇒ in many regions: random walk (similar to moleculardiffusion)
magnitude of eddy diffusion depends sensitively on processescausing random walk; on time scales longer than a few weeks,mesoscale eddies dominate horizontal eddy diffusion ⇒κ ≈ 102–103 m2 ·s−1
huge CM arrays are required to assess spatial variability ofabyssal flows ⇒ CM data are not usually suited to assess eddydiffusive dispersal
⇒ need Lagrangian (flow-following) data to assess dispersal in thedeep ocean (except in a few locations of strong mean flows)
Thurnherr: Abyssal Dispersal – p.13/24
Continuous Lagrangian Tracers
0.5cm/s
Ledwell (2000)
different tracers for different time scales (SF6, fluorescin, &c)
+ most similar to dispersing small neutrally buoyant particles
+ single release can be used to assess both advection & eddy diffusionover range of time scales
+ provides information on vertical eddy diffusion
- expensive & work intensive (compared to CM)
- provides only single snapshot; representativeness must be assesseddifferently (e.g. with CM time series)
Thurnherr: Abyssal Dispersal – p.14/24
Discrete Lagrangian Tracers
179˚E 180˚ 179˚W 178˚W 177˚W 176˚W 175˚W 174˚W 173˚W
25˚S
24˚S
23˚S
22˚S
21˚S
20˚S
19˚S
18˚S
17˚S
16˚S
15˚S
14˚S
A
A.M. Thurnherrwww.ldeo.columbia.edu/~ant/LAUB-FLEX
179˚E 180˚ 179˚W 178˚W 177˚W 176˚W 175˚W 174˚W 173˚W
25˚S
24˚S
23˚S
22˚S
21˚S
20˚S
19˚S
18˚S
17˚S
16˚S
15˚S
14˚S
A
A.M. Thurnherrwww.ldeo.columbia.edu/~ant/LAUB-FLEX
179˚E 180˚ 179˚W 178˚W 177˚W 176˚W 175˚W 174˚W 173˚W
25˚S
24˚S
23˚S
22˚S
21˚S
20˚S
19˚S
18˚S
17˚S
16˚S
15˚S
14˚S
A
A.M. Thurnherrwww.ldeo.columbia.edu/~ant/LAUB-FLEX
different types of floats with different capabilities & costs
+ multiple floats can be released at the same time/place to mimic continuous tracer
+ time-series releases can be carried out to assess representativeness
+ largely autonomous operation
- not affected by small-scale flows⇒ no information on vertical eddy diffusion
- need to return to the surface for data retrieval (and, possibly, positioning)
Thurnherr: Abyssal Dispersal – p.15/24
Progressive Vector Diagrams
-250
-200
-150
-100
-50
0
-250 -200 -150 -100 -50 0M
erid
iona
l Qua
si-D
ispl
acem
ent [
km]
Zonal Quasi-Displacement [km]
often, progressive vector diagrams are constructed by time-integratingEulerian velocity measurements ⇒ pseudo-Lagrangian “trajectories”
construction of PVDs is based on assumption that there is no lateralvariability in the velocity field ⇒ cannot be used to assess dispersal1. in regions where topography affects the flow (e.g. all of Demidova’s
abyssal CM data)2. over scales that are larger/longer than those of the eddies that
dominate eddy-diffusive dispersal (i.e. on time scales longer than afew weeks and space scales larger than a few 10s of km)
Thurnherr: Abyssal Dispersal – p.16/24
Dispersal on Weekly Timescales
0
5
10
15
20
25
0 10 20 30 40 50 60
Weekly Pseudo-Displacement [km]
Max. distance in any direction a neutrally buoyant particle in bottom500 m is likely to travel over time scales of 1 week?
dispersal dominated by mesoscale-eddy advection (stirring bymesoscale eddies can be ignored because integral time scale > 1week & the dispersal length scale is shorter than a typical eddy scale)
typical dispersal displacement O(10 km); any direction possible, exceptin regions with very strong mean flows
Thurnherr: Abyssal Dispersal – p.17/24
Dispersal on Yearly Timescales
0.5cm/s
Ledwell (2000)
2500m /s
1300m /s2
2mm/s
5mm/s
1cm/s
300
400
500
600
0 365 730 1095 1460 1825
Dis
tanc
e [k
m]
Time [days]
Eddy DiffusionMean−Flow Advection
0
200
100
away from topography, advective & eddy diffusive dispersal of similarmagnitude [O(100 km)]; near topography, “anything goes”
whether dispersal has preferred direction depends on speed oflow-frequency advection & on regional horizontal eddy diffusivity
quantifying the typically weak abyssal low-frequency flows with CMsrequires many years of continuous measurements
quantifying eddy diffusion requires Lagrangian trajectories
Thurnherr: Abyssal Dispersal – p.18/24
Dispersal on Monthly Timescales
Jac kson, Ledwell, Thurnherr (in prep.) Jac kson, Ledwell, Thurnherr (in prep.)
most difficult regime because 1 month is greater, but not much greater,than a typical integral time scale of the mesoscale eddy field ⇒advection is not dominated by single eddy any more, but not enougheddies for statistical treatment
⇒ dispersal cannot be viewed as simple superposition of advection &(mesoscale) eddy diffusion
Thurnherr: Abyssal Dispersal – p.19/24
The LADDER Experiment
LArval Dispersal along the Deep East pacific Rise
biological/physical NSF project
PIs: Mullineaux, Thurnherr, Ledwell, McGillicuddy, Lavelle
goal: assess dispersal near EPR crest on time scales relevantfor larvae
approach:sample larvæ (surveys & time-series)colonization experiments30–40 day tracer-release experimentPO surveys (CTD, LADCP, microstructure)PO moorings (CM & profilers)regional numerical circulation models with larvæ
Thurnherr: Abyssal Dispersal – p.20/24
PO Moorings
104˚ 45'W 104˚ 30'W 104˚ 15'W 104˚ 00'W 103˚ 45'W9˚ 00'N
9˚ 15'N
9˚ 30'N
9˚ 45'N
10˚ 00'N
NA
CA
SA
WF
EF
W1W3
-3300
-3200
-3100
-3000
-2900
-2800
-2700
-2600
-2500
-2400-Depth [m]
extensive array required to assess horizontal variability of regionalvelocity field
profilers used to assess vertical structure & diapycnal mixingThurnherr: Abyssal Dispersal – p.21/24
PO Surveys
105˚ 15'W 105˚ 00'W 104˚ 45'W 104˚ 30'W 104˚ 15'W 104˚ 00'W 103˚ 45'W
9˚ 00'N
9˚ 15'N
9˚ 30'N
9˚ 45'N
10˚ 00'N
LADDER-1 (2450m) 5cm/s
carefully planned repeat-sampling required to reduce tidal aliasing
provides high-res. snapshot of spatial variability of flow field
Thurnherr: Abyssal Dispersal – p.22/24
Tracer Release
Jac kson, Ledwell, Thurnherr (in prep.) Jac kson, Ledwell, Thurnherr (in prep.)
only nearly 100% convincing method to provide a ground-truthsnapshot of dispersal from an impulsive point source
remaining uncertainty:possible vertical motion due to adsorption on particlesnon-synopticity of samples
Thurnherr: Abyssal Dispersal – p.23/24
Main Points
abyssal dispersal on ecologically relevant timescales involvesboth advection & eddy diffusion
circulation schemes do not generally provide useful informationon the relevant time scales
Lagrangian data are required on all but the longest time scales(when eddy diffusion can be treated statistically)
many years of measurements are required to determine astatistically representative description of the flow field
topography important ⇒ should be studied regionally
Thurnherr: Abyssal Dispersal – p.24/24