Dispersal on Weekly to Yearly Time Scales in the Abyssal ... and...CM velocities are inﬂuenced by...

Dispersal on Weekly to YearlyTime Scales in the Abyssal

Pacific Nodule ProvinceA.M. Thurnherr

[email protected]

Lamont-Doherty Earth Observatory

Thurnherr: Abyssal Dispersal – p.1/24

Nodule Area


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?


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?


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)


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)




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




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)


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)


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!)


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


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)


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)


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


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


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)


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)


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


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


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


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


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

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-2800

-2700

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-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


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


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


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Dispersal on Weekly to Yearly Time Scales in the Abyssal ... and...CM velocities are inﬂuenced by...

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