Role of eddies in ocean circulation

TOPEXe.g. vanishing of ‘Deacon Cell’

What can we infer from observations?What can we infer from observations?

Doos and Webb

Danabasoglu, McWilliams

• Models suggest eddies play major role in tracer, momentum and vorticity budgets

But

• Ocean is full of geostrophic turbulence

h212

WOCE SR3

Schematic: Sverdrup

Tracer distributions

Salinity

vR v v

Residual mean

Eulerian mean

Bolus transport

Rhines

Rintoul

Circulation inferred from tracers is the ‘residual flow’

Circulation inferred from tracers is the ‘residual flow’

vh

hAntarctica

vhh

What is the role of eddy transfer in setting up ?

What is the role of eddy transfer in setting up ? vR

vR

Infer Eddy transport from observations

Winds

Surface buoyancy fluxes

v

vR

1

v

2 3 Momentum balance

Buoyancy transport

Eddy diffusivity,

Compare eddy stress to wind stress

KHow can we estimate a K from observations?

vR v vv

vDiabatic

Deacon Cell

Speer,Rintoul & Sloyan

Buoyancy Transport

Rewrite buoyancy budget:

Stream-wise average

2d (i)

(ii)

y

vb

z v v by w w b

z B z

vb

b z

+ dia

Andrews and McIntyre

Held & Schneider

1

wb

b y

Residual mean theoryResidual mean theory

Gent and McWilliamsIf eddies are ‘adiabatic’

Cross-stream transport inferred from air-sea fluxes

from winds

Speer, Rintoul & Sloyan

R by Bs from air-sea

buoyancy fluxes

Neglecting diffusion & diabatic eddy fluxesNeglecting diffusion & diabatic eddy fluxes

f

David Marshall

Cross-stream transport implied by the wind is not the same as that implied by air-sea buoyancy fluxes

Cross-stream transport implied by the wind is not the same as that implied by air-sea buoyancy fluxes

Air-sea buoyancy fluxWind Stress

NCEP

SOC DaSilva

53.553.5

0.1

0.14

0 Pole Pole 60oS60oS 4646

0

NCEP

Bs x Nm 2

In to ocean

Out of ocean

Cross-Stream transports

Buoyancy fluxes

Can eddies achieve such a large transport?Can eddies achieve such a large transport?

R Bs by

R

58 54 5062S 46

20

40

20

40

0

Sv

Ekman

‘Inferred’ bolus transport

Taka ItoPole

WOCE SAC climatology

Gouretski and Jancke

Eddy mixing

If so, how large is the K?

2

Use ‘idealized tracer’ driven by observed surface geostrophic flow to yield KUse ‘idealized tracer’ driven by observed surface geostrophic flow to yield K

Can we characterize eddies with a diffusivity, K?

Device to yield diffusivity from observations

‘idealized tracer’

altimetry

Large-scale v drives small scale mixing

Advection-diffusion in 2-dNakamura

‘small-scale’ diffusion

q t v q k 2q

In ‘area’ coordinates problem can be rephrased as:

k A

Aq,t

|q|2dA

q t

A K qA

where K qA

2

Haynes and Shuckburgh

kqkq

‘dissipation rate’ of qTheory ‘intersects’ with observations very nicely

Theory ‘intersects’ with observations very nicely

Drive tracer with altimetric data

Assumption: large-scale K is independent of small-scale

q t v q k 2q

Topex dataevery 10 days

Emily Shuckburgh (DAMPT)Helen Jones (MIT)

k 100m2s 1

16

16

Planning to repeat at130

130

Resolution

k 10m2s 1with

k

Eddy diffusivity

K m2s 1

2000

1000

50 40 3060 S

500

Latitude

Near surface

Phillipskm

m2s 1

SCM3mooring

K

1500

Implications for parameterizationImplications for parameterization

K

R Bs by

Deducing from

isopycnal slope at mixed layer base

WOCE SAC climatology

20

40

20

40

0

Sv

58 54 5062S 46

K

s b y

b z

R

Gouretski and Jancke

Pole vb

b z

Ks

fvR F vqPV mixing

PV mixing

PV front

vvv

Momentum balance

wind

eddy

surface0

Johnson and Bryden eddy wind R 0if

at 150m

Nm 2

v

0.1 0.20 0.1 0.2

3

Eddy stress

Eddy f fKs

wind z

Eddy z

Eddy PV flux

Mapping down to depth

salinity

No bottom water

Karsten, Marshall

UpperCircumpolarDeep Water

(Sv) R

IntermediateWater

Salinity

vv

Conclusions1

2

2-d mixing of idealized tracer

3

must be large and oppose

Estimated from altimetry

yields near-surface

Eddy stresses are as large as wind stress

4 Eddies are hugely important in ACCand elsewhere…..

500 to 2000K m2s 1

K

Transport implied by and are very different

30 Sv

and consistent pattern of

Bs

Estimate of global eddy stress

assuming K = 1000

Nm 2

using WOCE SAC climatology

At 300m

m2s 1 Parameterized in our global climate models!

Parameterized in our global climate models!

0.1 0.2 0.1 0.2 0

ObservationsTheory

Modeling

ReviewsNowlin and Klink, 1986Rintoul, Hughes and Olbers, 2001

Bryden,1979Gille,1997Phillips and Rintoul, 2000Speer, Rintoul and Sloyan, 2000Karsten and Marshall, 2002Gille, 2002

Munk and Palmén, 1951deSzoeke and Levine, 1981Marshall, 1982Johnson and Bryden, 1989Gent and McWilliams, 1990Marshall, Olbers, et al, 1993Nakamura, 1996David Marshall, 1997McDougall,1998Haynes and Shuckburgh, 2001Olbers and Ivchenko, 2002Marshall and Radko, 2002

McWilliams, Holland and Chow, 1978Semtner, Chervin, 1992Döös and Webb 1994Danabasoglu, McWilliams 1994Ivchenko, Richards, Stevens 1996