mixed layer lateral eddy fluxes mediated by air-sea interactions

8/14/2019 Mixed Layer lateral eddy fluxes mediated by air-sea interactions

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Mixed layer lateral

eddy fluxesmediated by air-sea

interaction

Guillaume Maze

John Marshall

David Ferreira

Emily Shuckburgh

March 9, 2007



Most of the eddy heat is

transported poleward in the top

200m by diabatic eddies

global high

resolution model

How ?

ECCO project



Oceanic

mesoscale

Rapid

atmospheric

synoptic scale

A snapshot reveals 2

different scales

Qnet, daily mean

May 5th 2003

Surface heat flux

modulation by eddies



Jan, 27th 2006

CLIMODE Experiment

Air-sea fluxes mesured by cruises

(here from the Atlantis, WHOI)

MicroWave SST

NCEP v,T,q, bulk formulae

Qnet



Schematic of theprocess studied

T+ T-

Eddies advect

anomalous

warm/coldwater

Cooling

which are

stronglydamped

leading tolateral

eddy flux

How can the lateral eddy flux be

estimated and what are its implications ?



1. Model description and method used

2. Eddy fluxes

3. Eddy diffusivities

4. Role of air-sea fluxes

5. In the Southern Ocean

Results from a global high resolution model

MITgcm.org



1/5

Model description

• Global, 1/8x1/8 degrees, lat/lon grid

• 50 vertical levels with KPP mixed layer

• Atmospheric boundary layer with NCEP v,T,q

• bulk formulae allow the ocean-atmosphere interactions

• 3 years of simulation from 2001 to 2003



U = U−U

sc

−[U]

T = T − T sc − [T ]

Q

net= Qnet −Qsc

net− [Qnet]

1/5

Method

∂ T

∂ t + v ·T =

Qnet

ρoC poH

Extract the eddy part

into the temperature

equation

leads to the T variance equation:

D

DtT 2

2

+ [v

T

]·[T ] =

1

ρoC poH [Q

netT

]

Eddy temperature

flux Modulation of Q by T’



2/5

Temperature eddy flux

Poleward

Surface

intensified

Localized on the

turbulent path

of the GS

[UT

] [T ]and



2/5

Air-sea eddy heat flux

Q’ and T’ of

opposite sign

Systematic

damping on the

turbulent path of the GS

[Q

netT ] [T ]and



K airsea =−1

ρC poH

[Q

netT ]

[T ] ·[T ]K

eddy = −[v

T ][T ]

[T ] ·[T ]

3/5

Eddy diffusivity

D

Dt

T 2

2

+ [vT ] ·[T ] =

1

ρoC poH [Q

netT ]

Eddy temperature flux,

direct computation of K:Q modulation

by T’

[vT ] = −K ·[T ]



3/5

Diffusivity from

air-sea eddy flux

High values

south of the Gulf Stream,

low values in the core

and

log10105

103

0

m2 /sK

air

sea[T ]



3/5

Surface diffusivity from

eddy temperature flux

log10

High values

south of the Gulf Stream,

low values in the core

105

103

0

m2 /s

But we didn’t removed the

rotational part of [v’T’]

...

A useful insight is obtained whenaveraging along isotherms

andK eddy [T ]



K eddy(z, T ) =

T

−[vT ][T ]

[T ] ·[T ]dS

Surface enhancement of Keddy

Mixed Layer

Depth

K eddy

increases

3/5

Diffusivity “section” from




Keddy(z) for various T:

6, 10, 14, 18oC

and all T class average

3/5

Diffusivity profiles from


K eddy strongly increases

at the surface

K eddy(z, T i) =

T

−[vT ][T ]

[T ] ·[T ]dS



K eddyinterior =

H

−400

K eddy

K eddymld =

0

H

K eddy

K air-sea is almost half

the K eddy in the mixed layer

4/5

Role of air-sea heat fluxes

Modulation

of Q by T’:

K air-sea

K eddy in the

interior:

small scales

processes:everywhere

K eddy in the

mixed layer:

South North



∂ q

∂ t+ v ·q = k2q

No damping

Marshall et al, JPO 20065/5

In the Southern Ocean

q: t=0

q: t=1y

Eddy diffusivity

from tracer analysis

K eff

after 1year

trigger of the

K eddy in the interior

previously shown



Qnet −λ(T − T ∗)

With damping5/5


∂ q

∂ t+ v ·q = k2q − λ(q − q∗)

new, damping of the

tracer...

representing the air-sea heat fluxinteractions:



With damping5/5


∂ q

∂ t+ v

·q = k2q − λ(q − q∗)

K eff

Damping time

scale (months)

Once again:

the eddy diffusivity

is enhancedby eddy air-sea

interactions



Conclusion

• All over the ocean, most of the meridional eddy heat transport is achieved in the top 200m

by mesoscale eddies

• The net air-sea heat flux is strongly modulated at the eddy scale, as seen in a high

resolution model and in observations (CLIMODE)

• Induced damping of eddies triggers an additional poleward heat transfert which has beenanalyzed through coefficients of diffusion

• Eddy diffusivity is shown to be strongly intensified at the surface

• Half of the surface diffusivity is attributed to eddy air-sea damping

• This is also found in the Southern Ocean from tracer analysis

• This process is believed to occur all over places of strong eddy activity and then may have

implications for example in:

‣ eddy parametrisation in coarse resolution models

‣ mechanisms with key role of air-sea fluxes, especially water mass formation (CLIMODE)



Thank you



Global:

Air-sea eddy heat flux

[Q’net T’]



Q



[Q

netT ]

ρC poH = −λ[T 2]

Qnet

ρC poH = −λ(T − T ∗)

1

λ= −ρC poH

[T 2]

[Q

netT ]

Dissipation time scale

GS core:

< 50 days

Lower part:

>100 days

mixed layer lateral eddy fluxes mediated by air-sea interactions

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