mixed layer lateral eddy fluxes mediated by air-sea interactions
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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
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Most of the eddy heat is
transported poleward in the top
200m by diabatic eddies
global high
resolution model
How ?
ECCO project
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Oceanic
mesoscale
Rapid
atmospheric
synoptic scale
A snapshot reveals 2
different scales
Qnet, daily mean
May 5th 2003
Surface heat flux
modulation by eddies
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8/14/2019 Mixed Layer lateral eddy fluxes mediated by air-sea interactions
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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
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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 ?
8/14/2019 Mixed Layer lateral eddy fluxes mediated by air-sea interactions
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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
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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
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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’
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2/5
Temperature eddy flux
Poleward
Surface
intensified
Localized on the
turbulent path
of the GS
[UT
] [T ]and
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Air-sea eddy heat flux
Q’ and T’ of
opposite sign
Systematic
damping on the
turbulent path of the GS
[Q
netT ] [T ]and
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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 ]
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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 ]
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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 ]
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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
eddy temperature flux
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Keddy(z) for various T:
6, 10, 14, 18oC
and all T class average
3/5
Diffusivity profiles from
eddy temperature flux
K eddy strongly increases
at the surface
K eddy(z, T i) =
T
−[vT ][T ]
[T ] ·[T ]dS
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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
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∂ 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
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Qnet −λ(T − T ∗)
With damping5/5
In the Southern Ocean
∂ q
∂ t+ v ·q = k2q − λ(q − q∗)
new, damping of the
tracer...
representing the air-sea heat fluxinteractions:
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With damping5/5
In the Southern Ocean
∂ 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
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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)
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Thank you
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Global:
Air-sea eddy heat flux
[Q’net T’]
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Q
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[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