Contributions to SST Anomalies in
the Atlantic Ocean
[Ocean Control of Air-Sea Heat Fluxes]
Kathie Kelly
Suzanne Dickinson and LuAnne Thompson
University of Washington, Seattle WA USA
SST Science Team, Seattle, WA 2013
Applied Physics Laboratory
University of Washington
Motivation
Want to understand origins of low frequency
(climate-related) ocean anomalies
Kelly et al 2013
Atlantic Multidecadal Oscillation
• 1st EOF of SST
• Tripole
structure
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Contributions to SST
•
•
•
•
Surface heating
Vertical penetration
Upwelling & mixing
Horizontal advection/diffusion
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Mixed Layer Heat Budget
Mixed layer temperature (here interchangeable with SST):
¶T Qnet
2
hp
=
- ( hpu ) ·ÑT - (T - T- ) we + hpKÑ T
¶t rc p
Problems:
• Heat penetration hp not mixed layer depth
• Time varying hp not known
• Advection difficult to compute
• Temperature below mixed layer not known
• Entrainment velocity unknown
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Estimate Mixed Layer Heat Budget
• compute climatogical penetration depth by
regression with net surface flux (Qnet)
• ML advection from surface currents
• also Ekman layer advection from wind stress
• residual of ML heat budget includes vertical
contributions and errors
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Estimate Penetration Depth
Regress:
¶T
Q
hc
=r
¶t
rc p
Then
hp = rhc
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Penetration Depth Ratios
Heat penetration
deeper than MLD
around 20N or 20S
Shallower in center
of gyres
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Compare Advection Estimates
Total from OSCAR currents and SST
Ekman from wind stress and SST
- ( hpu ) ·ÑT
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- ( hpu ) ·ÑT
( )
- UE ·ÑT
Mixed Layer Heat Budget
40N, 25W
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Relative Term Sizes
Advection too
large particularly
in energetic
current regions
- ( hpu ) ·ÑT
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Examine Heat Budget in Tripole Regions
• north
• middle
• south
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ML Heat Budget by Region
north
middle
south
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SST Versus Heat Content
• SST used in most climate studies (bulk flux formulas)
• SST decorrelates seasonally, but heat content does not
(Deser et al., 2003)
• Interannual air-sea interaction depends on heat
content (Kelly et al, 2010)
T1
T2
T1
T2
H
EOFs of SST and Heat Content
First mode of SST and heat content similar
Heat content from SSH has only 20-year record
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Water Column Heat Budget
¶H Qnet
2
=
- ò u ·ÑT dz + ò KÑ T dz
¶t rc p
where
H º ò T dz
• No vertical terms
• Advection/diffusion estimated as a residual
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Compare Heat Budgets
Mixed layer
¶T Qnet
2
hp
=
- ( hpu ) ·ÑT - (T - T- ) we + hpKÑ T
¶t rc p
Total column
¶H Qnet
=
- ò u ·ÑT dz + ò KÑ2T dz
¶t rc p
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Heat Content from Altimetric Sea Level
Assuming sea level anomalies are primarily from heat content
h
H = rc p
a
where η is sea surface height anomaly
α coefficient of thermal expansion
ρ density of sea water
cp heat capacity of sea water
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Contributions to Altimetric Sea Level
h
H = rc p
a
thermosteric sea level
altimetric sea level
mass & salinity
Kelly et al, J. Climate, accepted
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Column Heat Budget by AMO Region
north
middle
south
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Meridional Heat Advection
Meridional
advection
heat loss in
subtropics
Kelly et al 2013, J. Climate,
accepted
Gulf Stream
regions 2 &
3
Summary
• Mixed layer budget complicated (vertical
distribution of heat in water column)
• Mixed layer has large advection terms and large
residual (vertical or errors?)
• Column heat content budget simple: surface
heating vs. advection
• EOF 1 (AMO) similar for SST and SSH
AMO dominant term is:
ML: vertical distribution or errors?
Column: advection
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Take Home
• SST has longest record for computing oceanrelated climate indices
• Easier to determine contributions to heat
content than to SST
• Heat content dominates air-sea interaction on
longer (climate) time scales
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