Ocean Surface heat fluxes
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Transcript of Ocean Surface heat fluxes
Ocean Surface heat fluxes
Lisan Yu Robert A. Weller
Department of Physical Oceanography Woods Hole Oceanographic Institution
NOAA Climate Observation Division 5th Annual System Review
Silver Spring, Maryland. June 5-7, 2007
water vapor & latent heat Precipitation Longwave Sensible heat
Solar
Ocean heat fluxes: a key climate variable
Salinity Temperature
Evaporation
Energy budget: Net heat flux = Solar – Longwave – Latent heatLatent heat – Sensible heat
Water cycle: Freshwater flux = Precipitation – Evaporation
Estimating ocean heat fluxes
Latent heat flux: QLH = Le ce
U ( qs – qa )
Sensible heat flux: QSH = cp cp
U ( Ts – Ta )
• There are NO direct measurements of global surface heat fluxes.• There are direct measurements for most air-sea variables.• Fluxes are computed from bulk parameterizations.
Uqa, Ta
qs, Tsqa and Ta are not available from satellites, but are provided by NWP reanalyses.
Objectively Analyzed air-sea Fluxes (OAFlux)
website: http://oaflux.whoi.edu/
• OAFlux products are computed from COARE 3.0 bulk flux parameterization with surface meteorology determined from an objective analysis of satellites and atmosphere reanalyses (ECMWF, NCEP).
• Data currently available for the years 1958-2006 on global 1 grid:
– latent heat
– sensible heat
– evaporation
– wind speed
– near surface air humidity
– near surface air temperature
– sea surface temperature
Mean and Differences: 2006 versus 2005
Mean 2006
Difference: 2006 - 2005
Yearly averaged ocean heat fluxes and SST:
1958 - 2006LHF+SHF
LHF
SHF
SST
Evaporation increases in concert with the rise of SST
The Clausius-Clapeyron equation
Linear trends in latent heat flux and SST: 1981-2006
Heat fluxes are nonlinear functions of SST:
QLH ~ U ( qs(TS) –
qa )
QSH ~ U ( Ts – Ta )
Decadal variations of latent heat fluxdepicted by model and ship based products
Except for ERA40, all flux products show an increase of latent heat loss since the early 1980s.
Products
OAFlux Sat+NWP
NCEP NWP
ERA40 NWP
COADS (da Silva)
Ship
NOC Ship
Decadal variations of short- and longwave radiation
ProductsISCCP
(Rossow et al.)Satellit
e
GEWEX-SRB(Stackhouse et
al.)
Satellite
NCEP NWP
ERA40 NWP
COADS (da Silva)
Ship
NOC Ship
Mean Net Radiation 1984-2004: Differences are large
All components
No QLW
x No QLW and QSW
Locations of in situ measurements
Mean Diff(Product – Buoy)OAFlux+ISCCPERA40NCEP1NCEP2
LHF
SHF
SW
LW
NET
Product - buoy comparison
Importance of long-term flux buoys: quantify bias in NWP fluxes
Fluxes comparison statistic based on daily means---------------------------------------------------------------------- QNET QLH+QSH QSW+QLW
----------------------------------------------------------------------Buoys 50 -110 160OAFlux&ISCCP 54 ( +4) -113 ( -3) 168 ( +8)NCEP1 -14 (-64) -144 (-34) 130 (-30)ERA40 47 ( -3) -124 (-14) 171 (+11)----------------------------------------------------------------------
Stratus buoy (693 days, 10/08/00 to 08/31/02 )
NWP fluxes are poorly compared with buoys,but how can they achieve a global balance?
Products Qnet (Wm-2) Qsw+lw | Qlh+sh
NWP NCEP 3.6 119.2 | -115.6
NWP ERA40 6.1 126.1 | -120.0
Ship COADS (da Silva)
28.5 132.2 | -103.7
Ship NOC 30.5 130.3 | - 99.8
Sat+NWP OAFlux+ISCCP 30.9 138.1 | -107.2
Averages over global ocean grid points
Correction made on the NCEP net heat flux by data assimilated ECCO models
Differences (ECCO – NCEP)
NCEP Mean Qnet
NCEP overestimates the ocean heat gain at high latitudes, while underestimate it in mid and low latitudes.
Does NCEP have a right global heat balance?
Item 8: “Surface fluxes”, Ocean Reanalysis Evaluation.ECMWF, Aug 31 – Sep 1, 2006
Differences between the 9 annual-mean fields
The surface fluxes from the ocean models are dependent upon the model’s ability to resolve the ocean frontal dynamics.
Summary
• Does NCEP or ERA40 global heat balance represent the right balance? – Unknown.
• Has the global ocean evaporation been increasing since the 1980s? – It is depicted by all products except ERA40, but the magnitude of increase is uncertain.
• How has the global radiation (short plus longwave) been changing during the same period?
– Uncertain.• Long-term sustained flux observations, particularly in extra tropical regions, are highly needed
– to reduce bias in flux products – to provide accurate reference for accessing long-term climate change in air-sea exchanges – to reconcile differences between ocean reanalyses, atmospheric reanalyses, and global flux analyses – to, eventually, achieve and understand the global heat balance