1. 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

2. Ocean heat fluxes: a key climate variable
Solar
water vapor &
latent heat
Precipitation
Longwave
Sensible heat
Evaporation
Salinity
Temperature
Energy budget:
Net heat flux = Solar – Longwave – Latent heat – Sensible heat
Water cycle:
Freshwater flux = Precipitation – Evaporation

3. Estimating ocean heat fluxes
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.
Latent heat flux: QLH =  Le ce U ( qs – qa )
Sensible heat flux: QSH =  cp cp U ( Ts – Ta )
qa, Ta U
qa and Ta are not available from satellites, but are provided by NWP reanalyses.
qs, Ts

4. Objectively Analyzed air-sea Fluxes (OAFlux) website: http://oaflux
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 on global 1 grid:
– latent heat
– sensible heat
– evaporation
– wind speed
– near surface air humidity
– near surface air temperature
– sea surface temperature

5. Mean and Differences: 2006 versus 2005

6. Yearly averaged ocean heat fluxes and SST: 1958 - 2006
LHF+SHF
LHF
SHF
Evaporation increases in concert with the rise of SST
SST
The Clausius-Clapeyron equation

7. 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 )

8. Decadal variations of latent heat flux depicted by model and ship based products
OAFlux
Sat+NWP
NCEP
NWP
ERA40
COADS
(da Silva)
Ship
NOC
Except for ERA40, all flux products show an increase of latent heat loss since the early 1980s.

9. Decadal variations of short- and longwave radiation
Products
ISCCP
(Rossow et al.)
Satellite
GEWEX-SRB
(Stackhouse et al.)
NCEP
NWP
ERA40
COADS
(da Silva)
Ship
NOC

10. Mean Net Radiation 1984-2004: Differences are large

11. Locations of in situ measurements
All components
No QLW
x No QLW and QSW

12. Mean Diff Product - buoy comparison LHF SHF SW LW (Product – Buoy) NET
OAFlux+ISCCP
ERA40
NCEP1
NCEP2
NET

13. Importance of long-term flux buoys: quantify bias in NWP fluxes
Stratus buoy (693 days, 10/08/00 to 08/31/02 )
Fluxes comparison statistic based on daily means
QNET QLH+QSH QSW+QLW
Buoys
OAFlux&ISCCP ( +4) ( -3) ( +8)
NCEP (-64) (-34) (-30)
ERA ( -3) (-14) (+11)

14. Averages over global ocean grid points
NWP fluxes are poorly compared with buoys, but how can they achieve a global balance?
Averages over global ocean grid points
Products
Qnet (Wm-2)
Qsw+lw | Qlh+sh
NWP
NCEP
3.6
119.2 |
ERA40
6.1
126.1 |
Ship
COADS (da Silva)
28.5
132.2 |
NOC
30.5
130.3 |
Sat+NWP
OAFlux+ISCCP
30.9
138.1 |

16. 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

17. 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.

18. Summary Long-term sustained flux observations, particularly
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