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

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

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

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 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

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

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 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.

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

Mean Net Radiation 1984-2004: Differences are large

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

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

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 50 -110 160 OAFlux&ISCCP 54 ( +4) -113 ( -3) 168 ( +8) NCEP1 -14 (-64) -144 (-34) 130 (-30) ERA40 47 ( -3) -124 (-14) 171 (+11)

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 | -115.6 ERA40 6.1 126.1 | -120.0 Ship COADS (da Silva) 28.5 132.2 | -103.7 NOC 30.5 130.3 | - 99.8 Sat+NWP OAFlux+ISCCP 30.9 138.1 | -107.2

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 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