ocean surface heat fluxes lisan yu robert a. weller department of physical oceanography woods hole...

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