The SOC Global Air-Sea Heat and Momentum Flux Climatology

climatological net heat flux

Reliable estimates of the air-sea fluxes of heat and momentum are vital to improve our understanding of the coupled ocean-atmosphere system. A new global air-sea heat and momentum flux climatology has recently been developed at the Southampton Oceanography Centre. In generating the climatology, estimates of the fluxes have been obtained from in situ reports within the COADS 1a (Comprehensive Ocean Atmosphere Dataset 1a), a global dataset containing of order 30 million surface observations from ships and buoys collected over the period 1980 - 93. The major difference between the new climatology and previous ones based on in situ data is that the reported meteorological variables used to calculate the fluxes have been corrected for various observational biases using additional information about measurement procedures which has been blended in from the WMO47 list of ships. By doing this we hope to reduce the errors in the derived flux fields.

climatological annual mean wind stress field

The climatological annual mean net heat flux and wind stress fields that we obtain from our analysis are shown above. Heat gain is concentrated in the tropics, where shortwave heating dominates, while the major heat loss regions are over the western boundary currents, which are regions of strong latent and sensible heat loss, and the high latitudes. The principal feature in the wind stress field is the zone of intense stress, of order 0.1 N/m^2 on average, encircling the globe in the Southern Ocean with secondary features being the enhanced wind stresses under the main westerly flows in the North Atlantic and North Pacific and under the Trade Wind Belts.

It should be noted that these are preliminary results and that much analysis remains to be done. The use of in situ reports alone has the disadvantage of poor sampling in regions such as the Southern Ocean and South Pacific and the challenge ahead will be to combine satellite and model estimates of the fluxes with the in situ values to improve the quality of the fields in such areas.

The climatological monthly mean fields are available here as a dataset, which includes downloadable data files and a data viewer. We encourage users to inform us by email if they transfer the files giving a contact address and a short summary of research interest. By doing this we will be able to inform you of future modifications/updates to the fields.

It is important to note that the quality of the fields has a strong spatial dependence which reflects the global distribution of ship observations. Quality is likely to be high in the well sampled North Atlantic & North Pacific but to decrease in the Southern Hemisphere. In particular, south of 40 S the errors in the fields are likely to be large and we recognise the existence of spurious features which have been generated during the objective analysis of the original raw fields. We stress that caution must be taken when interpreting the fields in this region.

In addition, note that the current version of the fields does not give closure of the global heat budget, the imbalance being a global mean net heat gain by the ocean of 29 W/m^2. Ongoing work is being carried out to identify regions in which we believe the net heat gain has been overestimated. Results from several regional comparisons against high quality meteorological buoy data indicate that in those regions for which comparisons have been possible the SOC net heat flux estimates agree well with independent buoy measurements. Hence, we have not applied global adjustments to the heat flux components in order to balance the heat budget at this stage of our analysis.

Results from the analysis and evaluation of the climatological fields are currently being written up as a journal paper. A comprehensive description of the field characteristics will be given in a forthcoming climatological atlas. For discussion of an earlier version of the climatology see:

Josey, S.A., E.C.Kent, D.Oakley and P.K.Taylor, 1996: A new global air-sea heat and momentum flux climatology International WOCE Newsletter, 24, p.3 - 5.

the forthcoming paper:

Josey, S. A., E. C. Kent and P. K. Taylor, 1998 : Ocean - atmosphere heat exchange : a global climatology derived from in situ meteorological observations. Journal of Climate, in preparation.

If you have queries or require further information about the fields contact Simon Josey.

Funding has been received from the Hadley Centre, UK Meteorological Office for the production and analysis of this dataset. Thanks to Peter Gleckler, LLNI for preparation of the netCDF version.