AN EVALUATION OF ECMWF-BASED CLIMATOLOGICAL WIND STRESS-FIELDS

A new mean monthly wind stress climatology based on seven years (1980- 1986) of operational weather analyses by the European Centre for Mediu m-Range Weather Forecasts (ECMWF) has been derived by Trenberth et al. This climatology (referred to here as the TLO climatology) potentiall y represents a significant improvement over climatologies derived only from conventional wind observations. An attempt is made here to quant ify the absolute accuracy of the TLO climatology by comparison with gl obal wind stress fields constructed from vector winds measured by the Seasat-A Satellite Scatterometer (SASS) during 1978. From a simulated SASS dataset, it is shown that the magnitudes of the SASS stresses mus t be increased by about 7% to account for a systematic error that can be attributed to the scatterometer spatial- and temporal-sampling char acteristics. After applying this correction, differences between the T LO climatology and SASS winds in the tropics are most likely related t o known limitations of the ECMWF analyses. At latitudes south of 50-de grees-S, interannual variability and uncertainties in the operational weather analyses are so large that it is not possible to evaluate the TLO climatology on the basis of comparisons with SASS data. Outside of these equatorial and high southern latitude bands, the TLO stresses a re shown to be systematically stronger than SASS by almost 50%. It is found that this difference can be entirely accounted for if the 1980-1 986 ECMWF 1000-mb analyses are not interpreted as 10-m winds, as they were in constructing the TLO climatology. This conclusion is supported by an independent comparison of the synoptic ECMWF wind speed estimat es with coincident buoy observations.