A comparative study of observed and GCM-simulated turbulent surface fluxes at the positions of Atlantic weatherships

Abstract

If oceanic models are to be driven with transient atmospheric input, data from standard daily analyses or from an atmospheric GCM simulation can be used. The question arises whether these “FF-data” ( FF = field forcing ) are appropriate to be used as a realistic oceanic forcing in the form of wind stress and turbulent heat fluxes. A series of different FF-data sets is compared with respective “LF-data” ( LF = local forcing ) derived from in situ weathership observations. We believe these LF data to be the most accurate and reliable long year maritime time series. The study is restricted to 8 Atlantic weatherships and January conditions and to fields obtained with the Hamburg University GCM or derived from analyses of the German Weather Service (DWD). It turns out that DWD based FF data sets are suitable only if long year mean values are required. In general, the interannual and synoptic scale variability is too small for all FF data sets. With respect to the windstress, the empirical formulae to obtain the surface wind (from the sea level pressure field) together with the usage of a windspread dependent drag coefficient yield the best though still unsatisfying results. The approach using generalized similarity theory gives worse results with respect to the synoptic scale and interannual variability. The GCM simulated data set is systematically biased over wide regions which is partly due to a shift in the model's quasistationary Icelandic Low and an increased temperature at the model's lowest level. The transients are simulated at some positions even poorer than those analysed by the DWD, but at other positions superior though still weaker than the LF data's.