NORTH-ATLANTIC AND EUROPEAN SEASONAL PREDICTABILITY USING AN ENSEMBLEOF MULTIDECADAL ATMOSPHERIC GCM SIMULATIONS

An initial study of seasonal predictability for the North Atlantic and European regions is presented, using an ensemble of six integrations of the Hadley Centre atmospheric climate model (HADAM1) for the period 1949-1993. The model was forced by the Hadley Centre's observed globa l sea-ice and sea-surface temperature data set (GISST). The model repr oduces with considerable skill the patterns of many of the main modes of mean sea-level pressure (MSLP) variance over this region, including the North Atlantic Oscillation (NAO). For MSLP, the most predictable modelled seasons over the North Atlantic-European sector as a whole ar e spring and winter (although random variability is still substantial) , and for the NAO specifically, the model shows significant skill from winter to spring. At this time of year the observed NAO is related to SST patterns mainly in the North Atlantic, and in years of extreme At lantic SST anomalies, reasonably skilful statistical simulations of th e sign of the NAO anomaly can be made. For the UK, the highest simulat ion skill is for temperature, which we hypothesize is related directly to local SST anomalies. The effects of ENSO have also been investigat ed. Over the North Atlantic, the model produces a strong 500 hPa heigh t response to extreme El Nino events in winter and spring, similar to that observed, whereas over Europe influences are weak. The response t o La Nina is very weak in both observed and simulated data. (C) Britis h Crown Copyright 1997.