INTERNAL VERSUS SST-FORCED ATMOSPHERIC VARIABILITY AS SIMULATED BY ANATMOSPHERIC GENERAL-CIRCULATION MODEL

The variability of atmospheric flow is analyzed by separating it into an internal part due to atmospheric dynamics only and an external (or forced) part due to the variability of sea surface temperature forcing . The two modes of variability are identified by performing an ensembl e of seven independent long-term simulations of the atmospheric respon se to observed SST (1970-1988) with the LMD atmospheric general circul ation model. The forced variability is defined from the analysis of th e ensemble mean and the internal variability from the analysis of devi ations from the ensemble mean. Emphasis is put on interannual variabil ity of sea level pressure and 500-hPa geopotential height for the Nort hern Hemisphere winter. In view of the large systematic errors related to the relatively small number of realizations, unbiased variance est imators have been developed. Although statistical significance is not reached in some extratropical regions, large significant extratropical responses are found at the North Pacific-Alaska sector for SLP and ov er western Canada and the Aleutians for 500-hPa geopotential height. T he influence of SST variations on internal variability is also examine d by using a 7-year simulation using the climatological SST seasonal c ycle. It is found that interannual SST changes strongly influence the geographical distribution of internal variability; in particular, it t ends to increase it over oceans. Patterns of internal and external var iability of the 500-hPa geopotential height are further examined by us ing EOF decompositions, showing that the model realistically simulates the leading observed variability modes. The geographical structure of internal variability patterns is found to be similar to that of total variability, although similar modes tend to evolve rather differently in time. The zonally symmetric seesaw dominates the internal variabil ity for both observed and climatologically prescribed SST. The Pacific -North American (PNA) and Western Pacific (WP) patterns, on the other hand, are the dominant modes associated with patterns of SST variabili ty; the latter is related to Atlantic anomalies, while the former resp onds to both El Nino events and extratropical forcing.