AGCM Internal Variability and the Responses to an Extratropical SST Anomaly

The January and February responses of a GCM to an imposed extratropical Pacific SST anomaly are compared with the patterns of the model's internal variability. Relevant patterns of internal variability are diagnosed from ensembles of model control runs by regressing monthly-mean geopotentials and temperatures against low-level temperatures in the vicinity of the SST anomaly and by EOF analysis. These patterns are found to play a significant role in determining the local and the remote responses to the SST anomaly. The GCM response may be considered as comprising a direct linear response to low-level heating, that is local to the forcing and baroclinic, and an eddy-forced component that closely resembles patterns of the model's internal variability - it is equivalent-barotropic and extends over the entire hemisphere. The results suggest that in order for a warm SST anomaly over the Kuroshio Extension to induce an equivalent-barotropic ridge immediately east of the anomaly, the internal variability must have a well-defined center of action over the central Pacific. In our GCM this is nearly true in February but not in January. Similar analyses of the observed flow suggest that the natural variability in both January and February should favor the development of an equivalent barotropic ridge in response to a warm SST anomaly.