Relationships between Atmospheric Internal Variability and the Responses to an Extratropical SST Anomaly

The responses of a GCM to an imposed extratropical 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 a Pacific SST anomaly. Different responses to the SST anomaly in the GCM's January and February climates are largely explained by the differences in the regression patterns and in the leading EOF. The results suggest that in order for a warm SST anomaly over the Kuroshio Extension to induce an equivalent-barotropic ridge the internal variability must have a well-defined center over the Pacific. In our GCM this is largely true in February but not in January. Similar analyses are performed for the observed flow to determine the patterns of variability in nature and thereby to suggest the potential response to SST forcing. The natural variability in January and February has a strong large-scale center over the Pacific, which, according to our model results, should favor the development of an equivalent barotropic ridge in response to a warm SST anomaly.