Mechanisms Determining the Atmospheric Response to Midlatitude SST Anomalies

Previous GCM experiments demonstrated that a model atmosphere produces two different responses to a midlatitude warm SST anomaly over the Pacific under perpetual January and February conditions. To elucidate the mechanisms responsible for the different GCM responses and their dependence on the background flow, experiments with two idealized models are conducted. Experiments with a linear baroclinic model reveal that the GCM responses at equilibrium are primarily maintained by the anomalous eddy forcing. The anomalous flow induced directly by an idealized initial heat source exhibits little sensitivity to the background flow. Eddy feedbacks on the heating-induced anomalous flow are examined using a linear storm track model. The anomalous eddy forcing produced by the storm track model is sensitive to the basic state. The eddy forcing in January acts to shift the heating-induced upper-level ridge toward the northeast of the Gulf of Alaska, while in February it acts to reinforce the ridge. This suggests that the differences in the GCM responses are primarily associated with differences in the response of synoptic eddies to the presence of an anomalous ridge at the end of the Pacific storm track. The idealized model experiments are also performed with the observed winter mean flow. The eddy feedbacks depend on the position of the heating relative to the storm track. With the heating centered over the western Pacific the eddy-driven anomalous flow reinforces the ridge over the Pacific, similar to that in GCM February, but much stronger. No such reinforcement by the transients is found with the heating shifted over the eastern Pacific. These results suggest that SST anomalies over the western Pacific perhaps play a more active role in midlatitude atmosphere–ocean interactions.