How Does the Atmosphere Respond to Extratropical SST Anomalies? - the Role of Transient Eddy Feedbacks

Abstract
Much progress has been made over the past decade on the dynamics determining the atmospheric responses to extratropical SST anomalies. Most importantly, GCM studies suggest that transient eddy feedbacks play a determining role in defining the extratropical SST-induced responses. Hence, the nature of the simulated responses often depends sensitively on background intrinsic variability associated with different seasons and models.

The role of eddy feedbacks is examined here in particular in determining the NAO response to the North Atlantic SST tripole. Large (100-member) ensembles of GCM experiments reveal that the SST tripole induces a significant NAO response in late winter (Feb-Apr) when the model's intrinsic variability projects strongly on the NAO. The tripole-induced NAO response exhibits also an asymmetry about the sign of the SST anomaly, with a weaker response in the positive case than in the negative case. Mechanisms for developing and maintaining the GCM responses are elucidated through diagnostic experiments using a linear baroclinic model and a statistical storm track model based on GCM intrinsic variability.

The NAO-like symmetric response is primarily maintained by a dipolar anomalous eddy forcing that results from interactions between the heating-forced anomalous flow and the Atlantic storm track, as expected from an eddy-feedback mechanism. To account for the asymmetry of the responses about the sign of the SST tripole, a nonlinear eddy-feedback mechanism is considered. The model results demonstrate that, due to its nonlinear self-interaction, the tripole heating induces a weaker response in the positive phase than in the negative phase. Interactions of these nonlinear heating-forced anomalous flows with the Atlantic storm track result in symmetric eddy vorticity forcings that in turn sustain asymmetric eddy-forced anomalous flows in the two cases.

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