The relative roles of tropical and extratropical forcing on atmospheric variability

A fast atmospheric General Circulation Model is used to generate three ensembles of atmospheric circulation. While the first ensemble is forced by global SST anomalies, the forcing is confined to the tropics and extratropics, respectively, in the other two ensembles. The tropical SST anomalies have the largest impact on atmospheric variability. However, also the extratropical SST has a systematic effect on the atmosphere, especially in the Southern Hemisphere, where the extra-tropically forced signal reaches the strength of the tropically forced one. The atmospheric response to global SST anomalies can be understood as a nearly linear superposition of the reactions to tropical and extratropical forcing, respectively, or, in a different view, as a superposition of a thermodynamic response to the global-mean SST anomaly and a dynamic response to local SST anomalies, the most important being El Nino.