A fast atmospheric General Circulation Model is used to generate three ensembles of atmospheric circulation. While the first ensemble is forced by global sea surface temperature (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. In the Southern Hemisphere the extratropically forced signal reaches the strength of the tropically forced one on decadal time scales. 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 Ni¿o.