Contrasting the Evolution of the Tropical Pacific SST Responses to Time-Invariant Extratropical Forcings in the Two Hemispheres

Abstract

We compare the equatorial Pacific sea surface temperature (SST) responses to radiative forcings in the extratropics of each hemispehre, with heating or cooling imposed in either hemisphere using a fully coupled climate model. In the initial 3 years, the equatorial SST responses exhibit an opposite sign to the forcings in the northern extratropics but align with those in the southern extratropics. At this stage, heating the northern extratropics is more effective at cooling the equatorial Pacific than cooling the southern extratropics. This occurs because the anomalous warming in the northern extratropics is blocked by the rainband and can only enter the equatorial Pacific from the west, triggering Bjerknes feedback more effectively. Over a decade, all experiments show enhanced equatorial responses aligning with the signs of the forcings. The south-perturbed cases experience stronger equatorial SST responses, suggesting the significant control of the southern extratropics on tropical Pacific on decadal timescales.