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

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. Plain Language Summary The center of the Hadley circulation is north of the equator and the oceanic subtropical cell in the Southern Hemisphere is stronger than that in the north. These cause an uneven response in equatorial Pacific sea surface temperatures (SST) to forcings from the Northern and the Southern Hemispheres. We use a set of idealized experiments to compare the tropical Pacific responses to radiative forcings from the extratropics of the two hemispheres. Our findings show that the northern extratropical region has a stronger impact on the equatorial Pacific via a teleconnection pathway influenced by air‐sea fluxes. The northward‐displaced climatological rainband blocks the anomalous SST from the northern extratropics and forces the anomalies entering the tropical Pacific region from the west. This reinforces the meridional gradient in the eastern Pacific and the zonal gradient over the equatorial central Pacific, leading to stronger atmospheric circulation and SST responses. In contrast, the southern extratropics is efficiently connected to the equatorial Pacific region via the oceanic circulation for the delayed responses. The mechanisms discussed in our study are relevant for interpreting the cooling trend in the equatorial Pacific in recent decades and for predicting the increasingly important control from the Southern Ocean. Key Points Transient responses of tropical Pacific are sensitive to the hemisphere in which the extratropic