Contrasting responses to hemispheric forcing govern the evolution of the CO2-induced equatorial Pacific warming pattern

Understanding how greenhouse gases (GHGs) perturb the tropical Pacific warming pattern is crucial due to its impact on circulation and the hydrological cycle. A global CO2 increase is known to initially induce cooling in the tropical Pacific, particularly in the eastern basin, which gradually evolves into equatorially peaked warming amplified in the eastern basin. To disentangle the mechanisms driving the evolution of the CO2-induced equatorial Pacific warming pattern, we construct large-ensemble climate model experiments with CO2 increases confined to discretized latitudinal bands. On a fast timescale (years 1-3), Northern Hemisphere off-equatorial forcing (NH_OFFEQ) induces the basin-wide equatorial Pacific cooling due to intensified trade easterlies associated with a northward ITCZ shift. Local equatorial forcing (EQ) drives eastern equatorial Pacific cooling through enhanced climatological upwelling. In contrast, Southern Hemisphere off-equatorial forcing (SH_OFFEQ) leads to basin-wide equatorial Pacific warming, with an amplified response in the eastern basin due to the weakening of the southern subtropical ocean cell (STC). The effect of NH_OFFEQ and EQ forcing on equatorial Pacific SST changes diminishes over time due to dynamical ocean adjustments. Consequently, the fast strengthening of the zonal SST gradient induced by NH_OFFEQ and EQ forcing transitions into a weakening driven by SH_OFF forcing, which becomes dominant during the slow response. Our results suggest that the recent cooling in the eastern tropical Pacific could be part of the CO2-driven fast response.