Non-monotonic future changes in the North Atlantic warming hole under a fast CO2 emission scenario

Despite global warming, observations and climate models have demonstrated a relative surface cooling in the subpolar North Atlantic (SPNA), a phenomenon known as the North Atlantic warming hole (NAWH). Here, based on a series of 100-year Community Earth System Model version 2 (CESM2) experiments prescribed with different CO 2 emission rates, we find that the NAWH shows distinct future responses to different global warming rates. While the SPNA sea surface temperature (SST) generally exhibits a cooling trend in the slow- and normal-warming experiments, it displays a distinct non-monotonic behavior in the fast-warming scenario, characterized by initial cooling followed by warming. In response to the initial NAWH, a shallow atmospheric anticyclone develops over the SPNA, weakening the local surface wind speed and thus reducing the surface heat fluxes. On the one hand, this reduced evaporation lowers oceanic salinity, then weakens deep convection in the Labrador and Irminger Seas, and finally intensifies the NAWH. On the other hand, the weakened heat fluxes reduce oceanic heat loss, prompting local SST warming. In general, the former cooling effect overwhelms the latter warming, but the cooling fades away after about 50 years as the weakening deep convection in the fast-warming scenario collapses permanently, and the thermodynamic warming effect then becomes dominant. We further suggest that the internal atmospheric processes may affect the SST cooling process during the early stage. The ensembles with an atmospheric cyclonic trend over the SPNA tend to have a stationary SST initially but show an abrupt SST cooling later due to the continuous NAWH forcing on the deep convection.