Nonmonotonic Future Changes in the North Atlantic Warming Hole under a Fast CO2 Emission Scenario

Abstract 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-yr Community Earth System Model, version 2 (CESM2), experiments prescribed with different carbon dioxide (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 nonmonotonic behavior in the fast-warming scenario, characterized by initial cooling followed by strong 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. The former cooling effect overwhelms the latter warming in the begining, but the cooling fades away after about 50 years as the 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 possibly due to the continuous NAWH forcing on the deep convection. Significance Statement This paper investigates the future responses of the North Atlantic warming hole (NAWH) to different warming rates based on ideal Community Earth System Model, version 2 (CESM2), experiments. We find that the subpolar North Atlantic (SPNA) sea surface temperature (SST) is projected to cool first and warm later in the fast carbon dioxide (CO 2 ) emission scenario, in contrast to other mitigated experiments. This is due to an alternation between two chain reactions triggered by the NAWH-induced surface heat flux changes through wind speed: one is cooling by reduced salinity and vertical mixing, which works only at the early stage before deep convection collapse, and the other is warming caused by decreased oceanic heat loss. Our results provide an avenue to better understand the future change of NAWH in a warming world.