Nonstationarity of the Atlantic Meridional Overturning Circulation's Fingerprint on Sea Surface Temperature

Sea surface temperature (SST) has been increasing since industrialization with rising greenhouse gases. However, a warming hole exists in the North Atlantic where SST has cooled by 0.4 K/century during 1900-2017. It has been argued that this cooling is due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), and subpolar North Atlantic SST has thus been utilized to estimate AMOC variability. We assess the robustness of subpolar North Atlantic SST as a proxy for AMOC strength under historical forcing, abrupt quadrupling of CO2, and a medium future emissions pathway, finding that AMOC's fingerprint on SST depends upon forcing scenarios. AMOC is important in warming hole development during significant warming periods, although SST may introduce uncertainties for AMOC reconstruction in stabilized regimes due to diverse forcing mechanisms and decadal variability. Our results caution against using SST alone as a proxy for AMOC variability-both on paleoclimatic and contemporary time scales. As greenhouse gas concentrations increase, the global average sea surface temperature (SST) has risen in response at a rate of 1.4 K/Century since industrialization; however, there exists an area in the North Atlantic decreasing in SST, the North Atlantic Warming Hole (NAWH). Some have argued that the cooling is due to a slowdown of the Atlantic Meridional Overturning Circulation (AMOC), a global circulation current that transports warm equatorial surface water northward. We analyze forcing scenarios using a global climate model under three differing forcing mechanisms, one similar to the real world, one where CO2 initially starts at four times pre-industrial levels and then held constant, and one based on a medium level future emissions scenario. From this, we study the quality of North Atlantic SST as a proxy for AMOC strength. Our study finds that the impact of the AMOC on SST changes between scenarios, and thus we believe that North Atlantic SST may introduce uncertainty into paleoclimatic studies attempting to reconstruct AMOC change. Additionally, we found that a slowdown of the AMOC plays an important role in the cooling of the North Atlantic when AMOC is slowing rapidly (or "shutting down"), although has a more minimal impact in less extreme scenarios. Sea surface temperature based fingerprint of Atlantic Meridional Overturning Circulation (AMOC) changes under different forcing scenarios AMOC in steep decline significantly contributes to North Atlantic warming hole formation, largely due to cloud radiative forcing AMOC plays a smaller role in North Atlantic SST variability in historical period and after CO2 stabilization than during significant warming