Distinct Mechanisms for Subpolar North Atlantic Surface Climate Change and Roles of Internal Variability under Different External Forcings

The North Atlantic climate exhibits considerable low-frequency variability that involves internal and/or external mechanisms. In this study, we investigate the impacts of external forcings and internal variability on long-term sea surface temperature (SST) changes in the subpolar North Atlantic (SPNA) using the Community Earth System Model version 2 (CESM2) Single Forcing Large Ensembles where only the forcing of interest is time-evolving. To understand the physical processes responsible for the annual-mean SST trends, we apply a temperature change decomposition framework derived from surface heat balance equations. We find that in response to historical greenhouse gas (GHG) forcing, the SPNA SST on average has significantly decreased by 0.63 K/century, primarily due to changes in radiative processes, especially enhanced shortwave cloud radiative forcing (SW CRF) associated with more cloudiness and increased cloud albedo. Moreover, these changes in radiative processes can be explained by the Atlantic Meridional Overturning Circulation (AMOC) weakening, which on average is -3.27 Sv/century. The weakened AMOC also leads to a reduction in oceanic heat transport convergence and a substantial cooling in the SPNA, but the cooling is eventually efficiently damped by surface turbulent heat flux feedback, resulting in little SST change. In contrast, the AMOC has significantly strengthened in response to the 20th-century anthropogenic aerosol (AAER) forcing, which warms the SPNA, mostly by increasing oceanic heat transport convergence into the region and partially by SW CRF feedback. Notably, although the magnitude of the AAER-induced AMOC strengthening is twice that of the GHG-induced AMOC weakening, the magnitude of AAER-induced SPNA warming is only half of that of the GHG-induced SPNA cooling, indicating that processes independent of AMOC might exist to offset AMOC’s impact under AAER forcing. In addition to external forcing, internal variability also plays an important role in driving long-term SST changes. Specifically, the SPNA SST trend shows a standard deviation (STD) of 0.82 K/century across the GHG ensemble, which mostly originates from radiative processes, i.e., SW CRF and surface clear-sky longwave irradiance, while the spread induced by oceanic heat transport convergence is offset by that from surface turbulent heat fluxes. Moreover, about 75% of the spread in this radiative feedback can be explained by the AMOC trend (STD=0.94 Sv/century). However, the ensemble spread in the SPNA SST trend is much smaller under AAER forcing and is poorly explained by the AMOC trend. Our findings suggest that the dominant mechanisms for SPNA SST changes are forcing-dependent, but internal variability can have notable impacts under GHG forcing.