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The North Atlantic subpolar gyre (SPG) plays a crucial role in determining the regional ocean surface temperature (SST), which has profound implications on the surrounding continental and coastal climate. Here, we analyze the Max Planck Institute-Grand Ensemble global warming experiments and show that the SPG can evolve in two distinct phases under continuous global warming. In the first phase, as the global mean surface temperature approaches 2-K warming, the eastern SPG intensifies in combination with a weakening Atlantic meridional overturning circulation (AMOC), accompanied by a cooling of subpolar North Atlantic SST, known as the warming hole. The associated oceanic fingerprint matches with the observations over the last 15 years, where an intensification and cooling of the eastern SPG is related to salinity reduction at the eastern side of the SPG. However, for further warming beyond 2 K, in spite of a continuous decline in the AMOC, a northward shift of the mean zonal wind extends the subtropical gyre northward with an associated disruption of the eastern SPG intensification, resulting in the cessation of the warming hole. Therefore, a shift from the initially dominating oceanic drivers to the atmospheric driver results into a two-phase evolution of the North Atlantic Ocean SPG circulation and the associated SST under continuous global warming. [ABSTRACT FROM AUTHOR]

To examine the atmospheric responses toArctic sea ice variability in the Northern Hemisphere cold season (from October to the following March), this study uses a coordinated set of large-ensemble experiments of nine atmospheric general circulationmodels (AGCMs) forcedwith observed daily varying sea ice, sea surface temperature, and radiative forcings prescribed during the 1979-2014 period, together with a parallel set of experiments where Arctic sea ice is substituted by its climatology. The simulations of the former set reproduce the near-surface temperature trends in reanalysis data, with similar amplitude, and their multimodel ensemble mean (MMEM) shows decreasing sea level pressure over much of the polar cap and Eurasia in boreal autumn. The MMEM difference between the two experiments allows isolating the effects of Arctic sea ice loss, which explain a large portion of theArctic warming trends in the lower troposphere and drive a small but statistically significant weakening of the wintertime Arctic Oscillation. The observed interannual covariability between sea ice extent in the Barents-Kara Seas and lagged atmospheric circulation is distinguished from the effects of confounding factors based on multiple regression, and quantitatively compared to the covariability in MMEMs. The interannual sea ice decline followed by a negative North AtlanticOscillation-like anomaly found in observations is also seen in the MMEM differences, with consistent spatial structure but much smaller amplitude. This result suggests that the sea ice impacts on trends and interannual atmospheric variability simulated byAGCMs could be underestimated, but caution is needed because internal atmospheric variability may have affected the observed relationship. [ABSTRACT FROM AUTHOR]