More positive North Atlantic Oscillation cools the subpolar North Atlantic in the past century

Abstract

Abstract Sea surface temperature (SST) in the subpolar North Atlantic has significantly decreased at a rate of -0.39 (\(\pm 0.23\)) K/century during 1900–2020, which runs counter to global warming due to anthropogenic forcing. The cooling in the subpolar North Atlantic, known as the North Atlantic cold blob, could be driven by a host of mechanisms involving both the ocean and atmosphere. Here, we present evidence that changes in the atmospheric circulation over the North Atlantic, in particular a centennial trend towards a more positive phase of the North Atlantic Oscillation (NAO), could have contributed to the cold blob. The positive NAO intensifies the surface wind over the subpolar North Atlantic and induces excessive heat loss from the air-sea interface. According to an idealized mixed layer heat balance model, the NAO induced heat loss alone cools the subpolar North Atlantic by 0.26 K/century, which explains 67% of the observed cold blob SST trend. Thus, besides ocean circulation, including the slowdown of the Atlantic Meridional Overturning Circulation, the large-scale atmospheric circulation might have played an equally important role in prompting the century-long SST changes in the subpolar North Atlantic.