Mechanisms Governing the Development of the North Atlantic Warming Hole in the CESM-LE Future Climate Simulations

A warming deficit in North Atlantic sea surface temperatures is a striking feature in global climate model future projections. This North Atlantic warming hole has been related to a slowing of the Atlantic meridional overturning circulation (AMOC); however, the detailed mechanisms involved in its generation remain an open question. An analysis of the Community Earth System Model Large Ensemble simulations is conducted to obtain further insight into the development of the warming hole and its relationship to the AMOC. It is shown that increasing freshwater fluxes through the Arctic gates lead to surface freshening and reduced Labrador Sea deep convection, which in turn act to cool Labrador Sea sea surface temperatures. Furthermore, the resulting changes in surface ocean circulation lead to enhanced transport of cooled Labrador Sea surface waters into the interior of the subpolar gyre and a more zonal orientation of the North Atlantic Current. As a result, there is an increase in ocean advective heat flux divergence within the center of the subpolar gyre, causing this warming deficit in North Atlantic sea surface temperatures. These local changes to the ocean circulation affect the AMOC and lead to its slowdown.