On the North Atlantic Warming Hole

Abstract The North Atlantic Warming Hole (WH) is examined using the Norwegian Earth System model. The WH is a distinctive cool region in sea surface temperature anomaly, lying south of Iceland. It is present now, and intensifies in the climate simulation examined here. Previous studies have suggested the feature reflects declining oceanic heat transport, or increased surface cooling due to changes in the atmospheric circulation. In the present simulations, the primary driver instead is an influx of freshwater from high latitudes. The freshwater flows into the subpolar gyre and subsequently pools in the Icelandic basin. The structure cools in the interior, and as such the WH has only a weak signature in density. Rather, it has a strong "spice" anomaly, indicating density compensation. The turbulent fluxes indicate that eddy restratification acts to counteract buoyancy production from the freshwater, reducing the lateral density gradients. Thus the WH is fundamentally an advective feature in the ocean and could not be generated by atmospheric processes alone. Furthermore, the WH is not the direct result of reduced heat transport into the subpolar gyre. Rather, the freshwater simultaneously weakens the Atlantic Meridional Overturning Circulation (AMOC), so that AMOC declines in concert with the cooling of the NAWH. The findings underscore the significant role of freshwater fluxes from high latitudes, from melting Arctic sea ice and increased runoff, in altering the regional and global ocean circulation.