A New Role for the AMOC During the Ice Ages

Abstract

The Atlantic's Meridional Overturning Circulation (AMOC) collapsed during the last glacial termination, and the collapse led to warmings in both hemispheres. It is not clear, however, how the warmings occurred. The warmings are attributed here to a component of the AMOC that has received relatively little attention—the upshifts of the AMOC's return flow. The upshifts occur off Namibia and Peru, where the upshifted water is very cool. The cool water is warmed up as it spreads across the ocean basins, and the warmed-up water is then drawn away to the North Atlantic. The net result is a cooling of the tropics. The cooling would be eliminated when the AMOC collapses, and the demise of the cooling would allow the tropics and both hemispheres to warm together. The AMOC is also shown to keep the tropics and both hemispheres relatively cool when the summer insolation over the Northern Hemisphere is at a minimum in the 23-kyr precessional band. The cooling is then weakened as the summer insolation increases. Thus, cooling gives way to warming in much the same way that it did during the last glacial termination. These findings lead to a new description of how warming and cooling occurred during the ice ages.