Assessing AMOC over the Last 40,000 Years from Seawater Density Gradients

Abstract:

It has long been postulated that cold periods in the North Atlantic on glacial-interglacial and millennial time scales are associated with a weakening in the Atlantic Meridional Overturning Circulation and the associate heat transport into the region. Here I will review the work we have done to reconstruct seawater density gradients, which are related to flow via the thermal wind relation. We find support for the idea that the Last Glacial Maximum, and the Younger Dryas deglacial cold interval are associated with a weaker AMOC. However, the case seems more complicated for Heinrich events, the surges of icebergs into the North Atlantic Ocean that punctuated the last glacial period. While freshwater from the melting icebergs is thought to have interrupted the Atlantic meridional overturning circulation, we infer a substantial reduction of flow during Heinrich Event 1, but little change during Heinrich Events 2 and 3, which occurred during an especially cold phase of the last glacial period. It may be that the freshwater forcing had little additional effect because glacial circulation was already weakened before the onset of Heinrich Events 2 and 3.