Estimated strength of the Atlantic overturning circulation during the last deglaciation

Abstract

The Atlantic meridional overturning circulation affects the latitudinal distribution of heat, and is a key component of the climate system. Proxy reconstructions, based on sedimentary 231 Pa= 230 Th ratios and the difference between surface- and deep-water radiocarbon ages, indicate that during the last glacial period, the overturning circulation was reduced during millennial-scale periods of cooling 1‐5 . However, much debate exists over the robustness of these proxies 6‐8 . Here we combine proxy reconstructions of sea surface and air temperatures and a global climate model to quantitatively estimate changes in the strength of the Atlantic meridional overturning circulation during the last glacial period. We find that, relative to the Last Glacial Maximum, the overturning circulation was reduced by approximately 14 Sv during the cold Heinrich event 1. During the Younger Dryas cold event, the overturning circulation was reduced by approximately 12 Sv, relative to the preceding warm interval. These changes are consistent with qualitative estimates of the overturning circulation from sedimentary 231 Pa= 230 Th ratios. In addition, we find that the strength of the overturning circulation during the Last Glacial Maximum and the Holocene epoch are indistinguishable within the uncertainty of the reconstruction. The Atlantic meridional overturning circulation (AMOC) is an important feature of the Earth’s climate system. It is characterized by northward-flowing water masses at the surface ocean, deepwater formation in the North Atlantic and southward-flowing water masses in the intermediate to deep ocean. This overturning cell is complemented by an underlying, generally weaker, reversed overturning cell that originates in the Southern Ocean. In the following, we use AMOC to refer only to the upper overturning cell. Owing to the large heat capacity of water, the northwardflowing near-surface waters contribute effectively to the heat transport from the tropics to the mid- and high latitudes and therefore affect the regional climate around the North Atlantic 9,10 . Hence, changes of the AMOC strength can substantially influence climate in the north, especially in Europe. The depth of the southward-flowing waters and the strength of the circulation have changed in the past. During the last glacial period, abrupt warming events of several degrees within a few decades followed by slower cooling were detected in Greenland ice cores 11 . As effects of the so-called DansgaardOeschger events were found far beyond the Greenland ice sheet 1214 with patterns reminiscent of varying inter-hemispheric heat distribution 15,16 , they are believed to be the result of rapid changes of the AMOC strength 17 . Other examples of a reduction or even a complete shutdown of the