Unstable Atlantic Meridional Overturning Circulation during Glacial Intervals and Millennial Variability: The Role of Mean Sea Ice Extent

Abstract:

A striking feature of paleoclimate records is the strong irregular variability with an approximately 1500 yr period, known as the Dansgaard-Oeschger (D-O) events, which punctuate the last glacial interval but disappear during the Holocene. Many theories, modeling and data suggest that these events, seen as abrupt warming episodes in Greenland ice cores and sea surface temperature records in the North Atlantic, are linked to reorganizations of the Atlantic Meridional Overturning Circulation (AMOC). In this study, using a new low-order ocean model, we are able to reproduce a realistic power spectrum of this millennial variability, which emerges in the model as a result of unstable AMOC dynamics rather than due to external freshwater forcing. Within this model we explore differences in the AMOC stability between glacial and interglacial intervals of the 100 kyr glacial cycle of the Late Pleistocene. Previous modeling studies show that the edge of sea ice in the North Atlantic shifts southward during glacial intervals, moving the region of the North Atlantic Deep Water formation and the entire AMOC also southward. Here we demonstrate that, by altering the precipitation structure that the AMOC feels, such an expansion of sea ice cover makes the system unstable, which explains chaotic millennial variability during the glacials and the persistence of stable ocean conditions during the interglacials.