The Younger Dryas (YD) represented a rapid return to glacial conditions which occurred between 12.9 ka and 11.7ka. The rapid entrance into the YD has been conclusively demonstrated to have been caused by a release of meltwater from proglacial lakes into the Arctic Ocean through the Mackenzie River outlet. This freshwater was transported to sites of North Atlantic Deep Water (NADW) formation where it caused a pronounced slowdown of the Atlantic Meridional Overturning Circulation (AMOC). The recovery from the YD led to a series of climate variations known as the Preboreal Oscillation (PBO). This event was equally rapid but is as yet unexplained. Previous attempts to model the YD have all had various inaccuracies. These include using incorrect boundary conditions, releasing volumes of freshwater that violate eustatic sea level (ESL) constraints, and failing to couple the ocean model to an appropriate atmospheric model. Using the University of Toronto version of CCSM4, we model the YD using appropriate boundary conditions, as well as a volume of freshwater forcing that is compatible with not only the ESL constraints but also the much more stringent constraints imposed by estimates of the volume of proglacial lakes. We find that the approximately 1000-year duration of the YD arises naturally. Equally important, we recover the rapid return to deglacial conditions as well as a pronounced overshoot of AMOC strength, which could potentially lead to further climate fluctuations that characterize the PBO. Equally important, is the observation that the recovery from the YD occurs in precisely the same way as it has been argued that the transition from stadial to interstadial conditions occurs during Dansgaard-Oeschger events: intense vertical mixing below the sea ice causes a massive polynya to open in the Irminger Sea. This allows for a dramatic exchange in heat between the ocean and the atmosphere, which rapidly warms the Northern Hemisphere.
Host: Christian DiMaria