How extreme was northern hemisphere seasonality during the Younger Dryas?

Abstract

In explaining the rapid transitions associated with the Younger Dryas cooling, a reduced meridional overturning circulation has traditionally been invoked, but such a scenario has been difficult to reproduce in model studies without adding excessive amounts of freshwater to the North Atlantic. More recent studies challenge this view and indicate that the role of an extensive sea ice cover may have been as important in promoting abrupt climate change as reorganisations of the North Atlantic Ocean [Gildor, H., Tziperman, E., 2003. Sea-ice switches and abrupt climate change. Philosophical Transactions Of The Royal Society Of London Series A—Mathematical Physical And Engineering Sciences 361, 1935–1942]. Based on glacier evidence from eastern Greenland [Denton, G.H., Alley, R.B., Comer, G.C., Broecker, W.S., 2005. The role of seasonality in abrupt climate change. Quaternary Science Reviews 24, 1159–1182] suggest that the seasonal temperature amplitude increased by about 20 °C during the Younger Dryas. Such a ‘switching of seasonality’ lends support to the idea of a fast-expanding sea ice cover, because it allows for extremely cold winters that are balanced by relatively mild summers. However, climatic interpretations based on the geometry and length of glaciers under such conditions as in Scoresby Sund is not well understood, and equilibrium-line-altitude (ELA) estimates should, therefore, be regarded as tentative. Here, we discuss the absolute seasonal amplitude during the Younger Dryas by taking winter precipitation into account and show that the changes in seasonality may have been limited to 10 °C. We propose that by reducing the seasonal response in Greenland compared with western Europe we better understand the hinged-door modus operandi [COHMAP, 1988. Climatic changes of the last 18,000 years: observations and model simulations. Science 241, 1043–1052] of the polar front and sea-ice cover, where the absolute southward migration of sea-ice is highest in the East Atlantic region.