Mechanisms for a spatial fingerprint of Younger Dryas warming in the southeastern United States

Abstract:

The Younger Dryas was an abrupt climate event bringing cold conditions to much of the Northern Hemisphere during the last deglaciation, occurring ca. 12,900 to 11,700 years ago, that is often attributed to a freshwater induced reduction of the Atlantic Meridional Overturning Circulation (AMOC). Lacustrine fossil pollen and branched glycerol dialkyl glycerol tetraether temperature reconstructions from eastern North America demonstrate that sites north of ~35°N cool at the Younger Dryas onset, while sites south of this latitude demonstrate no cooling, with sites in Florida warming. We analyze three climate models to assess possible atmospheric and oceanic mechanisms for this spatial fingerprint. Each of the three atmosphere-ocean general circulation models (Geophysical Fluid Dynamics Laboratory's CM2.0 at coarse resolution, Hadley Centre Coupled Model V3, and Community Climate System Model V3) were forced with differing amounts of freshwater into the North Atlantic and simulate a reduction in the AMOC, cooling much of the Northern Hemisphere except eastern North America where all models demonstrate warming of varying magnitude and spatial extent. In all three models, reduced AMOC is associated with an increased pressure gradient between the Icelandic Low and Azores High, much like modern winters with a large positive North Atlantic Oscillation index. This enhanced pressure gradient produces a more zonal configuration of the midlatitudinal jet stream which drives southerly winds over the Gulf of Mexico into the southeastern United States. These winds advect warm and moist air into the southeastern United States, consistent with hydrological proxies that reconstruct increased precipitation in Florida. The HadCM3 and GFDL CM2Mc models also simulate an increase in the strength of the subtropical ocean cell, which may have further intensified warming in Florida. Hence, these simulations suggest two distinct and complementary mechanisms that, in the southeastern United States, offset the Younger Dryas cooling reported elsewhere in the Northern Hemisphere and contributed to the observed warming in Florida.