The importance of tropical sea surface temperature patterns in simulations of last glacial maximum climate

Abstract

Prescribed SST experiments are performed using the National Center for Atmospheric Research's Community Climate Model version 3 general circulation model to isolate the contribution of the tropical SSTs reconstructed by the Climate, Long-range Investigation, Mapping and Prediction study (CLIMAP) to the modeled global atmospheric circulation anomalies at the Last Glacial Maximum (LGM). The changes in tropical SST patterns cause changes in tropical convection that force large (>300 m in 500-mb geopotential height) changes in Northern Hemisphere wintertime circulation. These midlatitude circulation changes occur despite the small (1°C) change in the mean tropical SST between the present and the CLIMAP reconstruction. In fact, the midlatitude circulation changes due to the difference in the tropical SST pattern between the present and the CLIMAP reconstruction are greater than the circulation changes due to a uniform tropical SST cooling of 3°C or those due to the presence of the LGM ice sheets. The circulation anomalies due to the change in tropical SST patterns result in a wintertime warming (cooling) of 8°C (8°C) over the Laurentide (Fennoscandian) ice sheet and a decrease (increase) in annual mass balance of over 1000 mm yr-1 (800 mm yr-1) along the southern margin of the ice sheet. These results demonstrate that detailed knowledge of tropical SST patterns is needed in order to produce reliable simulations of LGM climate. In the appendix, it is shown that the lion's share of the midlatitude circulation changes are due to SST gradients in the northern Tropics, and that the physics involved in the teleconnection between tropical SST forcing and midlatitude circulation changes is rich and highly nonlinear.