Atmospheric response to modified CLIMAP ocean boundary conditions during the Last Glacial Maximum

Global climate simulations are conducted with the National Center for Atmospheric Research (NCAR) Community Climate Model version 3 (CCM3) to examine the sensitivity of the Last Glacial Maximum (LGM) climate to prescribed sea surface temperatures (SSTs) that are modified from the CLIMAP (Climate: Long-range Investigation, Mapping, and Prediction) study. Based on the consensus from various LGM proxy data, the SSTs are cooled by 4oC uniformly in the Tropics (30oN(30oS) relative to CLIMAP and the high latitude sea ice extent is reduced. The modified LGM SSTs cause significant opposing changes in the synoptic and regional scale atmospheric circulation, which are most pronounced in the winter hemisphere. For instance, there is significant weakening of the midlatitude circulation and reduction of 500-hPa eddy kinetic energy and midlatitude precipitation resulting from the decreased meridional temperature gradient in the modified SST simulation. In contrast, reduced sea ice extent during the boreal winter causes increased regional baroclinicity and enhanced atmospheric circulation in the western North Pacific and the North Atlantic. Cooled tropical SSTs also increase the land/ocean temperature contrast, which strengthens the Asian summer monsoon circulation. Both LGM simulations produce enhanced low-level convergence and increased precipitation along the South Pacific Convergence Zone (SPCZ) relative to present day despite the cooler LGM climate. The SPCZ orientation and intensity are closely linked to the distribution of South Pacific SSTs. Comparison of surface temperature estimates from land and ocean-based proxy data with model output indicates that uniform cooling of the tropical SSTs and modification of the high latitude sea ice extent may be sufficient to accurately simulate first order characteristics of the LGM climate.