SOUTHERN OCEAN SEA-ICE CONTROL OF THE GLACIAL NORTH ATLANTIC THERMOHALINE CIRCULATION

The National Center for Atmospheric Research-Community Climate System Model (NCAR-CCSM) is used in a coupled atmosphere-ocean-sea-ice simulation of the Last Glacial Maximum (LGM, around 21,000 years ago) climate. In the tropics, the simulation shows a moderate cooling of 3C over land and 2C in the ocean in zonal average. This cooling is about 1C cooler than the CLIMAP sea surface temperatures but consistent with recent estimates of both land and sea surface temperature changes. Subtropical waters are cooled by 2-2.5C, also in agreement with recent estimates. The simulated oceanic thermohaline circulation (THC) at the LGM is not only shallower but also weaker than the modern with a migration of deep-water formation site in the North Atlantic as suggested by the paleoceanographic evidences. This glacial North Atlantic THC changes are caused by the enhanced Antarctic Bottom Water (AABW) formation and an accompanying increase of oceanic vertical stability in the Atlantic Ocean, which is triggered by sea-ice change and an associated haline density flux increase in the Southern Ocean. From the coupled climate perspective, the Southern Ocean control of the glacial North Atlantic THC is caused by the stronger sea-ice sensitivity to the glacial climate forcing in the Southern Ocean than in the North Atlantic, triggered by the enhanced equatorward sea-ice transport by increased westerlies in the Southern Ocean at the LGM.