Last Glacial Maximum Ocean Circulation Simulated by PMIP3 Climate Models

Ocean circulation during the Last Glacial Maximum (LGM; 21,000 before present) may have been much different from today as suggested by various geochemical proxy records, with possible implications for climate and the carbon cycle. The Paleoclimate Modeling Intercomparion Project 2 (PMIP2) showed that climate models produced widely varying results concerning the Atlantic Meridional Overturning Circulation (AMOC) strength and structure. Here we analyze data from new model simulations that have recently been performed as part of PMIP3. In contrast to PMIP2, the five models that have so far been analyzed all show a strengthening in the flow of the North Atlantic Deep Water current. The models are less consistent with respect to the Antarctic Bottom Water in the Atlantic, however; three models simulate an increase whereas two simulate a decrease. Inflow of Circumpolar Deep Water into the Indian and Pacific oceans is either increased or not changed much. The barotropic circulation does not change much in the models except in the North Atlantic, where the subtropical gyre is stronger in all models. The Antarctic Circumpolar Current is enhanced in one model but not in the others. We explore different mechanisms that have been proposed to control the MOC, such as changes in winds, sea ice, and surface buoyancy fluxes in order to understand the reasons for the model responses.