Evaluation of the roles of thermal, freshwater, and wind forcing in the Atlantic Meridional Overturning Circulation during the Last Glacial Maximum

Abstract. We evaluated the relative impacts of thermal, freshwater, and wind forcing on the Atlantic Meridional Overturning Circulation (AMOC) during the Last Glacial Maximum (LGM) by performing ocean general circulation model simulations with surface boundary-condition anomalies obtained from 17 PMIP models. Thermal conditions of the LGM weakened the AMOC, while wind stress changes strengthened it, whereas freshwater flux had a relatively minor effect. The cooling over the Southern Ocean weakened the AMOC by increasing the density of Antarctic Bottom Water (AABW), and differences in Southern Ocean cooling among the PMIP models can contribute to differences in simulated AMOC strength. In contrast, cooling over the North Atlantic had two opposing effects: sea-ice expansion reduced air–sea heat exchange and weakened the AMOC, whereas enhanced heat loss in the open ocean strengthened it. The LGM wind stress anomaly strengthened the AMOC through changes in sea-ice transport and wind-driven ocean circulation, and its effect depended on the thermal background state. These results demonstrate that differences in surface boundary conditions among PMIP models can lead to substantial differences in the simulated AMOC through both thermal and dynamical processes.