Simulated Southern Ocean Upwelling at the Last Glacial Maximum and Early Deglaciation: The Role of Eddy-Induced Overturning Circulation

We explore the changes in Southern Ocean upwelling patterns during the last glacial maximum (LGM) and early deglaciation using a fully coupled climate model with ocean eddies parameterized via the Gent-McWilliams scheme. Relative to the preindustrial climate, the model simulates an equatorward shift of the upwelling of the residual circulation in the Southern Ocean during the LGM, which is consistent with paleo-evidence of export production. This change in upwelling primarily arose from an altered eddy-induced meridional overturning circulation (MOC) that resulted from the expanded sea ice and enhanced buoyancy loss around Antarctica during the LGM, which modulated the slope of buoyancy surface in the Southern Ocean. We have also found that there was a poleward shift of the upwelling of the residual circulation in the Southern Ocean during the early deglaciation, which was different than the change in the wind-driven upwelling and primarily driven by variations in the eddy-induced MOC. Plain Language Summary Upwelling in the Southern Ocean exerts a huge influence on the climate and oceanic ecosystem. This upwelling is set up by a balance between the surface westerly winds, surface buoyancy forcing, and eddy activity. However, the role of eddy activity in the Southern Ocean upwelling patterns during the last glacial maximum (LGM) and last deglaciation remains unclear. In this study, we explore the changes in Southern Ocean upwelling during the LGM and early deglaciation using a fully coupled climate model. The model simulates an equatorward shift of the upwelling of the residual circulation in the Southern Ocean during the LGM relative to preindustrial climate, which is consistent with changes in upwelling inferred from paleo-evidence. A poleward shift of the upwelling of the residual circulation in the Southern Ocean was also found to occur during the early deglaciation. The change in the residual upwelling cannot be explained by the changes in wind-driven upwelling but was mainly driven by variations in the eddy-induced vertical velocity field. Key Points A climate model simulates an equatorward shift in the Southern Ocean upwelling at last glacial maximum relative to preindustrial climate The coupled model also simulates a poleward shift in the Southern Ocean upwelling during the early deglaciation The change in upwelling primarily arose from an altered vertical velocity field that was eddy-induced rather than wind-driven