Roles of the wind and buoyancy forcing in the Southern Ocean upwelling during the last deglacial

Based on the TraCE-21000 simulation, roles of the wind and buoyancy forcing have been investigated in the Southern Ocean upwelling during the last deglacial. In the zonal integral over the Antarctic Circumpolar Current (ACC) region, the wind-driven Ekman pumping induces an upwelling flow within the unblocked layers, which decreases from the Last Glacial Maximum (LGM) to the early Holocene, with an abrupt and short-term rebounding during BØlling-AllerØd (BA). As the topography interrupts the circumpolar path, the buoyancy forcing comes to work in the blocked gyres. Fresh water discharge from the sea-ice melting around the Antarctic reduces the deep ocean stratification, which makes the upwelling flow extend deeply during the last deglaical. With the combination of the wind and buoyancy forcing, the depth-integrated upwelling has slight changes during the last 22,000 years, except a sudden increase at BA. On the other hand, the local upwelling in the downstream of the submarine ridges shows a significant enhancement during the last deglacial, which is consistent with previous results from the proxy records. From the wind forcing, the local Ekman pumping induces an either decreasing or increasing upwelling flow in the Ekman layer. While below the Ekman layer, vertical flows are greatly modified by the planetary vorticity advection, in order to satisfy the vorticity conservation. Due to the topography steering, southward flows are generated in the downstream of the submarine ridges to induce a negative planetary advection on each level, which leads to a vertical stretching of the water column and undermines the upwelled motions in the deep layers. However, the southward flows and their associated planetary vorticity advection has greatly decreased during the deglacial, as a result of reduced baroclinity by the buoyancy forcing. Therefore, the local depth-integrated upwelling has a robust increase in its magnitude during the deglacial transition.