Changes in Southern Ocean Circulation Across Climate Transitions and Their Impacts on Carbon and Heat Fluxes.

Today, the Southern Ocean (SO) is the primary sink of anthropogenic heat and carbon, yet anthropogenic warming and ozone depletion have altered its circulation and stratification, creating uncertainty about the persistence of this sink. Evidence from the last glacial period and deglaciation shows that the SO played a central role in past climate transitions by releasing heat and carbon to the atmosphere through sea-ice retreat and enhanced ventilation of deep and abyssal waters. Antarctic warming during weak Atlantic Meridional Overturning Circulation (AMOC) phases was likely amplified by SO feedbacks, such as increased deep-ocean convection, and a strengthening/poleward shift of Southern Hemisphere westerlies. However, changes in the abyssal circulation, its interaction with North Atlantic Deep Water, and its impact on oceanic carbon during the deglaciation remain debated. Finally, although sediment and modeling evidence suggests that Antarctic Ice Sheet (AIS) discharge can significantly affect climate and biogeochemistry, clear signals remain scarce in existing proxy records-either masked by dominant AMOC-driven variability or unresolved due to limited temporal resolution. Improved model representation and proxy records are needed to clarify the role of abyssal circulation and the interaction between the ocean and AIS.