Title: Recent Changes in Southern Ocean Circulation: The Role of Ocean Heat Uptake
Abstract:
The ocean plays a crucial role in influencing the Earth’s climate by absorbing and redistributing large amounts of heat, freshwater, and carbon, with over 90% of excess heat stored in its depths. The Southern Ocean, defined as the ocean south of about 30˚S, has taken up a substantial amount of anthropogenic heat, particularly north of the Subantarctic Front (SAF) of the Antarctic Circumpolar Current (ACC). Utilizing the lengthening satellite altimetry and Argo float records, we find that the Southern Ocean eastward flow had accelerated significantly between 48˚S and 58˚S. This acceleration can be reproduced in a hierarchy of climate models, including an ocean-eddy-resolving model, and has been confirmed to be of anthropogenic origin.
Previous studies of changing Southern Ocean circulation have focused mostly on the response to changes in westerly winds, which are shifting poleward and intensifying. We find that anthropogenic ocean warming is the dominant driver, as large (small) heat gain in the downwelling (upwelling) regime north (south) of the SAF causes zonal acceleration on the northern flank of the ACC and adjacent subtropics due to increased baroclinicity. Notably, in the Drake Passage, little warming occurs and the associated transport remains largely unchanged. These results also highlight the critical role of ocean heat uptake in shaping future circulation patterns in the Southern Ocean. Given the continuing role of the Southern Ocean in ocean heat uptake and equatorward heat transport, we expect the zonal acceleration to continue and even increase in the future, facilitating property exchange between ocean sectors and increasing ventilation of the subtropical thermocline as water moves along the accelerating SAF and enters the gyres.