Investigating Changes in Southern Ocean Circulation through the Use of Transient Tracers

The Southern Ocean is a region characterised by the upwelling of deep water and subsequent formation of new water masses such as Antarctic Bottom Water (AABW) and Antarctic Intermediate Water (AAIW). These subducting water masses are of particular interest to the climate system due to their capacity to remove heat and carbon from the atmosphere and store it in the deep ocean. The large-scale circulation of the region is controlled by buoyancy forcing and the strength of the Southern Hemisphere westerly winds. As models suggest a strengthening of the winds in a warming climate, it is important to improve our understanding of how this circulation may change, and the effect this will have on the oceanic uptake of heat and carbon from the atmosphere. This study uses ship-based measurements of CFCs, SF₆, nutrients and other tracers from repeats of 3 hydrographic sections in the Southern Ocean and South Atlantic. We build surface time histories of these tracers from global datasets and apply a Maximum Entropy Method to the data to estimate where and when water was last ventilated. The produced Green’s function then allows for the theoretical propagation of properties such as anthropogenic CO₂, whose signal is hard to separate from background levels of natural CO₂, into the ocean interior. By using this method, we obtain estimates of present-day uptake and circulation patterns, providing a basis for improved future predictions of how uptake of heat and carbon will change in coming decades.