Water Mass Transformation in the Cape Cauldron

The Cape Basin is a turbulent region off the west coast of South Africa where warm and salty Indian Ocean waters mix with cooler, fresher South Atlantic waters. This injection of heat and salt is known as Agulhas leakage and has been tied to the Atlantic Meridional Overturning Circulation’s (AMOC) strength, stability, and variability. Paleoclimate data and model hindcasts point to an increase in Agulhas leakage as the climate warms, but quantifying real-world leakage is very difficult owing to its turbulent nature. This is further complicated by the basin’s four dynamical regions: eddies, eddy-eddy interactions, topography, and filamentation which can influence and transform the water masses through the water column. Their properties and circulation are critical to understand because they can influence where and how this leakage is transported into the South Atlantic. Using data from the ARGO float array, we identified six water masses in the region using neutral density and calculated their heat and salt transports along the leakage corridor.  Tropical Surface Waters (TSW, <25.5), Subtropical Surface Waters (STSW, 25.5-26), South Atlantic Subtropical Mode Waters (SASTMW, 26-26.5), and the Upper North Atlantic Deep Waters (UNADW, 27.92-28.08) have a combined heat and salt transport of 6.85x10-11 PW and 11.8 kg/s respectively into the leakage corridor. South Indian Mode Waters (SICW, 26.5-27) and the Intermediate Waters (IW, 27-27.92) have a combined heat and salt transport of 3.36x10-11 PW and 47.6 kg/s respectively through the leakage corridor. As a result, approximately 30% of heat and 80% of salt are transported into the South Atlantic primarily from the SICW and the IW masses. Previous studies have primarily focused on the intermediate water masses as the contributor, yet our results show that Mode Waters can have a significant impact on Agulhas leakage’s transport and variability. To improve our understanding of Agulhas leakage and its impact on the AMOC, we must turn to improving our understanding on the basin’s seasonal variability and its potential mode water formation.