Origin of the Antarctic Intermediate Water in the Southern Ocean identified by a distance metric

Antarctic Intermediate Water (AAIW), occupying a vast area in the intermediate depths of the southern hemisphere oceans, is important for heat and freshwater redistribution in the global oceans. However, questions remain regarding its sources at the surface and pathways to the intermediate depths. To answer these questions, a recently-defined distance metric, which can distinguish similar water parcels, is applied to Argo data. Results show that the Pacific and Atlantic AAIW originates from only limited regions at the surface, specifically near the Subantarctic Front in the southeast Pacific and the Falkland Plateau in the southwest Atlantic. Further investigation indicates that through subduction in the deep mixed layer in these regions, low-salinity water penetrates into the intermediate depths of the Pacific and Atlantic Ocean. In the Indian Ocean, such a fast pathway for low-salinity to reach the intermediate depths is absent. Instead, the AAIW here, characterized by lower oxygen concentrations, consists of a mixture of locally slow downward spreading low-salinity water and AAIW inflow carried by the Antarctic Circumpolar Current from the Atlantic Ocean. The AAIW in the three southern hemisphere oceans exhibits slightly different physical properties, reflecting their different origins. Additionally, results indicate that mixing with surrounding saline waters is crucial in the transformation of surface waters into the AAIW cores. Our findings confirm the reliability of the distance metric and emphasize the importance of localized physical processes in the penetration of AAIW into the ocean interior.