Interior water-mass variability in the southern-hemisphere oceans during the last decade

Using an Argo dataset and the reanalysis ECCOv4 , a volume budget was performed to address the main mechanisms implied in the volume change of the interior water masses in the southern-hemisphere oceans between 2006 and 2015. The subduction rates, and the isopycnal and diapycnal water-mass transformation were estimated in a density-spiciness (σ-τ) framework. Spiciness, defined as thermohaline variations along isopycnals, was added to the potential density coordinates to discriminate between water-masses spreading along isopycnals. The main positive volume trends were found to be associated with the Subantarctic Mode Waters (SAMW) in the south Pacific and Indian basins revealing a lightening of the upper waters in the Southern Hemisphere. The SAMW exhibits a two-layer density structure in which the upper layers gained volume as a result of local subduction while the volume of the lower layer reduced as a result of isopycnal and diapycnal transformation. The Antarctic Intermediate Waters (AAIW), defined between the 27.2 and 27.5 kg m^-3 isopycnals, showed the strongest negative volume trends. This volume loss can be explained by negative transformation along isopycnals southward from the ACC into the fresher and colder Antarctic Winter Waters (AAWW) and northward into spicier tropical/subtropical Intermediate Waters. The AAWW is destroyed by obduction back into the mixed layer so its net volume change remains nearly zero. The proposed mechanisms to explain the transformation within the Intermediate Waters are discussed in the context of Southern Ocean dynamics. It is shown that the σ-τ decomposition provides new insight on the spatial and temporal waters-mass variability and driving mechanisms over the last decade.