Upper-Ocean Salinity Variability and Impact in the Southeast Indian Subantarctic Mode Water Formation Region during the Argo Era

Abstract Salinity is a state variable that determines the fundamental physical properties of the ocean, and understanding its changes and effects has important implications for the regional and global climate system. Based on Argo observations, changes in upper-ocean salinity and their impacts in the Southeast Indian Subantarctic Mode Water (SEISAMW) formation region are investigated from 2004 to 2022. The upper-ocean salinity in this region shows significant interannual variability, being relatively salty during 2010–16 and fresh during 2005–08 and 2019–22, respectively. A diagnostic analysis indicates that the interannual variability of winter mixed layer depth and stratification are mainly affected by upper-ocean salinity rather than temperature. In the SEISAMW formation region, higher upper-ocean salinity acts to weaken the stratification and deepen the mixed layer. Conversely, lower upper-ocean salinity acts to strengthen the stratification and shallow the mixed layer. A mixed layer salinity (MLS) budget analysis reveals that horizontal advection, predominantly influenced by meridional Ekman advection, governs the interannual variability of MLS. Furthermore, the interannual variability of meridional Ekman advection is mainly affected by the meridional Ekman velocity associated with zonal wind stress.