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 2009–2016 and fresh during 2004–2008 and 2018–2022, respectively. A diagnostic analysis indicates that the interannual variability of subduction rate, mixed layer depth, and stratification are mainly affected by upper ocean salinity rather than temperature. A relatively salty surface in the SEISAMW formation region weakens the stratification and deepens the mixed layer, which further affects lateral induction and thus subduction rate. Conversely, a lower surface salinity strengthens the stratification and shallows 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 zonal wind stress, which is positively correlated with the El Niño-Southern Oscillation index and negatively correlated with the Southern Annular Mode index during the Argo period.

Salinity,mode water,subduction