Subtropical sea surface salinity (SSS) maximum is formed in the subtropical South Indian Ocean (SIO) by excessive evaporation over precipitation and serves as the primary salt source of the SIO.Spacebe SSS measurements by Aquarius satellite during September 2011-May 2015 detect three disconnected SSS maximum regions (＞35.6) in the east (105°E-115°E,38°S-28°S),central (60°E-100°E,35°S-25°S),and west (25°E-40°E,38°S-20°S) parts of the subtropical SIO,respectively.Such structure is however not seen in gridded Argo data.Analysis of Argo profile data confirms the existence of the east maximum patch and also reveals SSS overestimations of Aquarius near the west and east boundaries.Although subjected to large uncertainties,a mixed-layer budget analysis is employed to explain the seasonal cycle of SSS.The east and central regions reach the highest salinity in February-March and lowest salinity in August-September,which can be well explained by surface freshwater forcing (SFF) term.SFF is however not controlled by evaporation (E) or precipitation (P).Instead,the large seasonal undulations of mixed layer depth (MLD) is the key factor.The shallow (deep) MLD in austral summer (winter) amplifies (attenuates) the forcing effect of local positive E-P and causes SSS rising (decreasing).Ocean dynamics also play a role.Particularly,activity of mesoscale eddies is a critical factor regulating SSS variability in the east and west regions.