Interannual variations of mixed layer temperature and salinity in the South Indian Ocean salinity maxima region

The study explores mixed layer temperature (MLT) and salinity (MLS) variability in the salinity maxima region present in the South Indian Ocean (SIO) on interannual time scale using ECCOv4r4 and Argo observations. It is observed that MLT and MLS are in tandem with surface heat flux and evaporation changes during the austral summer and winter seasons. Although the monthly evolution of high salinity in the SIO shows that the high salinity core is primarily located near 30 degrees S, with notable seasonal variability south of 30 degrees S. The region exhibits high interannual variability in MLT compared to MLS. Covariance and budget analysis show that net heat flux is the primary and significant component that contributes to the mixed-layer heat budget. However, changes in MLS are mainly attributed to meridional advection and entrainment. Furthermore, MLT variability is separated into two phases (I) 1992-2006, where the temperature is mostly below climatological value and (II) after 2007, the temperature is seen increasing with a hiatus-like signature from 2010-2015. During phase I, the MLT tendency is driven by meridional advection followed by net heat flux. However, in phase II, net heat flux mainly drives the temperature tendency, and meridional advection plays a secondary role. Whereas, salinity tendency is mainly driven by meridional advection. Further, in 1992-2006 period, downward Ekman pumping results from the strengthening of wind stress curl, led to the deepening of the mixed layer, while after 2006 MLD shoals due to weakening of wind stress curl. Additionally, during the second phase, the reduced meridional velocities in the mixed layer contribute to warming and salinification in the region.