The Mixed Layer Salinity Budget in the Central Equatorial Indian Ocean

The oceanic surface mixed layer salinity (MLS) budget of the central and eastern equatorial Indian Ocean during boreal fall and winter is studied using in situ and remote sensing measurements. Budgets on roughly 100 km scale were constructed using data from two DYNA mics of the M adden–Julian O scillation and two R esearch Moored A rray for African‐Asian‐Australian M onsoon A nalysis and Prediction moorings near 79°E during September 2011 to January 2012. The horizontal advective salinity flux plays a significant role in the seasonal variation of equatorial MLS. In boreal fall, the equatorial and 1.5°S MLS increases due to horizontal advection and turbulent mixing, despite the freshening surface flux associated with MJOs. In boreal winter, with larger sub‐monthly variation and uncertainties, the decreasing of equatorial MLS is accounted by freshening zonal advection and surface flux, abated by salty meridional advection; the 1.5°S MLS is explained by the combination of freshening meridional advection and surface flux, and salty zonal advection. Budgets between 2011 and 2015 are investigated using data products from Tropical Rainfall Measuring Mission, Aquarius, Ocean Surface Current Analyses Real‐time, Objectively Analyzed air‐sea Fluxes, and Argo mixed layers over a wider region. The eastward development of the equatorial salinity tongue in the central to eastern Indian Ocean in boreal fall and the westward retreat in boreal winter are largely determined by the equatorial zonal current. The meridional migration of ITCZ rainfall plays a secondary role. In order to improve model prediction skills of MLS changes in the equatorial Indian Ocean, both zonal and meridional salinity advective fluxes, at a spatial scale of 1° longitude and latitude and a timescale less than days, need to be properly simulated.