Mixed Layer Salt Budget over Different Sectors of the Indian Ocean

Oceanic mixed layer (ML) has important implications to the air-sea interaction processes such as convection, monsoons, formation and intensification of tropical cyclones, etc. The mixed layer depth (MLD) depends on thermo-haline structure of the upper oceanic layers and found to vary in range of 20-200 m. Salinity variability is one of the crucial parameters that plays an important role in control of MLD variability, particularly in regions where the freshwater flux dominates in stratification process. For example, in the Bay of Bengal (BoB) the freshwater flux is large due to contributions from several rivers draining into the BoB, and the excess precipitation over evaporation. The low-saline water on the surface of ocean leads to formation of barrier layer (BL) and maintains higher sea surface temperature (SST), which supports convection and formation of cyclonic systems. Therefore, understanding MLD variability and factors affecting these variations are important to investigate. In this study we used observations of temperature and salinity profiles and numerical simulations from Modular Ocean Model (MOM) with data assimilation. Various terms in the mixed layer salt budget, namely salinity tendency, freshwater flux, horizontal advection, diffusion, and vertical processes are calculated over the northern Indian Ocean domain. The observed variability in salinity tendency over different sectors of the Indian Ocean such as Arabian Sea, BoB, equatorial Indian Ocean studied. Relative role of various processes in controlling salinity tendency has been investigated by examining contributions of different terms of the salt budget at a given time. Seasonal variations in salt budget terms over different sectors of the Indian ocean, and their role in MLD variability has been discussed.