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Seasonal evolution of the barrier layer (BL) and temperature inversion in the northern Bay of Bengal and their role on the mixed layer temperature (MLT) is examined using observations from a single Argo during December 2013 to July 2017. During fall, low salinity at surface generates BL in this region. It thickens to almost 80 m in winter enhanced by deepening of isothermal layer depth due to remote forcing. During winter, surface cooling lowers near-surface temperature, and thus, the subsurface BL experiences a significant temperature inversion (similar to 2.5 degrees C). This temperature inversion diffuses to distribute heat within ML and surface heating begins deep penetration of shortwave radiation through ML during spring. Hence, the ML becomes thermally well stratified, resulting in the warmest MLT. The Monin-Obukhov length attains its highest value during summer indicating wind dominance in the ML. During spring and fall, upper ocean gains heat allowing buoyancy to dominate over wind mixing.

Plain language Summary The northern Bay of Bengal is well known for its fresh ocean surface due to low salinity input from rivers. This study investigates the hydrography from the temperature and salinity profiles obtained from an Argo float, which drifted for 44 months in the northern Bay of Bengal. The mixed layer depends on the local processes and isothermal layer depth depends on the remote forcing through propagating Rossby waves. Thermal inversion within thick barrier layer is observed during winter. Wind stress is the dominant forcing in setting up the mixed layer during summer and winter. The barrier layer and temperature inversion play important roles on the mixed layer temperature. The state-of-the-art models failed to reproduce the temperature inversion layer due to low vertical resolution. While this is an integrated view from a single Argo, high-frequency high-resolution sampling would be necessary to understand turbulent mixed layer processes and quantify relative roles of haline buoyancy to thermal buoyancy.