Importance of Seasonally Evolving Near‐Surface Salinity Stratification on Mixed Layer Heat Budget During Summer Monsoon Intraseasonal Oscillation in the Northern Bay of Bengal in 2019.

The discharge of freshwater from major rivers into the northern Bay of Bengal (BoB) increases dramatically during the summer monsoon season, reaching a peak in August–September, and there is a corresponding increase in the vertical salinity gradient in the upper ocean. Here we study the impact of seasonally evolving near‐surface salinity stratification on the response of ocean mixed layer temperature (MLT) to Summer Monsoon Intraseasonal Oscillations (MISO), using accurate surface fluxes and high vertical resolution (∼2 m) hydrographic measurements from a mooring in the northern BoB (17.8°N, 89.5°E) during June–September 2019. Prominent MLT warming and cooling with a range of 1.5°C is observed between suppressed (clear skies, calm winds) and active (cloudy, windy) phases of MISO convection. However, the intraseasonal MLT response to the active phase of a late‐season MISO event is minimal compared to MISO events in early summer. We infer this is primarily due to the much smaller contribution from oceanic vertical processes (∼6 Wm−2) in late summer 2019, compared to their role in early summer (−15 to −55 Wm−2). During the active phase of the MISO event of late summer 2019, the combined effect of reduced entrainment and weak vertical temperature gradients associated with a barrier layer inhibits near‐surface cooling. Conversely, the near‐surface salinity stratification and the barrier layer are weak during MISO events in the early summer of 2019—these hydrographic conditions lead to enhanced MLT cooling in response to MISO, apparently through a freer turbulent exchange of cool thermocline water with the surface layer. Plain Language Summary: It is important to accurately represent the upper ocean and air‐sea interaction processes that modulate sea surface temperature variations associated with Summer Monsoon Intraseasonal Oscillation (MISO) in coupled models used for monsoon forecasting. The freshwater input to the Bay of Bengal (BoB) from continental rivers increases in May and reaches its peak in August–September during the summer monsoon. Hence, near‐surface salinity stratification is not uniform throughout the summer, and this can affect the relative contribution of different processes in the mixed‐layer heat budgets of the late‐season versus early season MISO events. The present study examines the difference in mixed‐layer temperature response to MISO with respect to seasonally evolving near‐surface salinity stratification in the northern BoB using accurate surface fluxes and high‐vertical resolution temperature, salinity, and current measurements (∼2 m) from a mooring in the northern BoB (17.8°N, 89.5°E). Our analysis reveals that cooling due to vertical processes in the ocean is much higher during early season MISO events than late‐season event, primarily due to strong near‐surface salinity stratification and a strong barrier layer that develops later in the monsoon season. Key Points: The impact of seasonally evolving near‐surface salinity stratification on the ocean mixed layer temperature response to MISO is examinedThe ML heat storage is positive during the suppressed and negative during the active phases of MISOThe cooling due to vertical processes in the ocean is much higher during early season MISO events than the late‐season event [ABSTRACT FROM AUTHOR]