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
JOURNAL OF GEOPHYSICAL RESEARCH-OCEANS(2023)
摘要
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 (similar to 2 m) hydrographic measurements from a mooring in the northern BoB (17.8 degrees N, 89.5 degrees E) during June-September 2019. Prominent MLT warming and cooling with a range of 1.5 degrees 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 (similar to 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.
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关键词
summer monsoon intraseasonal oscillation,bengal,mixed layer heat budget
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