A new diagnostic of air-sea interaction reveals ocean control of turbulent heat flux in the South Asian Summer Monsoon

Alex Kinsella , Amala Mahadevan

Air-sea interaction plays a central role in the South Asian summer monsoon, with processes such as surface turbulent heat fluxes, upper ocean mechanical mixing, surface buoyancy forcing, and surface moisture fluxes providing bridges between the ocean and atmosphere. The air-sea heat flux responds to internal variability in the ocean and atmosphere, as well as feedbacks arising from their interaction. Depending on whether SST drives turbulent heat flux or vice-versa, one may diagnose the air-sea heat exchange as being in an ocean-controlled regime or an atmosphere-controlled regime. We develop a method to quantify the air-sea interaction regime at each location and yearday by analyzing lagged correlations between SST and turbulent heat flux records from satellite data or reanalysis. We apply this method to the northern Indian Ocean and find that the basin exhibits a coherent shift from atmosphere control to ocean control during the summer monsoon onset and a return to atmosphere control during the monsoon retreat. This finding emphasizes the importance of oceanic processes such as entrainment and advection in monsoon dynamics. The Arabian Sea exhibits a more pronounced shift between atmosphere and ocean control than the Bay of Bengal, suggesting the importance of upper-ocean stratification and mixing in determining the turbulent heat flux in response to the monsoon wind. We find good agreement between the regimes diagnosed from blended satellite products and reanalysis, showing robustness of the results across datasets. Our method may provide a useful diagnostic for verifying air-sea interaction within coupled models of the ocean and atmosphere.