Air-Sea Interaction Regimes of the Northern Indian Ocean

Air-sea interaction plays an important role in the South Asian summer monsoon, with processes such as surface turbulent heat fluxes, wind-induced mixing of the upper ocean, and surface buoyancy fluxes providing a bridge between the ocean and atmosphere. The turbulent air-sea heat flux has contributions from internal variability from the ocean and atmosphere, as well as feedbacks from each subsystem to the other. Depending on the lagged correlations exhibited by the heat flux and sea surface temperature, one may define the system as in an ocean-controlled regime or an atmosphere-controlled regime. In regions in which the air-sea interaction is ocean-controlled, details of ocean mixing and advection may play a large role in the local surface heat fluxes, so that such regions require coupled models with realistic ocean components for accurate prediction.

We develop a method to algorithmically quantify the air-sea interaction regime at a given location and day of year given SST and turbulent heat flux records from satellite data or reanalysis. Symmetry properties of the lagged correlation curves reveal the extent to which a particular record is atmosphere- or ocean-controlled. We apply this method to daily data over the Northern Indian Ocean for the period 1985-2009 and find that the basin exhibits a coherent change from atmospheric control to oceanic control during the onset of the summer monsoon. During the pre-monsoon season, the basin is primarily atmospherically-controlled other than at the ocean margins, where upwelling and coastal processes are important for the heat fluxes. The regime shift to oceanic-control coincides with strengthening wind forcing throughout the basin that mixes the upper ocean, upwells deeper water, and increases the magnitude of turbulent surface fluxes. The Arabian Sea and Bay of Bengal exhibit differing patterns and strengths of oceanic control during the monsoon season, suggesting the importance of upper-ocean stratification in determining the turbulent heat flux response to an imposed windstress. With the retreat of the monsoon, the basin returns to atmospheric-control, with the transition initiated in the equatorial region. Our findings emphasize the importance of oceanic processes, particularly mixing and upper-ocean stratification, in the dynamics of monsoon air-sea interaction.