Estimation of oceanic sub-surface mixing under a severe cyclonic storm using a coupled atmosphere-ocean-wave model

A coupled atmosphere-ocean-wave model used to examine mixing in the upper oceanic layers under the influence of a very severe cyclonic storm Phailin over the Bay of Bengal (BoB) during 10–14 October 2013. Model simulations highlight prominent role of cyclone induced near-inertial oscillations in sub-surface mixing up to the thermocline depth. The inertial mixing introduced by the cyclone played central role in deepening of thermocline and mixed layer depth by 40 m and 15 m, respectively. A detailed analysis of inertial oscillation kinetic energy generation, propagation, and dissipation was carried out at a location in northwestern BoB. The peak magnitude of kinetic energy in baroclinic and barotropic currents found to be 1.2 m2 s−2 and 0.3 × 10−2 m2 s−2, respectively. The power spectrum analysis suggested a dominant frequency operative in sub-surface mixing was associated with near-inertial oscillations. The peak strength of 0.84 m2 s−1 in zonal baroclinic current found at 14 m depth. The baroclinic kinetic energy remain higher (> 0.03 m2 s−2) during 11–12 October and decreased rapidly thereafter. The wave-number rotary spectra identified the downward propagation, from surface up to the thermocline, of energy generated by inertial oscillations. A quantitative analysis of shear generated by the near-inertial baroclinic current showed higher shear generation at 40–80 m depth during peak surface winds. Analysis highlights that greater mixing within the mixed layer take place where the eddy kinetic diffusivity was high (> 6 × 10−11 m2 s−1). The turbulent kinetic energy dissipation rate increased from 4 × 10−14 to 2.5 × 10−13 W kg−1 on approaching the thermocline that dampened mixing process further downward into the thermocline layer.