[A-OS11] Ocean Mixing Frontiers

[AOS11-08] Convection enhances upper ocean mixing during a tropical cyclone

*Devang Falor¹, Bishakhdatta Gayen^1,2, Debasis Sengupta¹, Dipanjan Chaudhuri^3,1 (1.Indian Institute of Science, Bangalore, 2.University of Melbourne, Australia, 3.University of Washington, Seattle)

Keywords:Turbulent Mixing, Cyclone, Convection

Cyclones are extreme weather events accompanied by large wind shear and high latent heat loss on the ocean surface. Due to these large surface forcings, the ocean usually responds with enhanced upper ocean turbulence and a drop in the sea surface temperature (SST). This study investigates the upper ocean turbulent mixing in the Bay of Bengal from a category 5 tropical cyclone, Phailin, using both in-situ data and large-eddy simulations (LES). We show the presence of both diffusive convection and near-inertial shear in the upper ocean response. We are able to capture turbulent boundary layer dynamics and also quantify the associated turbulent mixing during the cyclone event for the first time. Our simulated mixed layer depth, SST, and other dynamical variables match the observed mooring data well. We estimate the mixing efficiency, η, which is defined as the energy sink ratio due to irreversible diapycnal mixing to the total mechanical energy sink. η was found to be close to 0.2 in the first half of the response, when the wind shear was picking up, while it increased to about 0.5 in the second half, as convection became dominant and the surface stress decreased. Our study further improves the mixing parametrization used by a large-scale ocean model to predict ocean response better during the cyclones.