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Volume 14, issue 2 | Copyright
Ocean Sci., 14, 259-272, 2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 03 Apr 2018

Research article | 03 Apr 2018

Estimation of oceanic subsurface mixing under a severe cyclonic storm using a coupled atmosphere–ocean–wave model

Kumar Ravi Prakash, Tanuja Nigam, and Vimlesh Pant Kumar Ravi Prakash et al.
  • Centre for Atmospheric Sciences, Indian Institute of Technology Delhi, New Delhi-110016, India

Abstract. A coupled atmosphere–ocean–wave model was 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. The coupled model was found to improve the sea surface temperature over the uncoupled model. Model simulations highlight the prominent role of cyclone-induced near-inertial oscillations in subsurface mixing up to the thermocline depth. The inertial mixing introduced by the cyclone played a central role in the deepening of the thermocline and mixed layer depth by 40 and 15m, respectively. For the first time over the BoB, a detailed analysis of inertial oscillation kinetic energy generation, propagation, and dissipation was carried out using an atmosphere–ocean–wave coupled model during a cyclone. A quantitative estimate of kinetic energy in the oceanic water column, its propagation, and its dissipation mechanisms were explained using the coupled atmosphere–ocean–wave model. The large shear generated by the inertial oscillations was found to overcome the stratification and initiate mixing at the base of the mixed layer. Greater mixing was found at the depths where the eddy kinetic diffusivity was large. The baroclinic current, holding a larger fraction of kinetic energy than the barotropic current, weakened rapidly after the passage of the cyclone. The shear induced by inertial oscillations was found to decrease rapidly with increasing depth below the thermocline. The dampening of the mixing process below the thermocline was explained through the enhanced dissipation rate of turbulent kinetic energy upon approaching the thermocline layer. The wave–current interaction and nonlinear wave–wave interaction were found to affect the process of downward mixing and cause the dissipation of inertial oscillations.

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Parameters at the sea surface are determined by the air–sea fluxes of heat, salt, and momentum. Surface wind speed drives the oceanic surface circulation and mixing of temperature and salinity up to a certain depth (mixed layer depth) from the sea surface. In this study, we examined the oceanic mixing process using numerical models under strong cyclonic winds. Results highlight the important role of inertial oscillations in subsurface mixing.
Parameters at the sea surface are determined by the air–sea fluxes of heat, salt, and momentum....