A tropical cyclone (TC) interacts with the ocean through the interchange of surface fluxes of heat and momentum at the air‐sea interface. The sustainability and intensification of TCs are maintained by the enhancement in the latent heat flux in the high wind speed regime. An accurate calculation of heat fluxes is vital to understand the dynamics of the TC. The sea surface roughness is a crucial parameter that modulates the air‐sea fluxes. A coupled ocean‐atmosphere‐wave model is used to simulate TC Vardah over the Bay of Bengal during 10–15 December 2016. The study focuses on the influence of sea surface roughness on the heat fluxes, track, and intensity of TC Vardah. Four numerical experiments are performed with different parameterization schemes for surface roughness on the basis of (i) frictional velocity (Charnock), (ii) wave age, (iii) wave steepness, and (iv) a combination of wave steepness and wave age. A change in ocean surface roughness results in the modification of the cyclone track and heat fluxes. However, the effect of sea spray on the simulation of wind intensity of TC Vardah is only marginal. Out of the four experiments, the wave age‐based roughness parameterization was found to improve cyclone track, wind, and, hence, the generation of wind waves on the sea surface. In the case study of TC Vardah, the dynamic sea spray reduces surface wind speed up to 1.5 m/s which is within the uncertainty range of the model. An intercomparison of model experiments reveals the impact of surface heat fluxes on the TC Vardha characteristics.
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 15 m, 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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