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

Abstract

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.

Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea

“…Thus, in order to account for the oceanic response in the Sargasso Sea, we restricted the study area to the deep waters to the right of Dorian's trajectory. Given that upwelling, cooling, and deepening of the isothermal layer are more intense on the right side of a TC's trajectory in the Northern Hemisphere (Price, 1981;Hanshaw et al, 2008;Gierach and Subrahmanyam, 2008;Fu et al, 2014), our asymmetric approach captures the strongest oceanic response. In addition to these Lagrangian measurements along Dorian's trajectory, we also investigated the oceanic response in a square area (Eulerian) located to the right of its trajectory (Fig.…”

“…3b and c). TCs typically give rise to upwelling flow underneath the storm centre and weak downwelling of the displaced warm water over a broad area beyond the upwelled regions (Price, 1981;Jullien et al, 2012;Fu et al, 2014). Overall, TC-induced upwelling by Ekman pumping plays a major role in inducing cooling under the TC centre (e.g.…”

“…Over oceans, TC-induced wind forcing mixes the surface layer, deepens the mixed layer, and uplifts the thermocline, leading to a decreased upper-ocean temperature and heat potential (Price, 1981;Shay and Elsberry, 1987;Trenberth et al, 2018). Vertical mixing and upwelling also lead to an increased abundance of surface phytoplankton due to entrainment of nutrient-rich waters from the nitracline to the ocean surface and/or entrainment of phytoplankton from the deep chlorophyll maximum (DCM) (Babin et al, 2004;Walker et al, 2005;Gierach and Subrahmanyam, 2008;Shropshire et al, 2016).…”

Oceanic response to the consecutive Hurricanes Dorian and Humberto (2019) in the Sargasso Sea

“…These authors found that during and immediately following a cyclone's passage, the spatially averaged MLD in the Sargasso Sea increases sharply by 7 m on average, while the positive anomaly rapidly diminishes during the subsequent days. Then, after the post-storm shoaling of the MLD, it can oscillate (shoaling/deepening) around the same mean depth (Foltz et al, 2015;Zhang et al, 2016;Prakash et al, 2018). However, this temporal evolution considers the oceanic response to an individual TC.…”

“…Although strong mixing due to TCs is mostly confined to the upper ocean, the cyclone-induced inertial waves affect the subsurface mixing too (Prakash et al, 2018). Part of the energy transferred to inertial currents by the storm may propagate below the mixed layer and finally drive turbulent mixing in the thermocline (Cuypers et al, 2013).…”