Suppression of submesoscale currents by a tropical cyclone

Abstract

Using high-resolution submesoscale-permitting Regional Ocean Modelling System (ROMS) simulations with and without river runoff, this study investigates how a category 5 tropical cyclone modulates submesoscale currents (SMC) in the Bay of Bengal (BoB). The analysis reveals a three-stage sequence: pre-storm genesis of SMC, their suppression during the passage of the cyclone, and partial post-storm recovery. Before the storm, offshore advection of a river plume by a cyclonic eddy generated sharp lateral buoyancy gradients and velocity shear along the plume edges that sustained fronts, filaments, and ageostrophic secondary circulations through frontogenesis and mixed-layer instabilities. During the in-storm stage, strong winds and vertical mixing disrupted these buoyancy gradients, reversed energy conversion from potential to kinetic energy, and diminished frontal jets as wind-stress-driven frictional forcing overwhelmed buoyancy control. After landfall, weaker but reappeared buoyancy gradients once again supported frontogenesis and the gradual re-emergence of SMC. These findings demonstrate a novel cyclone-plume-SMC coupling in the BoB with implications for vertical exchanges, cyclone intensity, and biogeochemical responses in freshwater-dominated basins.