Mixing, upwelling, and internal wave generation beneath Super Typhoon Mangkhut: a vorticity-divergence view of the ocean response to tropical cyclones

Tropical cyclones (TCs) are powered by heat fluxes across the air-sea interface, which are in turn influenced by subsurface physical processes that can modulate the intensity of these storms and thus introduce uncertainty to weather forecasts. This study uses data from an array of 6 profiling floats to produce a three-dimensional diagnosis of ocean dynamics beneath Super Typhoon Mangkhut as it swept over the Western North Pacific in September of 2018. Vertical profiles of temperature show the mixed layer deepen ahead of the storm and reveal an asymmetric cold wake of sea surface temperature (SST). The divergence of measured horizontal currents suggests upwelling velocities of roughly 8 mh−1 behind Mangkhut, marking the generation of a large amplitude (∼75 m crest to trough) near-inertial internal wave. Furthermore, density overturns provide indirect estimates of diapycnal diffusivities ^ ∼ 10−2 as the ocean mixed layer deepened near the TC’s eye. To explain these observations, we formulate the ocean’s mixed layer dynamics in terms of vorticity and divergence. This demonstrates that near-inertial oscillations transform wind-forced vorticity into divergence and thus control the timing and intensity of upwelling and internal wave generation behind fast-moving storms. Lastly, we find evidence that turbulent fronts propagated away from the storm track in phase with internal waves of frequency ∼ 2 5 . Our analyses provide a rare observational confirmation of theory and comprehensive review of the subsurface physical processes controlling air-sea interactions under fast-moving TCs.