A Vorticity-Divergence View of Internal Wave Generation by a Fast-Moving Tropical Cyclone: Insights From Super Typhoon Mangkhut

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 storm intensity. Here, we use data from 6 profiling floats to recreate 3D fields of temperature (T), salinity (S), and velocity around the fast-moving Super Typhoon Mangkhut (western North Pacific, September 2018). Observational estimates of vorticity (zeta) and divergence (Gamma) agree with output from a 3D coupled model, while their relation to vertical velocities is explained by a linear theoretical statement of inertial pumping. Under this framework, inertial pumping is described as a linear coupling between zeta and Gamma, whose oscillations in quadrature cause periodic displacements in the ocean thermocline and generate near-inertial waves (NIWs). Vertical profiles of T and S show gradual mixing of the upper ocean with diffusivities as high as kappa similar to 10(-1) m(2) s(-1), which caused an asymmetric cold wake of sea surface temperature (SST). We estimate that similar to 10% of the energy used by mixing was used to mix rainfall, therefore inhibiting SST cooling. Lastly, watermass transformation analyses suggest that kappa > 3 x 10(-3) m(2) s(-1) above similar to 110 m depth and up to 600 km behind the TC. These analyses provide an observational summary of the ocean response to fast-moving TCs, demonstrate some advantages of zeta and Gamma for the study of internal wave fields, and provide conceptual clarity on the mechanisms that lead to NIW generation by winds.