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 (ζ) and divergence (Γ) 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 ζand Γ, whose oscillations in quadrature cause periodic displacements in the ocean thermocline and generate near‐inertial waves (NIWs). Vertical profiles of Tand Sshow gradual mixing of the upper ocean with diffusivities as high as κ∼ 10−1m2s−1, which caused an asymmetric cold wake of sea surface temperature (SST). We estimate that ∼10% of the energy used by mixing was used to mix rainfall, therefore inhibiting SST cooling. Lastly, watermass transformation analyses suggest that κ> 3 × 10−3m2s−1above ∼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 ζand Γ for the study of internal wave fields, and provide conceptual clarity on the mechanisms that lead to NIW generation by winds. Near‐inertial internal waves (NIWs) are generated by winds and lead to oscillations in the internal structure of ocean currents and stratification. Turbulence induced by the vertical current shear in these waves helps sustain the upper ocean stratification and circulation. In this study, we use data from six autonomous floats deployed ahead of Super Typhoon Mangkhut to reconstruct the ocean's 3D response and compare it to output from a coupled air‐sea model. Patterns in NIW are explained using simple linear equations based on vorticity and divergence rather than current velocities, providing an alternative view of how TC winds help generate waves in the stratified ocean interior. Measurements of temperature and salinity detail how turbulence mixed rainfall and thermocline waters into the upper ocean. Our analyses indicate that turbulent mixing rates are greatest within 100 km of the typhoon eye but remain elevated hundreds of kilometers in the TC wake. Theory and observations presented here provide a comprehensive view of the ocean response to fast‐moving tropical cyclones. Float data, linear theory, and a 3D model reveal vorticity and divergence control on inertial pumping beneath a fast‐moving tropical cycloneRightward‐enhanced sea surface cooling of 1.2°C was dominated by mixing and modulated by rainfall, which suppressed cold water entrainmentEstimates of turbulent diffusivity explain sea surface cooling rates 0.1°C hr−1under the tropical cyclone eye and thermocline mixing in its wake Float data, linear theory, and a 3D model reveal vorticity and divergence control on inertial pumping beneath a fast‐moving tropical cyclone Rightward‐enhanced sea surface cooling of 1.2°C was dominated by mixing and modulated by rainfall, which suppressed cold water entrainment Estimates of turbulent diffusivity explain sea surface cooling rates 0.1°C hr−1under the tropical cyclone eye and thermocline mixing in its wake