A vorticity-divergence view of internal wave generation by tropical
cyclones: insights from Super Typhoon Mangkhut
Abstract
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 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 ζ and Γ, whose cycles 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 κ~10^-1 m^2 s^-1,
which caused an asymmetric cold wake of sea surface temperature (SST).
We estimate that rain layer destruction used ~ 10% of
energy used for mixing near the TC track, therefore inhibiting SST
cooling. Lastly, watermass transformation analyses suggest that κ
> 3x10^-3 m^2 s^-1 above ~110 m
depth and up to 600 km behind the TC. These analyses provide an
observational summary of the ocean response to TCs, demonstrate some
advantages of ζ and Γ for the study of internal wave fields, and provide
conceptual clarity on the mechanisms that lead to NIW generation behind
TCs.