Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array

Abstract

AbstractNear‐inertial internal waves (NIWs) are generated by inertially‐rotating winds under tropical cyclones (TC). Since NIWs are mostly horizontal, their vertical propagation out of the mixed layer is slow. However, mesoscale vorticity and shear increase vertical group speed by increasing near‐inertial frequency and horizontal wavenumber. To assess NIW propagation, a profiling float array under Super Typhoon Mangkhut in September 2018 made broad and persistent measurements in space and time of density in the upper 200 m and depth‐mean velocity. The TC wake was a region of positive vorticity on its southern side, displayed elevated shear, and thereby enhanced downward propagation of NIWs. The vertical energy flux is estimated as 0.04–0.11 W m−2, which is about 1%–4% (3%–8%) of the mean total (near‐inertial) wind work of 3.0 (1.3) W m−2 calculated from a high‐resolution TC model. Considerable uncertainties arise in the (a) estimated group speed based on wavelength, shear, and frequency and (b) energy density based on depth‐varying density and the NIW polarization relations, which are sensitive to frequency. Following the TC's passage, NIWs propagated southward and horizontal wavelengths decreased from 1,000 to 500 km, as time progressed. Also, we identify an interfacial wave at the mixed layer base, which displaces isopycnals vertically. The following process is suggested. As the North Equatorial Current flows over these displacements, which act as topographic obstacles, secondary NIWs propagate up‐/downward into the mixed layer/thermocline. These waves are 180° out of phase in the mixed layer and thermocline, which can enhance shear at the mixed layer base.