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Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array
Author(s) -
Johnston T. M. Shaun,
Wang Shuguang,
Lee ChiaYing,
Moum James N.,
Rudnick Daniel L.,
Sobel Adam
Publication year - 2021
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2020jc016749
Subject(s) - geology , wavelength , mesoscale meteorology , mixed layer , inertial wave , thermocline , typhoon , vorticity , mean flow , shoaling and schooling , wind shear , wake , energy flux , internal wave , geodesy , wavenumber , geophysics , mechanics , wave propagation , physics , wind speed , climatology , optics , vortex , oceanography , turbulence , longitudinal wave , mechanical wave , astronomy
Near‐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.