Near‐Inertial Wave Propagation in the Wake of Super Typhoon Mangkhut: Measurements From a Profiling Float Array. Issue 2 (7th February 2021)

• Record Type: Journal Article
• Authors: Johnston, T. M. Shaun; Wang, Shuguang; Lee, Chia‐Ying; Moum, James N.; Rudnick, Daniel L.; Sobel, Adam
• Is Part Of: Journal of geophysical research. Volume 126:Issue 2(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-02-07
• Subjects: internal waves -- profiling floats -- tropical cyclone -- wake; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2020JC016749
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: 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. Plain Language Summary: Winds in tropical cyclones (TC) generate near‐inertial internal waves (NIWs), which propagate slowly into the ocean's interior. The restoring forces on NIWs are a combination of gravity and Coriolis resulting in slow downward energy propagation into the interior, where breaking NIWs are significant contributors to mixing. We describe three mechanisms (two known, one new), that enhance vertical propagation. The well‐known key to downward propagation is the reduction of the horizontal wavelength, which occurs due to (a) rotation of mesoscale eddies and (b) their vertical current shear. A new result is that the TC wake aids downward propagation because it exhibits these two mesoscale properties itself. The third mechanism is new and arises from a NIW found between the mixed layer and ocean interior. This NIW alternately augments and opposes the North Equatorial Current, which leads to flow convergence and vertical excursions of the mixed layer base at a near‐inertial frequency, which, in turn, produces both up‐ and downward propagating NIWs. These measurements were made from profiling floats, which only measured temperature and salinity, and are identical to those widely used in the Argo program, although their mission profile was different. Key Points: Mean flow over isopycnals displaced by near‐inertial internal waves (NIWs) generates secondary, vertically‐propagating, and shorter NIWs The tropical cyclone (TC) wake's vorticity and shear alter NIW frequency and wavelength, allowing vertical propagation Downward NIW energy flux is ∼6% of this TC's energy flux from wind forcing to mixed layer near‐inertial kinetic energy