A Locally Generated High‐Mode Nonlinear Internal Wave Detected on the Shelf of the Northern South China Sea From Marine Seismic Observations

• Published in: Journal of geophysical research. Oceans 2018-02, Vol.123 (2), p.1142-1155
• Main Authors: Tang, Qunshu, Xu, Min, Zheng, Chan, Xu, Xing, Xu, Jiang
• Format: Article
• Language: English
• Subjects: Continental shelves; Deep water; Density stratification; Geophysics; Imaging techniques; Instability; internal solitary wave; Internal wave breaking; Internal waves; Kelvin-Helmholtz instability; Orbital velocity; Seismic activity; seismic oceanography; Seismic stability; Seismic surveys; Shelf edge; Splitting; Stratification; Tides; Water structure; Wave breaking
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1002/2017JC013347

In this work, a secondary nonlinear internal wave (NIW) on the continental shelf of the northern South China Sea is investigated using high‐resolution seismic imaging and joint inversion of water structure properties combined with in situ hydrographic observations. It is an extraordinary wave combination with two mode‐2 NIWs and one elevated NIW occurring within a short distance of 2 km. The most energetic part of the NIW could be regarded as a mode‐2 NIW in the upper layer between 40 and 120 m depth. The vertical particle velocity of ∼41 cm/s may exceed the critical value of wave breaking and thus collapse the strong stratification followed by a series of processes including internal wave breaking, overturning, Kelvin‐Helmholtz instability, stratification splitting, and eventual restratification. Among these processes, the shear‐induced Kelvin‐Helmholtz instability is directly imaged using the seismic method for the first time. The stratification splitting and restratification show that the unstable stage lasts only for a few hours and spans several kilometers. It is a new observation that the elevated NIW could be generated in a deepwater region (as deep as ∼370 m). Different from the periodical NIWs originating from the Luzon Strait, this secondary NIW is most likely generated locally, at the continental shelf break during ebb tide. Key Points Finescale structure of a nonlinear internal wave is imaged and is recovered from seismic and hydrographic data The thermocline is subjected to dual nonlinear processes of wave breaking and Kelvin‐Helmholtz instability within a distance of 1 km The shear‐induced Kelvin‐Helmholtz billows are directly imaged using marine seismic data for the first time