Can Tidal Forcing Alone Generate a GM‐Like Internal Wave Spectrum?. Issue 24 (30th December 2019)

• Record Type: Journal Article
• Authors: Chen, Zhiwu; Chen, Shaomin; Liu, Zhiyu; Xu, Jiexin; Xie, Jieshuo; He, Yinghui; Cai, Shuqun
• Is Part Of: Geophysical research letters. Volume 46:Issue 24(2019)
• Page Start: 14644
• Page End: 14652
• Publication Date: 2019-12-30
• Subjects: internal waves -- near‐inertial waves -- GM spectrum -- parametric subharmonic instability -- internal wave breaking -- northern South China Sea; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2019GL086338
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: An idealized two‐dimensional numerical model is utilized to study internal wave generation by tide‐topography interaction and wind forcing. Previous studies suggest that the quasi‐universal Garrett‐Munk (GM) internal wave spectrum can be generated and maintained only when energy is supplied from both the wind and the tides, while the present study shows that M2 tidal forcing alone is sufficient to generate a GM‐like spectrum in a latitude band roughly between 17° and 29°, with the steepness parameter ε > 1, N 0 h 0 / u 0 > 1 ( N 0 is buoyancy frequency, h 0 topographic height, and u 0 tidal amplitude), and a finite excursion parameter. The key lies in the generation of near‐inertial waves (NIWs) by breaking internal tides and triadic resonant interactions around supercritical topography. Remarkably, in the northern South China Sea, tidal forcing can contribute a comparable amount of near‐inertial energy as that contributed by the summer wind and thus is an important NIW generation mechanism. Plain Language Summary: Oceanic internal waves receive energy from both winds and tides and are thought to be central to ocean mixing, large‐scale circulation, and even the global climate. The most important conceptual understanding so far is that internal waves in the world oceans can be described by an empirical parametric model, the so‐called Garrett‐Munk (GM) spectrum, which describes the distribution of wave energy with respect to time and spatial scales. Previous studies suggest that a GM‐like spectrum can be created and maintained only when energy is supplied from both the winds and the tides, but we show here that wind forcing is not a must. This is because strong tidal flows over supercritical topography can induce wave breaking, thus generating near‐inertial waves that in turn interact with the internal tides to create a GM‐like spectrum. In the deep ocean where wind forcing is virtually absent, tidal forcing alone can create a GM‐like spectrum. Remarkably, in the northern South China Sea, near‐inertial energy contributed by tidal forcing can be of comparable magnitude as that contributed by the summer wind. The present study can significantly improve our understanding of the role of tidal forcing in generating a GM‐like internal wave spectrum. Key Points: Tidal forcing alone can generate a GM‐like internal wave spectrum in a particular parameter space Observations of the GM spectrum in the deep ocean are likely due to the effects of tidal forcing alone Tidal forcing is of equal importance to the summer wind in generating near‐inertial energy in the northern South China Sea