**1. Introduction**

Near-inertial waves (NIWs) are a kind of internal wave, which are ubiquitous in the global ocean. In the internal wave spectrum, NIWs appear as a predominant peak near the local inertial frequency, and hence, act as a dominant mode of high-frequency variability in the ocean [1]. NIWs play an important role in the oceanic energy budget [2,3], as they can cause intense shear [4,5] and contribute to the elevation of turbulent mixing [6,7].

There are various mechanisms that can cause NIWs, among which, the most important is synoptic wind forcing. According to previous estimations, the global power of wind-driven near-inertial motions is 0.3–1.5 TW [8–14], which is comparable to the global power converted from astronomical tides to internal tides [3,15,16]. Due to strong wind stresses and their compact size, tropical cyclones (typhoons and hurricanes) are an efficient generator of NIWs [1]. Moreover, nonlinear wave–wave interaction, including parametric subharmonic instability [17–19] and resonant triad interaction [20–22], lee waves [23], frontal jets [24] and mesoscale eddies [25], can also induce NIWs under some conditions.

The Western North Pacific is the region with the highest concentration of typhoons in the world [26]. According to the estimation of Nguyen et al. [27], there are an average of 22 typhoons per year and most of them pass through the South China Sea (SCS), the

**Citation:** Cao, A.; Guo, Z.; Pan, Y.; Song, J.; He, H.; Li, P. Near-Inertial Waves Induced by Typhoon Megi (2010) in the South China Sea. *J. Mar. Sci. Eng.* **2021**, *9*, 440. https:// doi.org/10.3390/jmse9040440

Academic Editors: Matt Lewis and SungHyun Nam

Received: 8 March 2021 Accepted: 15 April 2021 Published: 18 April 2021

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largest marginal sea in the Western North Pacific. The frequency of typhoons has shown an increasing trend in the SCS [28]. They import a significant amount of energy to the ocean and induce intense NIWs [29]. Several previous studies have reported typhooninduced NIWs in the SCS through analyzing in situ observations [30–40]. However, due to the differences in typhoon characteristics, mooring-measuring ranges, distances between typhoon centers and moorings, and local conditions, the NIWs induced by different typhoons usually exhibit different features and those induced by the same typhoon are site-dependent [31,32,34,35,37]. More importantly, because of the great difficulties and large costs of in situ observations, it is nearly impossible to simultaneously deploy sufficient moorings to observe NIWs induced by the same typhoon, which limits our understanding of NIWs to some extent.

In this study, through a comparison with in situ observations, we first showed that the hybrid coordinate ocean model (HYCOM) reanalysis results could reasonably reproduce the typhoon-induced NIWs. Thereafter, these data were used to investigate the characteristics of NIWs induced by typhoon Megi in 2010, with the aim of deepening our understanding of typhoon-induced NIWs in the SCS. The paper is organized as follows. Typhoon Megi, the HYCOM reanalysis results and corresponding data analysis methods are introduced in Section 2. A comparison between the HYCOM reanalysis results and in situ observations is performed in Section 3. After validation, the HYCOM reanalysis results are used to reveal the energy and modal characteristics of Megi-induced NIWs, which are shown in Sections 4 and 5, respectively. A discussion is presented in Section 6. Finally, the conclusions of this study complete the paper in Section 7.
