**1. Introduction**

Internal waves for which reduced gravity provides the restoring force widely occur in seas and are responsible for enhanced dissipation in the deep ocean [1]. Most of the internal wave energy is radiated from the source over a long distance, which can significantly affect the space-time distribution of energy available for global mixing [2–4]. The South China Sea may have most intense internal waves among all oceans. Internal solitary waves with peak-to-trough amplitudes over 150 meters have been reported in the northern South China Sea, and such waves are believed to impact other ocean processes notably [5,6]. Both observation and numerical simulation studies have suggested that the Luzon Strait is the main generation site of internal waves in the northern South China Sea [7–14]. Alford et al. depicted a cradle-to-grave picture of internal waves from the Luzon Strait to the continental shelf on a basin scale through the combination of in situ data and numerical simulation [15]. In situ observations show that the regularity and strength of internal solitary waves on the shelf of the northern South China Sea has a significant spring-neap variability [12,15,16]. Moreover, internal tides in the deep basin west of the Luzon Strait also show a spring-neap variability that has been demonstrated by moored current observations [17]. In addition, numerical simulations and remote sensing data suggest that internal solitary waves on the shelf are developed by nonlinear steepening and frequency dispersion of the baroclinic tides generated in the Luzon Strait by tide–topography interactions [8,9,12,15,18]. As Ramp et al. stated, "Most of the features of the wave arrival can be explained by the tidal variability in the Luzon Strait" [16]. Therefore, the generation of baroclinic tides in

**Citation:** Zhang, Z.; Chen, X.; Pohlmann, T. The Impact of Fortnightly Stratification Variability on the Generation of Baroclinic Tides in the Luzon Strait. *J. Mar. Sci. Eng.* **2021**, *9*, 703. https://doi.org/ 10.3390/jmse9070703

Academic Editors: Déborah Idier and Vengatesan Venugopal

Received: 30 March 2021 Accepted: 24 June 2021 Published: 26 June 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

the Luzon Strait, which is investigated in the present study, is important for forecasting internal wave characteristics in the South China Sea.

At least five different internal wave generation mechanisms exist including beam scattering, mixed region collapse, and lee–wave mechanism [18–22]. The timing and strength of the energy transfer process from barotropic to baroclinic mode thereof determine how the internal wave is generated at the beginning. In previous numerical studies concerning baroclinic tides in the Luzon Strait, the main focus has been on tides and topography and their interaction, for example, the spring-neap cycle, flood–ebb current, generation site, and resonance between two ridges [11–13,23,24]. Stratification is considered a minor factor in internal wave variability, for example, the seasonal variation of internal tides in the northern South China Sea is considered to be mainly modulated by the astronomical tides rather than by the seasonal thermocline in the Luzon Strait [17]. Some previous numerical simulations have shown that stratification can notably impact internal wave generation under specific conditions [25,26]. Recently, an extreme internal solitary wave with a maximum peak-to-though amplitude of 240 *m* was reported in the northern South China Sea [6]. The authors argue that the variability of stratification in the Luzon Strait may be a key factor for the generation of energetic internal tides and the formation of this extreme internal solitary wave event. As a result, the variability of stratification in the Luzon Strait obviously deserves more attention in baroclinic tide generation studies. In particular, stratification can be affected by many factors, such as surface heat flux and mesoscale eddy intrusion. To simplify this situation, we focus only on the tidal effect on stratification because the interaction between strong tidal flow and steep topography in the Luzon Strait can severely affect the local stratification and is thus the major source of baroclinic variability in our case. The specific questions we address are how stratification is affected by tide–topography interactions and how the baroclinic tides are affected by this stratification variability. Research on these questions will be helpful for improving the understanding of the internal wave generation process and variability in realistic situations.

In this paper, the MIT general circulation model (MITgcm) described by Marshall et al. is used for the three-dimensional hydrodynamic baroclinic simulation [27]. We focus mainly on the stratification variability and the generation of baroclinic tides in the Luzon Strait. Therefore, we decided to use the hydrostatic version of MITgcm as our main target processes can be adequately resolved by this model. The main objective is to determine how the stratification variability affects the internal wave generation in the Luzon Strait.

In Section 2, the model settings are presented. Subsequently, in Section 3, the model validation, the stratification variability, and the analysis of governing mechanisms in the Luzon Strait are presented. These sections are followed by an analysis of energy transfer and its effect on internal wave generation. Finally, the discussion and conclusions are presented in Sections 4 and 5, respectively.
