**1. Introduction**

Internal solitary waves (ISWs), characterized by their large amplitudes, high rate of occurrence and strong nonlinearity, are widely distributed in global oceans with crest lengths of up to 200 km [1]. As a result of tide-topography interactions, ISWs are especially active in marginal seas and around straits where strong tidal currents flow over steep topographies [2,3]. In those areas, ISWs induce horizontal current velocities exceeding 2 m/s, and in vertical currents, ISWs depresse isopycnal surfaces rapidly with fluctuating amplitudes of up to 240 m in 10 min [4–7]. Moreover, ISWs in the oceans often appear as multi-wave packets which contain a number of rank-ordered solitary waves in groups [3,7].

The South China Sea is a marginal sea with a high concentration and occurrence of ISWs [8]. ISWs in the northern South China Sea are among the strongest waves in global oceans. As such, recent years have seen growing attention paid to ISWs in the South China Sea. Xu and Chen reported a strong ISW in the northeast of Dong-sha Atoll, which had a velocity of 2.94 m/s, the largest among the ISWs in global oceans [9]. Cai et al. summarized the generation and evolution of ISWs in the South China Sea [10].

Sound propagation is the most effective method of information transmission in the ocean, and ISWs have a great impact on underwater sound propagation because they cause time-dependent spatial variations in the water temperature and sound speed [11–14]. The drastic variations in the sound propagation characteristics under ISWs can cause changes in arrival time, propagation path, transmission loss, horizontal refraction and other factors

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**Citation:** Li, J.; Shi, Y.; Yang, Y.; Huang, X. Noise of Internal Solitary Waves Measured by Mooring-Mounted Hydrophone Array in the South China Sea. *J. Mar. Sci. Eng.* **2022**, *10*, 222. https://doi.org/ 10.3390/jmse10020222

Academic Editor: Unai Fernandez-Gamiz

Received: 22 December 2021 Accepted: 4 February 2022 Published: 8 February 2022

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**Copyright:** © 2022 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/).

of acoustic waves [15,16]. Thus, ISWs are potentially destructive, decreasing the detection performance of sonar systems.

In addition to sound propagation characteristics, ISWs have been observed to contribute substantially to ambient noise by breaking waves on the sea surface, increasing the internal velocity of currents, and stirring marine sediments [12]. There are three types of noise induced by ISWs, which have been widely studied.

The first type of noise can be easily observed, because the emergence of ISWs forms intense surface rips and produces a noise that is identifiable to the human ear [17]. The collapse of surface waves brings air into the sea and forms a large number of bubbles. In the process of bubble growth and rupture, narrow-band pulses are emitted near the resonant frequency [18]. Rip-band noise can increase the ambient noise by 18 dB at frequencies of 5–15 kHz [17,18]. Near-bottom currents induced by ISWs generate the second type of noise by moving sediments [12,19]. After the sediment particles have been moved away from the seabed, they produce noise by colliding with each other and with the shells of near-bottom hydrophones. Sediment-generated noise is present at frequencies above 10 Hz, but a spectral maximum occurs between 2 kHz and 10 kHz [20–22].

When ISWs flow past a hydrophone and an entire mooring system, pressure fluctuations occur in the turbulence [12]. This noise is referred to as flow noise (the third type), usually below 100 Hz [23]. The intensity of flow noise decreases with increasing frequency. It is usually observable on a mooring-mounted hydrophone array in the cable, but not easily detected on a fixed hydrophone on the seabed [23]. Research on flow noise has a long history. As early as 1960, Willis and Dietz measured the flow noise associated with the tides at frequencies of 40–100 Hz in Narragansett [24,25]. Deane found that the flow noise of mooring instruments in shallow water was mostly below 50 Hz [26]. Strasberg and Webb proposed an infrasonic flow-noise model, but there is as yet no generalized flow-noise model for frequencies exceeding the infrasonic range (frequencies above 20 Hz) [27,28]. Flow noise is closely associated with the sensor size and the entire mooring system.

There have been a number of works on the effects that internal waves have on underwater noise [11–28]. However, noise induced by ISWs in the South China Sea has rarely been studied directly—which is also true of the mechanism of the noise. The strong noise caused by ISWs can drown the underwater acoustic signal and reduce signal-to-noise ratio. Therefore, studies of ISW noise would be of special interest to underwater continuous acoustic communication and monitoring with sonar system.

In this study, we observed the water temperature and flow velocity changes induced by ISWs as part of the 2016 Internal Solitary Wave Cooperative Observation Experiment in the South China Sea. Then, we analyzed the effects of ISWs on noise using acoustic data from a mooring-mounted hydrophone array. We found that the low-frequency flow noise was produced by the cable vibrating under the impact of ISWs, named vortex-induced vibration (VIV). This paper has been organized as follows. The experiment and data are introduced in Section 2. Analysis of noise induced by ISWs are presented in Section 3. A comparison between ISW observations in the 2016 experiment and the 2019 experiment in the South China Sea is provided in Section 4. Finally, Section 5 gives a conclusion.
