**1. Introduction**

Internal solitary waves (ISWs) are the nonlinear short-period waves widely distributed in the global oceans [1–8]. The largest observed ISW has an amplitude of 240 m and its vertical current reaches 0.64 m/s [9]. Featuring large amplitudes and strong currents, ISWs in the oceans are considered as one of the major threats to submarine navigation [10]. Specifically, ISWs have the ability to cause strong density disturbances that can suddenly decrease the buoyancy of a submarine, resulting in a large depth drop in a very short time. On the other hand, the strong downward current in front of large amplitude ISW can exert a huge force on the submarine and may drag it to seabed [11,12]. The well-known USS Thresher nuclear submarine disaster in 1963 was possibly caused by internal waves [13].

Connecting the Bali Sea (BS) with the Indian Ocean, the Lombok Strait (LS) features steep bottom topographies, and its southern portion is occupied by the Nusa Penida sill with an average depth of 200 m. Research based on Synthetic Aperture Radar (SAR) images [14–17] and in-situ observation [18] have revealed active ISWs around the LS area, which were generated by tidal currents flowing over the Nusa Penida sill. Recently, comprehensive high-frequency observations in the LS have captured almost continuously internal wave packets with a maximum amplitude of approximately 40 m [19]. The northwardpropagating ISWs would propagate into the BS and travel across the entire basin with an

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**Citation:** Wang, T.; Huang, X.; Zhao, W.; Zheng, S.; Yang, Y.; Tian, J. Internal Solitary Wave Activities near the Indonesian Submarine Wreck Site Inferred from Satellite Images. *J. Mar. Sci. Eng.* **2022**, *10*, 197. https:// doi.org/10.3390/jmse10020197

Academic Editor: Lev Shemer

Received: 7 December 2021 Accepted: 26 January 2022 Published: 1 February 2022

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average speed of about 2.0 m/s, and their crests could extend for hundreds of kilometers [17]. Numerical simulation results also showed that the occurrences of ISWs in the LS area varied significantly over monthly and interannual timescales under the modulation of thermocline structure adjustment, monsoons and the Indonesian Throughflow [20–22].

In the early morning of 21 April 2021 local time (near 4:30 AM), the Indonesian Navy's submarine (*KRI nanggala-402*) crashed in the BS and all 53 crew members were died. Public information reported that the submarine *KRI nanggala-402* crashed about 60 miles north of Bali Island, at a water depth of ~850 m. So far, Indonesian officials have not announced the specific cause of the submarine wreck. On grounds of the abundant ISW activities in the BS, it is necessary to investigate the ISW characteristics around the time when the *KRI nanggala-402* was wrecked, which will be helpful to clarify the reasons of the submarine incident. Fortunately, satellites photographed dense ISW signals over BS from 12–21 April 2021. In this study, we collected the optical remote sensing images covering BS during those days and investigated the distribution, propagation and underwater structure of ISWs around the time of the *KRI nanggala-402* wreck.
