**4. Conclusions**

By surveying remote sensing images over the BS, we found significant ISW activities near the submarine *KRI nanggala-402* wreck site in April 2021. Those ISWs were generated in the LS and then traveled along the north-western direction in the BS basin, passing through the submarine wreck site, and finally reached the continental shelf to the west of the Kangean Islands. The ISWs in April 2021 were mainly distributed in the area to the north of Bali Island, rather than north of Lombok Island as was revealed by previous studies, indicating a significant temporal variation of ISW distribution in the BS.

Along the propagation path, there were up to three wave packets simultaneously existing in the BS. The wave packet contained dozens of solitons, whose crest can extend for 200 km, within a meridional range of more than 60 km, covering a vast region of the BS. Those ISWs propagated at a mean speed of 2 m/s from the source region to the continental shelf, and the speed was as fast as 2.69 m/s in the BS basin and reduced to 0.71 m/s in the shallow water. On 19 April, about two days before the submarine incident, the amplitude of an ISW near the submarine wreck site was inversed to be 41 m according to satellite images, and the reconstructed underwater structure showed a maximum horizontal and vertical velocity of 65 cm/s and 10 cm/s, respectively. Moreover, it was inferred from the near-source evidence that ISWs were released within the southward barotropic tidal trough, and the variation of source tides revealed that two ISWs were generated with an interval of 11.7 to 12.3 h every day during the April tidal period.

The analyses presented here have provided necessary observational information on the ISW activities in the BS for the submarine wreck investigations, and whether or not the submarine *KRI nanggala-402* encountered with ISWs will be ascertained once the accurate time and location site of the submarine wreck becomes available in the future. In addition, it seemed that ISWs in the area to the north of Bali Island were extraordinarily active around the time of the submarine wreck in comparison with the statistical results from 2006 to 2011 and from 2014 to 2015 based on satellite images [14,29]. Such variability of ISW distribution is also an interesting topic worth further investigation, and long in-situ observations prove the necessary to improve the understanding of the ISWs in BS.

**Author Contributions:** Conceptualization, X.H., T.W., W.Z., S.Z., Y.Y. and J.T.; methodology, T.W. and X.H.; software, Y.Y.; validation, T.W. and X.H.; formal analysis, T.W.; investigation, T.W. and X.H.; resources, X.H. and W.Z.; data curation, T.W.; writing—original draft preparation, T.W.; writing—review and editing, X.H., W.Z., S.Z. and Y.Y.; visualization, T.W.; supervision, X.H.; project administration, W.Z.; funding acquisition, X.H., W.Z. and J.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This paper was supported by the National Natural Science Foundation of China (Grants 41976008, 91858203, 91958205), the Hainan Provincial Joint Project of Sanya Yazhou Bay Science and Technology City (Grant 120LH018).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** We acknowledge the use of satellite images from the NASA Worldview application (https://worldview.earthdata.nasa.gov, accessed on 28 November 2021). The WOA18 data was downloaded from the NOAA's National Centers for Environmental Information (https: //www.ncei.noaa.gov/products/world-ocean-atlas, accessed on 25 November 2021).

**Conflicts of Interest:** The authors declare no conflict of interest.
