**5. Conclusions**

In this study, oceanographic and acoustic data from the 2016 Internal Solitary Wave Cooperative Observation Experiment in the South China Sea were studied. We analyzed the noise during three large ISWs and found that ISWs produced strong noise at low frequencies. The analysis results could be used as a supplement to internal wave observations and provide a passive acoustic monitoring and warning method for high marine currents.

Within 15 min of the internal wave flowing past the subsurface mooring, the isothermal surface was depressed substantially, with a maximum amplitude reaching 160 m and the maximum velocity exceeding 2 m/s. The noise received by the mooring-mounted hydrophone array also increased.

Compared with the ambient noise, the results indicated that the ISW noise was higher than the ambient noise at frequencies below 2 kHz, and even higher than 20–40 dB within 100 Hz.

Through analysis of time–frequency spectra and power spectra, we interpreted the low frequency noise as vortex-induced vibration caused by ISWs flowing past the marine cable system owing to the significant harmonic component of the noise. The noise duration was basically consistent with the time of ISWs interacting with the mooring system. It was notable that the frequency fluctuation of noise was caused by the variation of internal wave velocities with time and space. Through comparison of the ISW noises received by hydrophones at different depths, we found that the power spectra of ISW noises were associated with the positions of the hydrophones fixed on the cable. The closer to the fixed end, the smaller the noise spectrum level, and vice versa.

For further evidence, the observation experiment of internal waves in the South China Sea in 2019 confirmed that the frequency fluctuation and harmonic components of low frequency noise induced by internal solitary waves still occurred under different experimental conditions.

The results of this study could establish a high marine current warning and realtime marine current monitoring system, because the observed acoustic characteristics, including harmonics, frequency fluctuation, and energy enhancement, were intrinsically related to the velocity and amplitude of internal waves. Oceanographic phenomena can be studied based on acoustic observations and noise analysis. In addition, when studying oceanographic phenomena, acoustic signal analysis technology can receive data through a single hydrophone on the mooring system, without the full ocean depth thermistor chains and ADCP. It would be both cheaper and more practical than other ocean instruments.

To our knowledge, few studies have examined the model of the marine cable system vibration under ISWs. Therefore, the low frequency noise induced by ISWs calls for further experimentation and theoretical exploration.

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

**Funding:** This research was funded by the National Natural Science Foundation of China, grant number 41906160, 11974286, 12174312.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data from 2016 Internal Solitary Wave Cooperative Observation Experiment presented in this study are available on request from the corresponding author.

**Acknowledgments:** We are grateful to Bingyong Guo for providing helpful suggestions about VIV in the manuscript.

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