**4. Comparison of ISW Noise in 2019 ISW Observation Experiment**

This section compares noise measurements from the 2019 Internal Solitary Wave Observation Experiment. The experiment was located on the continental shelf of the South China Sea with an average water depth of 360 m.

Figure 8 shows the temperature data and the noise of the same internal wave received by two hydrophones. The maximum amplitude of this ISW was 91.74 m which arrived at 19:42 UTC+8. Figure 8b shows the time–frequency spectrum below 100 Hz recorded by a hydrophone located at depth of 200 m, which was the middle hydrophone of the mooring-mounted linear array. The length of the cable in this experiment was about 300 m. The fundamental frequency of low frequency noise was 10.20 Hz, with uniform harmonic components. *J. Mar. Sci. Eng.* **2022**, *10*, x FOR PEER REVIEW 11 of 14

**Figure 8.** Temperature data and noise of the same ISW received by two hydrophones on 9 July 2019. (**a**) Temperature measured by the thermistor chains. (**b**) Time–frequency spectrum from a hydrophone at a depth of 250 m. The cable of the hydrophone array is 300 m long. (**c**) Time–frequency spectrum from the near–bottom hydrophone at a depth of 340 m. The cable of the single hydrophone is 30 m long. **Figure 8.** Temperature data and noise of the same ISW received by two hydrophones on 9 July 2019. (**a**) Temperature measured by the thermistor chains. (**b**) Time–frequency spectrum from a hydrophone at a depth of 250 m. The cable of the hydrophone array is 300 m long. (**c**) Time–frequency spectrum from the near–bottom hydrophone at a depth of 340 m. The cable of the single hydrophone is 30 m long.

A comparison between ISW observations in the 2016 experiment and the 2019 experiment is shown in Table 4. The amplitude of the internal wave in the 2019 experiment was slightly smaller than that observed in the 2016 experiment. Due to the shallow sea, the cable length of the mooring system was much smaller than that of the deep-sea experiment. This result showed that the noise intensity induced by ISW was related to the cable length, because the longer the cable, the stronger the VIV [30,35]. This was also related to The results confirmed that the flow noise induced by ISWs was indeed caused by the VIV of the cable system. The noise in Figure 8c was from the near-bottom hydrophone at a depth of 340 m, 17.8 km west of the mooring-mounted linear array. The length of the cable that held the hydrophone was 30 m. When the same ISW flowed past the near-bottom hydrophone, the noise intensity was substantially reduced. The fundamental frequency of low frequency noise was 6.15 Hz.

the position of the hydrophone attached on the mooring system. Overall, all noise datasets had uniform harmonics, which was in accordance with the characteristics of flow velocity. **Table 4.** The comparison between ISW observations in the 2016 experiment and 2019 experiment.  **2019 Experiment 2016 Experiment**  Water depth (m) 360 m 1000 m Cable length (m) 300 m 30 m 490 m 490 m 490 m Amplitude of ISW (m) 91.74 m 91.74 m 159.65 m 118.28 m 98.30 m A comparison between ISW observations in the 2016 experiment and the 2019 experiment is shown in Table 4. The amplitude of the internal wave in the 2019 experiment was slightly smaller than that observed in the 2016 experiment. Due to the shallow sea, the cable length of the mooring system was much smaller than that of the deep-sea experiment. This result showed that the noise intensity induced by ISW was related to the cable length, because the longer the cable, the stronger the VIV [30,35]. This was also related to the position of the hydrophone attached on the mooring system. Overall, all noise datasets had uniform harmonics, which was in accordance with the characteristics of flow velocity.

> 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

> 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 hy-

> 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

> 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

**5. Conclusions** 

Hz.

drophone array also increased.

Hydrophone location middle bottom middle bottom bottom Fundamental frequency (Hz) 10.20 Hz 6.15 Hz 12.72 Hz 9.16 Hz 8.18 Hz


**Table 4.** The comparison between ISW observations in the 2016 experiment and 2019 experiment.
