*3.3. Underwater Structure of ISWs Inferred from MODIS Images*

Understanding the underwater structure of ISWs near the wreck site is crucial for evaluating the impacts of ISWs on submarine navigation. About two days before the submarine disaster, a clear wave packet (S5-b) near the wreck site was captured by the MODIS at 12:55 on 19 April (Figure 1f), and its underwater structure was reconstructed here.

According to the MODIS image, the center of the leading wave of S5 propagated in a northwest direction with a half-wavelength of ~2 km at a depth of about 1200 m (Figure 4a), and its amplitude was inversed to be 41 m. Furthermore, the theoretical propagation speed *C* of this ISW can be acquired by *C* = *C*<sup>0</sup> + *αη*0/3. In this case, *C*<sup>0</sup> and *α* are 2.51 m/s and −0.0166 respectively, and the calculated *C* is 2.73 m/s, which is in good agreement with the mean propagation obtained from the satellite images in Figure 1e,f.

**Figure 4.** (**a**) Local amplification of the ISW occurring in the BS basin within the remote sensing image from 12:55 19 April local time. (**b**) Gray level changes along the red line in (**a**). (**c**) Climatological potential density and Brunt-Väisälä frequency profiles calculated from WOA18 data at the pink dot site in (**a**). (**d**) First-mode of the ISW.

Figure 5 shows the underwater structure of the wave S5-b. As shown in the figure, the maximum horizontal current velocity induced by the wave was 65 cm/s, and the flow core with a velocity greater than 50 cm/s existed in the upper 50 m and spent nearly 10 min passing by the site where the ISW crest located. Below 300 m, the horizontal current direction of the ISW was opposite to the wave propagation direction. Vertically, there were downward and upward currents respectively before and after the wave trough with a maximum velocity of 10 cm/s, and the vertical flow exceeding 6 cm/s extended for 800 m and 650 m in the horizontal and vertical directions, respectively.
