**7. Conclusions**

Based on the HYCOM reanalysis results, the NIWs generated by typhoon Megi in 2010 in the SCS were investigated in this study. Through a comparison with in situ observations at mooring UIB6, we first showed that the HYCOM reanalysis results can reasonably reproduce typhoon-induced NIWs, which can act as a supplement to in situ observations and provide us with an opportunity to better understand NIWs.

The results indicate that Megi-induced NIWs showed temporal and spatial variations in the SCS. The NIKE in the SCS was rapidly enhancedin response to typhoon Megi. However, the strongest NIKE appeared several days after the passage of Megi, rather than under its influence. Moreover, it is interesting to note that Megi-induced NIKE was mainly concentrated in the deep SCS basin where the water depth is greater than 1000 m, although typhoon Megi passed over both the deep SCS basin and shallow continental shelf and slope in the northern SCS. Through analysis, it was found that the continental slope in the northern SCS is supercritical to Megi-induced NIWs. In other words, when Megi-induced NIWs impinged on the continental slope, they were reflected and trapped in the deep SCS basin. This conclusion was validated by comparing the northward–downward and southward–downward components of Megi-induced NIWs. Moreover, it is found that Megi-induced NIWs could reach 1000 m depth in the vertical direction.

In situ observations have shown that Megi-induced NIWs dampened quickly at mooring UIB6 [32,37]. However, the HYCOM reanalysis results indicate that only in the region near Megi's track did the NIWs exhibit a similar feature, with the e-folding time generally being smaller than one week; whereas in two zones to the west of Luzon Island and the Luzon Strait, which are far away from Megi's track, the e-folding time of Megiinduced NIWs could be longer than 20 days. It was found that the NIWs generated at other sites could propagate to the two zones and influence the local NIKE, which accounted for the long e-folding time.

The modal content of Megi-induced NIWs was also explored in this study. Near Megi's track, Megi-induced NIWs were dominated by mode-2, which was followed by mode-3 and mode-1. The three modes accounted for 76% of the total NIKE. However, at the region far away from Megi's track, the NIKE of higher modes (mode-4 to mode-7) was enhanced several days after the passage of typhoon Megi. The cause of these higher modes still needs further exploration.

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

**Funding:** This research was funded by the Natural Science Foundation of Zhejiang Province through grant LY21D060005, the Joint Project of Zhoushan Municipality and Zhejiang University through grant 2019C81060, and the National Natural Science Foundation of China through grant 41621064. This research was also supported by the Laboratory for Regional Oceanography and Numerical Modeling, Qingdao National Laboratory for Marine Science and Technology through grant 2019A01.

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