Statistical Analysis of the Second- mode Internal Solitary Wave in the Continental Slope Area of the North-ern South China Sea

The mode-2 internal solitary waves in the continental slope region of northern South China Sea are analyzed based on the mooring-observed ocean current data. During the observation period, a total of 72 mode-2 solitary waves were found, which includes 101 mode-2 solitons. The mode-2 internal solitary waves are mainly upward in upper layer and downward in lower layer, and the maximum amplitude occurs mostly in the downward part, which accounts for 79.2%. The wave speed is calculated with the KdV equation and can be described with cosine function, which is large in summer and small in winter. From the velocityprofile, it can be found that the solitary wave velocity of the second mode has an obvious three- layer structure. In this paper，the maximum velocity depth and the turning depth of the upper and lower layers are calculated by manual discrimination. The statistical results show that the average turning depth of the upper internal wave flow is at 97.7 m, the maximum velocity depth of the middle internal wave flow is at 134.6m，and the average depth of the lower internal wave flow is at 204.2 m. The maximum flow velocity mostly occurred in the middle layer，accounting for 82.2%. The upper flow direction was mainly southeastward, and the velocity was mainly in the range of 0-0.6 m·s-1，accounting for 98.0%. The middle flow direction was mainly northwestward, and the velocity was mainly in the range of 0.2-0.8 m·s-1，accounting for 93.1%. The flow direction of the lower layer is mainly southeastward, and the velocity is mainly in the rangeof0-0.4 m. s7', accounting for 94.1%. This paper systematically presents the statistical characteristics of mode-2 solitary waves in the continental slope region of northern South China Sea, which provides a reference for future research.