Study on the characteristics of mode-2 internal solitary waves near the Hengchun Ridge

Most of the internal solitary waves discovered by seismic oceanography method are first-mode internal solitary waves. In this paper, two convex mode-2 internal solitary waves with large amplitude were captured in the Hengchun Ridge south of Taiwan Island by seismic oceanography method. The apparent phase speeds of ISW1 and ISW2 are about 0.7 m.s(-1) by using the pre-stack migration sections of the Common Offset Gathers. The apparent propagation direction is from S to N along the seismic survey line. The apparent phase speed of ISW1 with a larger maximum amplitude is larger. Results show that the observed amplitude values obtained from seismic oceanography section conforms to the theoretical amplitude values calculated by the theoretical equation. The variation of the fine structure of the mode-2 internal solitary wave group is observed by the prestack migration section. It is found that the back edge of the reflection event steepens, bifurcates and merges during the acquisition process. Because the center of the pycnocline deviates from half depth of water column, and the effect of background shear flow, the high frequency internal wave develops in the tail of the two second-mode ISWs, which enhances the energy dissipation. In addition, the formation and propagation characteristics of mode-2 internal solitary waves are affected by many factors, such as stratification, Kuroshio, background shear current and seabed topography.