Evolution of obliquely incident waves on the slope of the northern South China Sea

Internal solitary waves (ISWs) with distinct fronts were frequently captured by satellite images and were often obliquely incident to continental slopes in the region southwest of the Dongsha Islands in the northern South China Sea (NSCS), although few in situ observations and explanations were available. To study the generation and evolution of the wave front in this region, three moorings were deployed along the continental slope of NSCS from April to September 2009 and synthetic aperture radar (SAR) images were collected for ISWs analysis as well. It was found that ISWs normally came out in the form of two successive waves at an interval of approximately 3 h. Further research showed that, although the interval was short, the generation mechanisms of the two ISWs were completely different. The first arriving ISW had obviously different wave structure along the wave front and always appeared in advance of the internal tides (ITs) trough, which indicates that the first arriving ISW evolved from the interaction between the ITs and continental shelf topography. Furthermore, based on the Korteweg-de Vries (KdV) equation and Ostrovsky number, it was inferred that the appearance of the first ISW was determined by the relative importance of nonlinearity and rotation in the propagation of ITs. Comparatively, the second arriving ISW evolved solely from the ITs without interaction with topography in the deep basin between the Luzon Strait and continental shelf, and had the same wave structure along the wave front.