A 2D-numerical modeling of the generation and propagation of internal solitary waves in the Luzon Strait

The wide presence of internal solitary waves (ISWs) in the northern South China Sea (SCS) has been confirmed by both Synthetic Aperture Radar (SAR) images and in situ observations. These ISWs are believed being generated over the varying topography in the Luzon Strait. They typically propagate westwards into the SCS with a diurnal or semidiurnal period. Their generation sites are, however, not yet solidly identified. To obtain a clear picture of the ISWs, we designed numerical experiments to analyze the generation and propagation of the ISWs in the Luzon Strait using a 2-dimensional non-hydrostatic model. The model current is forced by barotropic or baroclinic currents imposed at open boundaries. The experiments show that the tidal current serves as a kind of triggering force for the ISWs over the submarine ridges in the strait. Under the forcing of tidal currents, depressions are formed near the ridges. The ISWs then split from the depressions through a process different from lee-wave generation mechanism. The appearance of the ISWs is influenced by the strength and period of the forcing current: the ISWs are more likely to be generated by a stronger tidal current. That is why the ISWs in the Luzon Strait are frequently observed during spring tide. Compared with diurnal tidal current, the ISWs generated by semidiurnal tidal current with the same amplitude is much more energetic. It is partly because that the wave beams in diurnal frequency have a larger angle with the vertical direction, thus are more likely to be reflected by the topography slope. The impact of the Kuroshio to the ISWs is also analyzed by adding a vertical uniform or shear current at boundaries. A vertically uniform current may generate ISWs directly. On the other hand, a vertically shear current, which is more realistic to represent the Kuroshio branch, seems to have little influence on the generation process and radiating direction of the ISWs in the Luzon Strait.