On the generation and evolution of internal solitary waves in the northwestern South China Sea

Recently, internal solitary waves (ISWs) were observed from remote sensing images of the northwestern South China Sea (SCS). Scrutinizing the crests of these ISWs, it was suggested that they were generated locally as a result of tide–topography interaction. To confirm this assumption, a 2-D, fully nonlinear, non-hydrostatic Massachusetts Institute of Technology general circulation model (MITgcm) was applied to investigate the generation process and mechanism of ISWs in the northwestern SCS. A series of numerical simulations were carried out to explore the influences from the bottom topography, strength of tidal forcing, stratification and different tidal harmonics on ISW generation. It can be concluded that the ISWs to west of the sills were generated when the tidal flow over the sills changed from an ebb tide to a flood tide, while the generation of those to the east of the sills was associated with the start of the ebb tide. Analyses of the Froude number and slope parameter, which govern generation regimes, indicate that the generation of ISWs in this area is subject to mixed lee waves rather than to a baroclinic tide regime or unsteady lee wave regime. Scrutiny of these ISWs reveals that the superposition of different internal wave modes exists around the sills, and well-developed first and second ISWs are pronounced further away from the sills. There are three sills in the model domain, labeled sills a, b and c from the west to the east, respectively. Numerical experiments suggest that ISWs to the west of these sills are generated by sills b and c, while sill a intensifies the nonlinearity of the internal waves and generates higher internal wave modes. ISWs to the east of these sills are generated by sill c only.