Generation of shoreward nonlinear internal waves south of the Hainan Island: SAR observations and numerical simulations

Abstract

The generation of shoreward nonlinear internal waves (NLIWs) on the continental shelf south of the Hainan Island (SHI) is investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. Two types of shoreward NLIWs are identified from SAR images according to their distinct geographic distribution. One type of NLIWs, named Type-N NLIWs, is distributed on the northern SHI, and the other one is named Type-S NLIWs, distributed on the southern SHI. The SAR-observed wave occurrence frequency during the spring and neap tides, combined with the calculated body force, suggests that the Type-N NLIWs originate from the Xisha Islands, whereas the Type-S NLIWs originate from both the Xisha Islands and the continental shelf break, and the shelf break has a larger contribution. The synergistic analyses of the internal tidal ray path, gamma parameter and earliest SAR-observed NLIWs reveal that the Type-N NLIWs are excited by the impingement of the diurnal internal tidal beams emanating from the Xisha Islands on the near-surface pycnocline close to the continental shelf. Based on the realistic shelf-slope topography and tidal forcing, the two-dimensional numerical simulations using the MITgcm suggest that the Type-S NLIWs result from the nonlinear disintegration of a mode-1 diurnal internal tide which develops from a lee wave formed at the continental shelf break. Furthermore, the sensitive numerical experiments show that the background current can greatly affect the nonlinear evolution of the internal waves generated at the shelf break.