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

The nonhydrostatic Regional Ocean Modeling System is applied to the nonlinear internal waves, or solitons, that are generated at the Luzon ridge in the South China Sea. The Luzon ridge near the Batan islands is represented by an idealized ridge with a height of 2.6 km on a flat bottom. Model runs are performed for various ridge shapes and (a)symmetric tidal forcings. The model is in the mixed tidal lee wave regime. The barotropic tide over the ridge generates first-mode waves through the internal tide release mechanism. Westward-traveling solitons emerge from these first-mode waves through nonlinear steepening. In the internal tide release mechanism, asymmetric tides with strong eastward currents can generate strong westward solitons. The eastward current creates an elevation wave with a higher energy density west of the ridge, and as soon as the current slackens, the wave is released westward. On its backslope strong solitons develop. The energy density is further enhanced by nonlinearities, such as differences in phase speeds and energy fluxes related to lee waves. A modal and harmonic decomposition shows the generation of vertical modes and higher temporal harmonics and indicates significant wave-wave interaction (e.g., triads). In the mixed tidal lee wave regime, more energy is contained in the first mode compared to the higher modes. Hence, linear internal tide beams are less well defined and strong solitons develop.