Internal solitary waves induced by flow over a ridge: With applications to the northern South China Sea

The generation of internal solitary waves by barotropic tides over a ridge is studied in a nonhydrostatic numerical model under idealized oceanographic settings. The experiments examine the effects of ridge width, barotropic tidal strength, and stratification on wave generation. The barotropic tidal flow produces internal wave beams emitting from the ridge top if the slope of the ridge exceeds a critical value equal to the slope of the wave beam. Reflection and refraction of a wave beam in an upper ocean waveguide associated with a strong shallow thermocline produce horizontally propagating internal tides. When the local Froude number over a ridge is not small, lee waves generated on the ridge convert enough energy from the barotropic tides to the internal tides to form tidal bores and solitary waves. Increasing stratification at ridge depths enhances the generation of internal waves, particularly at the diurnal periods. In the Luzon Strait, the slope of the wave beam decreases in spring and summer as stratification at the ridge depths increases, favoring the generation of internal tides. Without the presence of a strong shallow thermocline, internal solitary waves are not observed east of the Luzon Strait. In the northern South China Sea, internal solitary waves are likely observed from April to July when a strong shallow thermocline is present. A deep mixed layer in winter suppresses the production of internal solitary waves.