Energy of nonlinear internal waves in the South China Sea

Four sets of ADCP measurements were taken in the South China Sea (SCS); these results were combined with previous satellite observations and internal-tide numerical model results. Analysis suggests that strong internal tides are generated in Luzon Strait, propagate as a narrow tidal beam into the SCS, are amplified by the shoaling continental slope near TungSha Island, become nonlinear, and evolve into high-frequency nonlinear internal waves (NIW). Internal waves in the SCS have geographically distinct characteristics. (1) West of Luzon Strait the total internal wave energy (E-iw) is 10 x that predicted by Garrett-Munk spectra (E-GM) ( Levine, 2002). There is no sign of NIW. (2) Near TungSha Island E-iw = 13 x E-GM. Strong nonlinear and high-harmonic tides are present. Repetitive trains of large-amplitude NIW appear primarily at a semidiurnal periodicity with their amplitudes modulated at a fortnightly tidal cycle. The rms vertical velocity of NIW shows a clear spring-neap tidal cycle and is linearly proportional to the barotropic tidal height in Luzon Strait with a 1.85-day time lag, consistent with the travel time of internal tides from Luzon Strait to TungSha Island. (3) At the northern SCS shelfbreak E-iw = 4 x E-GM. Single depression waves are found, but no multiple-waves packets are evident. (4) On the continental shelf E-iw = 2 x E-GM. Both depression and elevation NIW exist.