Evolution of the semidiurnal (M-2) internal tide on the continental slope of the northern South China Sea

Kinetic energy spectra from a site on the continental slope in the South China Sea reveal that significant peaks appear at some nonlinear interaction frequencies, namely M-3 (M-1 + M-2) and fM(1) (M-1 + f), where f is the inertial frequency, M-1 is the diurnal internal wave, and M-2 is the lunar semidiurnal internal tide. A possible generation mechanism of M-3 is explored. Analysis of bicoherence and shear spectra suggests that strong M-3 is indirectly associated with parametric subharmonic instability ( PSI) of M-2. In another word, under the effect of PSI the energy of M-2 is first transferred to M-1; then via other nonlinear coupling, some nonlinear waves (e.g. fM(1), M-3) are generated. Moreover, M-1 is also present at another site near the bottom of the continental slope. The shear spectra from these two sites show, for the first time, that M-1 can be significantly distinguished from lunar diurnal O-1 and lunisolar diurnal K-1