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 .