Nonlinear interactions among internal tidal waves in the northeastern South China Sea

Abstract

We used a set of 75-day long ADCP data from the northeastern South China Sea (SCS) to investigate nonlinear interactions among freely propagating internal tidal waves. The kinetic energy spectra displayed significant peaks at some higher tidal frequencies, such as O1M2 (O1+M2), and M4 (M2+M2), where O1 is the lunar diurnal internal tide, M2 is the lunar semidiurnal internal tide, and M4 is the first higher harmonic frequency of M2. These higher tidal harmonic frequency peaks, as well as the fundamental tidal harmonic peaks, show a σ−2.3 spectral falloff rate with frequency. In addition, we explored the possible generation mechanism of higher tidal harmonics. Analysis on the rotary and bicoherence spectra suggests that strong forced non-resonant interaction induced by nonlinear advections was the dominant physical mechanism that induced these higher tidal harmonics. Moreover, the energetic, freely propagating semidiurnal (M2) internal tidal wave played the most crucial role in these interactions. These results indicate that strong nonlinear forced non-resonant interactions among internal tides can be one of the processes responsible for the redistribution of energy in the internal wave spectrum.