Observations of parametric subharmonic instability-induced near-inertial waves equatorward of the critical diurnal latitude

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Abstract

[1] Moored current observations of 75 days duration in the northeastern South China Sea (∼20°N) suggest that parametric subharmonic instability (PSI) of semidiurnal (D2) internal tides can not only generate waves of frequencies close to D2/2, but also excite near-inertial waves whose frequencies are different from D2/2. Time series of shear amplitudes clearly show a 14-day cycle. Although near-inertial and near-diurnal motions dominate the shear, this cycle is in phase with the fortnightly spring-neap cycle of D2-waves. After separation of near-inertial and near-diurnal waves using band-pass filters, shear magnitudes for both motions still follow this 14-day cycle, rather than that of diurnal internal tides or variations of the local wind field. This strongly suggests that PSI equatorward of the critical latitude for D2/2 waves (∼29°) not only transfers D2-energy to D2/2 waves, but also to high-mode near-inertial waves. Near-inertial waves (f) and another subharmonic (D2-f), together with D2 waves, compose a PSI-triad following strong interaction.

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