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Moored current observations of 75 days duration in the northeastern South China Sea (similar to 20 degrees N) suggest that parametric subharmonic instability (PSI) of semidiurnal (D(2)) internal tides can not only generate waves of frequencies close to D(2)/2, but also excite near-inertial waves whose frequencies are different from D(2)/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 D(2)-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 D(2)/2 waves (similar to 29 degrees) not only transfers D(2)-energy to D(2)/2 waves, but also to high-mode near-inertial waves. Near-inertial waves (f) and another subharmonic (D(2)-f), together with D(2) waves, compose a PSI-triad following strong interaction. Citation: Xie, X.-H., X.-D. Shang, H. van Haren, G.-Y. Chen, and Y.-Z. Zhang (2011), Observations of parametric subharmonic instability-induced near-inertial waves equatorward of the critical diurnal latitude, Geophys. Res. Lett., 38, L05603, doi: 10.1029/2010GL046521.

Moored current observations of 75 days duration in the northeastern South China Sea (similar to 20 degrees N) suggest that parametric subharmonic instability (PSI) of semidiurnal (D(2)) internal tides can not only generate waves of frequencies close to D(2)/2, but also excite near-inertial waves whose frequencies are different from D(2)/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 D(2)-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 D(2)/2 waves (similar to 29 degrees) not only transfers D(2)-energy to D(2)/2 waves, but also to high-mode near-inertial waves. Near-inertial waves (f) and another subharmonic (D(2)-f), together with D(2) waves, compose a PSI-triad following strong interaction. Citation: Xie, X.-H., X.-D. Shang, H. van Haren, G.-Y. Chen, and Y.-Z. Zhang (2011), Observations of parametric subharmonic instability-induced near-inertial waves equatorward of the critical diurnal latitude, Geophys. Res. Lett., 38, L05603, doi: 10.1029/2010GL046521.