Xiaohui Xie

Evolution of the semidiurnal (M2) 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.

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

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.

Variations of diurnal and inertial spectral peaks near the bi‐diurnal critical latitude

Three sets of ADCP data obtained from the upper ocean are used to examine possible influence of parametric subharmonic instability (PSI) at phi(c) approximate to 13 - 15 degrees N. Both kinetic-energy and shear spectra at phi(c) reveal that significant peaks appear at subharmonic frequencies of diurnal internal tides. Especially, the 0.5 K-1 (subharmonic of K-1 tides) can be trapped poleward of its critical latitude (14.52 degrees N), significantly distinguished from wind generated near-inertial internal waves. Moreover, the enhanced 0.5 K-1 motions are more or less subject to a fortnightly spring-neap circle. Relative to a higher latitude (say 18 degrees N), diurnal motions at phi(c) are greatly weakened. In contrast, small vertical-scale motions in the inertial band and the shear induced by these motions are significantly enhanced. Likely, PSI mechanism plays an important role in these observations, whilst trapped sub-inertial waves (0.5 K-1) may be associated with the presence of negative sub-inertial eddies. Citation: Xie, X.-H., X.-D. Shang, G.-Y. Chen, and L. Sun (2009), Variations of diurnal and inertial spectral peaks near the bi-diurnal critical latitude, Geophys. Res. Lett., 36, L02606, doi: 10.1029/2008GL036383.

Poleward propagation of parametric subharmonic instability‐induced inertial waves

This study presents two sets of current records obtained from the South China Sea and satellite altimeter data, and it suggests that near-inertial waves induced by parametric subharmonic instability (PSI) associated with internal tides can be transported poleward beyond their critical latitude phi(c) by background geostrophic flow (BGF). The two mooring locations were poleward of phi(c) (approximate to 14 degrees N) for diurnal subharmonics (0.5D(1); half diurnal frequency D-1); however, both of the current records revealed clear signals at 0.5D(1). The enhanced subinertial motion at 0.5D(1) exhibited a fortnightly spring-neap cycle but did not agree with that of D-1, indicating that it may not be generated via PSI associated with the local D-1. Observations from the altimeter data and a ray-tracing simulation suggested that these nonlocally generated 0.5D(1) waves may be excited near their phi(c), after which they propagated poleward under the role of the BGF to the observation site with a latitude higher than phi(c). The poleward propagation of near-inertial waves can produce elevated vertical shears; thus, it may play an important role in enhancing the local turbulent mixing.