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Keywords:

  • internal tides;
  • critical latitude;
  • parametric subharmonic instability

[1] Three sets of ADCP data obtained from the upper ocean are used to examine possible influence of parametric subharmonic instability (PSI) at ϕc ≈ 13–15°N. Both kinetic-energy and shear spectra at ϕc reveal that significant peaks appear at subharmonic frequencies of diurnal internal tides. Especially, the 0.5 K1 (subharmonic of K1 tides) can be trapped poleward of its critical latitude (14.52°N), significantly distinguished from wind generated near-inertial internal waves. Moreover, the enhanced 0.5 K1 motions are more or less subject to a fortnightly spring-neap circle. Relative to a higher latitude (say 18°N), diurnal motions at ϕc 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 K1) may be associated with the presence of negative sub-inertial eddies.