Observations of parametric subharmonic instability of the diurnal internal tide in the South China Sea

Shipboard observations are presented that suggest the occurrence of parametric subharmonic instability ( PSI) of diurnal K-1 and O-1 internal tides at "critical'' latitudes of 14.52 degrees N and 13.44 degrees N, respectively. In a transect spanning 12.5-18 degrees N, depth-mean shear squared shows sharp peaks at 14.52 degrees N ( elevated relative to that at 15 degrees N by a factor of ten) and at 13.44 degrees N (by a factor of 7). Wind speed measured from the ship and Quikscat scatterometer during and before the transect was < 10 m s(-1) at these latitudes. Eight-hour time series ( about 1/6 of an inertial period) of shear and isopycnal depth at 14.52 degrees N are sufficient to associate the elevated shear with alternating, clockwise-rotating layers analogous to those observed at the M-2 critical latitude of 28.8 degrees N.