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°N and 13.44°N, respectively. In a transect spanning 12.5–18°N, depth‐mean shear squared shows sharp peaks at 14.52°N (elevated relative to that at 15°N by a factor of ten) and at 13.44°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°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°N.