Characterizing the nonlinear internal wave climate in the northeastern South China Sea

Four oceanographic moorings were deployed in the South China Sea from April 2005 to June 2006 along a transect extending from the Batanes Province, Philippines in the Luzon Strait to just north of Dong-Sha Island on the Chinese continental slope. The purpose of the array was to observe and track large-amplitude nonlinear internal waves (NIWs) from generation to shoaling over the course of one full year. The basin and slope moorings observed velocity, temperature (T) and salinity (S) at 1-3 min intervals to observe the waves without aliasing. The Luzon mooring observed velocity at 15 min and T and S at 3 min, primarily to resolve the tidal forcing in the strait. The observed waves travelled WNW towards 282-288 degrees with little variation. They were predominantly mode-1 waves with orbital velocities exceeding 100 cm s(-1) and thermal displacements exceeding 100 m. Consistent with earlier authors, two types of waves were observed: the a-waves arrived diurnally and had a rank-ordered packet structure. The b-waves arrived in between, about an hour later each day similar to the pattern of the semi-diurnal tide. The b-waves were weaker than the a-waves, usually consisted of just one large wave, and were often absent in the deep basin, appearing as NIW only upon reaching the continental slope. The propagation speed of both types of waves was 323 +/- 31 cm s(-1) in the deep basin and 222 +/- 18 cm s-1 over the continental slope. These speeds were 11-20% faster than the theoretical mode-1 wave speeds for the observed stratification, roughly consistent with the additional contribution from the nonlinear wave amplitude. The observed waves were clustered around the time of the spring tide at the presumed generation site in the Luzon Strait, and no waves were observed at neap tide. A remarkable feature was the distinct lack of waves during the winter months, December 2005 through February 2006.