Soliton Arrival Patterns Over the Chinese Upper Continental Slope During June 2014: Reaffirmations and Exploded Myths

A very dense array of sensors measuring temperature, salinity, and velocity was deployed between the 386 m – 266 m isobaths northeast of Dongsha Island in the South China Sea. One complete fortnight was sampled at 90 s or faster intervals, which allowed nonlinear internal waves (NLIW) generated by a wide range of tidal forcing to be observed. Two “families” of waves were observed, the now familiar a-waves and b-waves. The correct way to categorize these waves is by their generation mechanism since their amplitudes and packet structures overlap. The a-waves were generated on the larger of the two ebb tides in the Luzon Strait and the b-waves on the flood tide that immediately preceded it. Details such as the amplitudes and arrival times of the incoming waves very closely tracked the details of the Luzon Strait forcing. Almost all the a-waves were present by the time the waves reached the 386m isobath, however many of the b-waves were formed locally by steepening and breaking of the leading edge of the upslope internal tide. This phenomenon commonly took place between the 342 m and 266 m isobaths. Breaking of individual NLIW was not observed since the orbital speed did not exceed the local propagation speed of 1.69 – 1.87 m/s in this depth range. The increasing number of waves per packet shoreward was instead consistent with the formation of a dispersive “tail” over gently sloping topography. Huge double a-waves two hours apart were observed near spring tide. The origin of these waves is unknown and deserves further study.