In-Situ Observations of Internal Waves on the Continental Slope and Shelf of the South China Sea

Large amplitude internal waves are a long recognized feature of the South China Sea. Remote sensing imagery often shows the sea-surface manifestation of these waves as they radiate from the Luzon passage into the deep water of the South China Sea. In situ observations have shown that the waves occur as soliton-like coherent packets, creating short wavelength depressions in the thermocline stratification. These waves are described as nonlinear, with their steepness often exceeding a slope of 1/50. In the deep basin west of Luzon, the waves are known to reach displacement amplitudes exceeding 100 m, and persist for several days as they propagate toward China and Vietnam. The packets convert to elevation anomalies as they shoal onto the continental slope, becoming stretched into longer trains of waves. Shoreward of the continental slope, there is little previous in-situ data available for use in quantifying wave energy. Here, we examine survey data from April 2005, allowing for direct assessment of waves on the continental shelf. Measurements of internal waves from the continental shelf of the South China Sea show persistent internal wave activity even during a period of neap tides. Trains of high-frequency waves were observed at multiple locations along the shelf with amplitudes typically reaching 25% of the water depth. Timing of wave packets was not obviously modulated by the tides, and intervals between wave packets were typically 0.5 to 2 hours. Measurements of Acoustic Doppler Current velocity and estimates of wave displacement allow for estimation of wave energy. Our analysis contrasts energy estimates at two sites: the 160-m isobath near the shelfbreak, and the 70-m isobath on the continental shelf. Elevated energy levels were found at the 160-m isobath site, where the depth-integrated baroclinic energy exceeded 10 kJ m-2. In contrast, we found depth-integrated energy levels were less than 1 kJ m-2 at the 70-m isobath. The normalization of energy based on internal tide generation at the shelfbreak suggests that the 160-m isobath observations are indicative of the presence of Luzon waves, while the 70-m isobath observations are indicative of locally generated internal waves. An independent hindcast of wave activity for the survey period supports our interpretation of the results. Our analysis suggests that it is possible to use an energy analysis to identify Luzon waves from local waves on the shelf.