Latitudinal Structure of Solitons in the South China Sea

Four current-meter moorings and 12 pressure sensor–equipped inverted echo sounders (PIES) were deployed during summer 2011 in the South China Sea. The goal of the experiment was to obtain synoptic observations of the large-amplitude nonlinear internal waves from the near field to the far field as they propagated west-northwest across the sea. The program was unique because it was the first to observe the latitudinal variability of the wave crests in addition to the transformations along a single east–west transect. The waves were strongest down the center of the PIES array along roughly 20°45′N and were weaker off axis in both directions. Both a-waves and b-waves arrived earlier in the south than the north, but with different lag times indicating different propagation directions and therefore different sources. The waves were classified by their arrival patterns and source locations and not by their amplitude or packet structure. The Stanford Unstructured Nonhydrostatic Terrain-Following Adaptive Navier–Stokes Simulator (SUNTANS) model, calibrated against the array, showed that the a-waves developed out of the internal tide spawned in the southern portion of the Luzon Strait and the b-waves originated in the north. The northern tides were refracted and suffered large dissipative losses over the shallow portion of the western ridge, whereas the southern tides propagated west-northwest unimpeded, which resulted in a-waves that were larger and appeared sooner than the b-waves. The results were consistent with previous observations that can now be understood in light of the full three-dimensional structure of the internal waves and tides in the northeastern South China Sea.