A study of nonlinear internal waves in the northeastern South China Sea

The generation, propagation, and polarity conversion of nonlinear internal solitary waves (ISWs) in the northeastern South China Sea (SCS) are studied based on satellite images, field measurements and numerical models. One hundred and sixteen ISW packets are observed in remote sensing images acquired from 1995 to 2001. Temperature profiles measured on the South China shelf in the ASIAEX 2001 are also studied.; In shallow water internal wave theory, ISWs in stratified oceans can be depression, elevation, and broadening waves, depending on the signs of nonlinearity coefficients. Satellite images can record in the spatial domain a continuous evolution process from depression, via broadening, to elevation ISWs, under conditions of bottom shoaling both in the propagation direction and in the transect direction. The conversion process can be obtained by transferring information from the spatial domain to the temporal domain. Ocean environment parameters can be estimated. The polarity conversion process is simulated using the variable coefficient EKdV equation.; ISWs in the northeastern SCS can fall into two categories: single-wave ISW packet and multiple-wave ISW packet. That all the 22 single-wave ISW packets occur in the deep water zone reveals their uniqueness. It is suggested that these ISWs originate from nonlinear steepening of internal tides, which are induced by tide topography interaction in the Luzon Strait. An ERS-2 SAR image records such an evolution process from an internal tide to a single ISW.; The role of tides in the generation of large amplitude ISWs is studied. The 26-day series of temperature profiles and the tide level data from tide stations are used to confirm a phase-locked relationship between internal tide and surface tide. It is found that the ISWs appear on the front faces of internal tide troughs, which correspond to the westward tidal current phases in the tidal current curve. It is also found that it takes ISWs about 62.4 hours to propagate from the Batan Islands to the Dongsha coral reef. These argue for the internal tide mechanism. All the characteristics of the ISWs and internal tides can be explained by examining the westward tidal current variation.