The Deformation of Shoaling Internal Waves Observed at the Dongsha Atoll in the Northern South China Sea

The deformation of nonlinear internal waves (NLIW) from depression to elevation waves during the shoaling process has been attributed to three key factors: wave amplitude, mixed-layer depth and water depth. This study examined the critical conditions for wave deformation by means of five strings of thermistors deployed at the Dongsha Atoll in the northern South China Sea (SCS). During a two-day experiment, four wave trains consisting of 78 solitons at five sites were identified. The parameters related to soliton transformation were formulated by applying the extended Korteweg-de Vries (eKdV) theory. The results indicated that the soliton energy dispersed dramatically along the sharp bottom slope, likely as a result of bottom friction and turbulence mixing. Large-amplitude depression waves were observed to transform into elevation waves before the thickness of the lower layer became equal to that of the upper layer. The ratio of the wave amplitude to the lower-layer thickness was found to be a good indicator of wave deformation. The critical conditions for the transition of the internal waves (IWs) occurred at the ratio approximately equal to 0.66 +/- 0.2. This means that a bottom-trapped elevation wave could form before passing through the theoretical critical point. The solitons felt the bottom and deformed when their amplitudes approached half of the lower-layer thickness. The solitons existed in the form of elevation waves when the waves were in contact with the bottom.