Observations of Shoaling Nonlinear Internal Waves: Formation of Trapped Cores

Large-amplitude nonlinear internal waves (NLIWs) shoaling on the continental slope in the northern South China Sea are observed. Observed NLIWs often reach the breaking limit, the maximum horizontal current velocity exceeding the wave speed, and trapped cores are formed with recirculating fluid. The conjugate flow does not form. The vertical position of the maximum horizontal velocity is displaced from surface to subsurface, via the formation of the trapped core. Trapped-core NLIWs are strongly dissipative and evolve rapidly into trains of NLIWs. The vertical overturning is as large as 75 m, and the turbulence kinetic energy dissipation rate is estimated as O(10^{-5}) W kg-1. We propose that the formation and the evolution of trapped cores catalyze the generation of the trains of NLIWs on the Dongsha plateau often captured by satellite images and by recent field observations. The generation, evolution, fission, dissipation, and energetics of observed trapped-core NLIWs will be discussed and compared with results of numerical models and laboratory experiments.