Shoaling Nonlinear Internal Waves In The Northeastern South China Sea

This dissertation deals with the process and implications of shoaling nonlinear internal waves over continental shelf regions in the northeastern South China Sea,where the nonlineear internal waves are energetic and ubiquitous.After decades' investigations,people have in-depth understanding on the generation mechanism and propagation characteristics of nonlinear internal waves.However,the understanding on the process and implication of shoaling nonlinear internal waves is still very limited.In this dissertation,through combing the field measurements,satellite observations,theoretical analysis,and numerical simulations,we analyze the features of the nonlinear internal waves,the shoaling process of nonlinear internal waves,the environmental impacts on the shoaling waves,and the implication of the shoaling nonlinear internal waves on the sea surface chlorophyll.Through field measurements,this dissertation points out the existence of nonlinear internal waves over the steep slope of the northeastern South China Sea.These waves are high-frequency internal waves in a period of about 6 minutes.Their statistical characteristics are revealed based on 12 years' satellite observations.Moreover,two types of nonlinear internal waves with a distinct scale of wave crest length and geographic distributions are identified:Type-I waves have larger wave crest lengths and span a large area from the southern Taiwan Strait to the northern South China Sea,while Type-II waves have smaller wave crest lengths and appear only at the steep slope of the northeastern South China Sea.Further analyses,based on an empirical model of internal wave propagation and on the calculations of the depth-integrated internal wave-generated body force,suggested that Type-I waves are mainly originated from the Luzon Strait,while Type-II waves are locally generated at the shelf break of the northeastern South China Sea.Using a two-layer weakly nonlinear theoretical model,the dissertation reveals that internal wave fission is the generation mechanism of those observed high-frequency internal waves.Additionally,fission is decided by stratification and the corresponding critical point of polarity conversion.Since a fully nonlinear model could provide more accurate simulations of nonlinear internal waves,we apply a fully nonlinear model named Internal Gravity Model(IGW)in this dissertation to investigate the environmental influence on shoaling nonlinear internal waves,such as the effects of initial wave amplitude,seasonal stratification,small-scale topography,multiple background current and viscosity on the shoaling nonlinear internal waves.All of these simulations are carried out in continuously stratified fluid.In addition to steady background shear current,this dissertation also studies the effect of time-varying barotropic tide on internal wave polarity conversion.The results suggest that strong cross-shelf tidal currents could significantly modulate the pycnocline depth,leading potentially to substantial cross-shelf variation of the critical point.It is expected to be shifted onshore by flood tide but offshore by ebb tide.As a result,during flood tide shoaling nonlinear internal waves convert their polarity further onshore of the nominal critical point,while during ebb tide polarity conversion occurs some distance offshore of the critical point.For typical M2 tidal currents on the northeastern shelf of the South China Sea,the critical point could be moved 10 km onshore and 2 km offshore when the maximum tidal current is 0.3 m/s over the continental shelf with the depth of 80 m.This dissertation also indicates that nonlinear internal waves could affect the distribution of chlorophyll concentration in the sea surface.Observations of ocean colour remote sensing show an asymmetric distributing feature of chlorophyll concentration around the Dongsha Atoll.The spatial distribution is suggested to be induced by the shoaling nonlinear internal waves.Based on numerical simulations,the dissertation illustrates that shoaling internal waves of depression could track surface particles to move towards the right side of shoaling path.High chlorophyll water near the atoll is transported offshore in the north of the Dongsha Atoll,but no source could support chlorophyll in the south of the atoll.As a result,the satellite observations show a center of high chlorophyll concentration in the north while low chlorophyll concentration in the south of the Dongsha Atoll.