Investigation On The Circulation And Coastal Trapped Waves In The Northern South China Sea

The continental shelf sea is rich of resources, and is closely related to Human life and economic development. It is really important to investigate the continental shelf circulation for the better development and use of resources on the continental shelf. The circulation on the shelf of the northern South China Sea have a greate effect on the transport of coastal low-salinity water, sediment, pollution, and also ecosystem in the northern South China Sea. There have been lots of research on the circulation in the northern South China Sea, however, during summer season there are still some controversy about the dynamic mechanism and interaction between the coastal current west of Guangdong, the Qiongzhou Strait current and circulation in the Gulf of Tonkin. During winter season, the driving mechanism of the counter-wind current in the northern South China Sea still need clarification. The coastal-trapped waves have a period of several days, and play an important role in the circulation of shelf. In addition, typhoons influence the northern South China Sea frequently. There are few research on the current and water exchange contributed by the coastal-trapped waves with a short time scle due to tropical storms. Based on the analysis of observed data and an advanced multi-scale high resolution three-dimensional baroclinic numerical model, a series of model experiments are constructed to investigate the dynamic mechanism of the circulation in the northen South China Sea, and explained the interaction and influence factors of along-shore current west of Guangdong, Qiongzhou Strait flow and circulation in the Gulf of Tonkin. We studied the driving mechanism of northeastward counter-wind current on the northern South China Sea shelf during wind relaxation. We raised some problems when modeling the South China Sea Warm Current over slope region under sigma coordinate models, and the improvement methods are also proposed. This thesis also discussed the general characteristics, propagation and influence factors of the coastal trapped waves in the northern South China Sea. Some main new results are summarized as follows:1. The long-time current measurement in the Gulf of Tonkin is analyzed and it shows that the low-frequency current during summer is not fully controlled by wind. Based on a Global-Gulf of Tonkin nested model, we investigated the forcing mechanism of the summer time circulation in the Gulf of Tonkin. There exists a cyclonic circulation in the Gulf of Tonkin during both winter and summer seasons. The circulation pattern in the Gulf of Tonkin during winter is consistent with the traditional view of cyclonic circulation, however, the observed low-frequency current is mainly toward northwest or west, which is in contrast with the traditional theory of wind-driven barotropic circulation pattern. The summertime cyclonic circulation in the northern Gulf of Tonkin forms as a result of the combination of stratified wind-driven circulation and tidal-rectified inflow. This paper also investigates the effect of Hong River on the Gulf of Tonkin circulation, and the influence of the interaction between southwest monsoon and the Hong River on the cyclonic circulation in the Gulf of Tonkin is also discussed. The Hong River has a significant effect on the summertime circulation in the Gulf of Tonkin. Since the bathymetry in the northern region is relatively flat, the river plume water could turn cyclonically northward with southwest monsoon forcing during the summer season. The interaction between the southwest monsoon and buoyancy-driven flow from the Hong River can significantly intensify the cyclonic circulation near the surface.2. The current system of coastal current west of Guangdong, the Qiongzhou strait current, and circulation in the Gulf of Tonkin is described and analysised systematically based on a 3-D baroclinic model focused on the continental shelf sea of the northern South China Sea. The dynamic mechanism and influence factors of the southwestward coastal current west of Guangdong in summer and the westward Qiongzhou Strait current are examined based on process-oriented numerical experiments:the Zhujing River plume plays an important role in the formation and development of southwestward coastal current west of Guangdong during summer. The summertime southwestward coastal current satisfies a geographic balance in which a dominant balance was observed between the combined effects of barotropic and baroclinic pressure gradient and Coriolis force. Not only the barotropic response of the current to sea level change is significant, but also the baraclinic response to buoyancy forcing contributes to the southwestward coastal current west of Guangdong. The contribution of the southwestward coastal current to the westward Qiongzhou Strait current can not be ingnored, but it is not the main driving machansim. Tidal rectification inside the strait plays a major part in the westward Qiongzhou Strait current.3. Our research reveals the dynamic mechanism for the transient northeastward counter-wind current on the northern South China Sea shelf during wind relaxation period:The intermittent counter-wind current on the shelf are mainly barotropic during wind relaxation; During the initial days of wind relaxation, the northeastward along-shelf pressure gradient provide the initial driving force for the northeastward counter-wind current. The counter-wind current gradually adjusts to reach a geographic balance following the sea elevation adjustment on the shelf, and at this time the cross-shelf pressure gradient forcing is considered to be the major forcing to maintain the northeastward counter-wind current. We also pointed out that the southwestward coastal current and slope current in the northern South China Sea affects the distribution and adjustment of sea elevation on the shelf, which contributes to the northeastward counter-wind current.4. When typhoon approach the South China Sea from western Pacific Ocean, southwestward along shore wind is formed in the northern South China Sea, which induces onshore Ekman transport and water piles up along the coast. Thus, the source of coastal trapped waves will be generated and the wave propagated southwestward along the coast. The wave split into two waves, with one wave continued propagating westward after passing through the Qiongzhou Strait, while the other wave was observed trapped by the Hainan Island and traveling anti-clockwise around the island in the model as an island trapped wave. Most the wave energy is limited around the coastal region. We examined the spatial structure and characteristics of the coastal trapped waves, and the different features of coastal trapped waves induced by winter storm and typhoons are also investigated. The coatal trapped waves propagating southwestward along the coast can be reinforced or decreased by the onshore or offshore transport due to local wind forcing. The abrupt change of the shelf width and coastline around Leizhou Peninsula and Hainan Island are responsible for strong scattering of CTWs from one mode into higher modes, which can be used to explain the change of propagation speed when the coastal trapped wave encounters the Qiongzhou Strait.