Study On The Structures And The Kinetics Characteristics For The Subsurface And Intermediate Water Masses In The South China Sea

The South China Sea (SCS) is the largest marginal sea in the North Pacific Ocean. Since its unique geographic location and the varied topography, the SCS shows its particularities in the aspects of water inherent features, the way and the pattern of motions. All of these attract the oceanographers. The subsurface and intermediate water masses of the SCS take on prominent characteristics in the vertical. The whole environment in the SCS is influenced by the distributions and variations of physical and chemical properties and also by the circulation pattern.Based on collecting and processing the historical cruise data, a climatological dataset of the hydrographic elements is accomplished in this thesis. Combined with the Argo float data, sea level anomaly, evaporation and precipitation reanalysis data, the seasonal and regional variations of the elements are discussed in this thesis, mainly focusing on the vertical and horizontal distribution for temperature, salinity, dissolved oxygen (DO) and nutrients (including nitrate, phosphate and silicate). At last, it is given of an updated circulation structures on the core depths of subsurface and intermediate water masses in the statistical sense.In the study of analysis of the SCS water masses, it is the first time to apply dissolved oxygen data to trace typical water body. There is a better relation between the DO concentration and the water masses than that between either temperature or salinity and the water masses in the deep sea. Moreover, DO concentration has a much higher resolution in the deeper layers.T-S curves showed a reversed "S" vertical structure for the water near the east of Luzon Strait (LS) and the inner SCS. The salinity reached its maximum and minimum value at the core layers of subsurface and intermediate waters respectively. It is similar to the vertical structure of dissolved oxygen. The DO maximum and minimum values corresponded well to the upper boundary of subsurface water and lower part of intermediate water. The nutrients first showed an increasing trend from the surface down to a certain depth and then kept steady in the abyss, and there were no clear seasonal variations in the distribution of nutrients.On the subsurface water level, the kinetic mechanism could be figured out in the seasonal variations of Kuroshio intrusion and hydrologic elements. The statistic results in this thesis show that the intrusion mainly occurred at the north of the LS and extending to the south of the SCS along the continental slope in the west. The intrusion path is different from the way mentioned in hypothesis of "loop current" in which it was believed that the Kuroshio branch came to the SCS through the south end of the LS and turned back through the north. It was also shown that the intrusion depended on the SCS monsoon. In winter and summer, it was strong and turned weak in spring and autumn. It also keeps the balance with the water near the west of Palawan Island which is characterized by the low temperature, low salinity and low dissolved oxygen. The characteristics of subsurface water could respond to the winter monsoon rapidly. The distributions of dissolved oxygen and nutrients content follow the regulations:DO concentration is higher in the Kuroshio Current region and lower in the SCS; the low concentration core in the SCS occurrs at the southeast. Additionally, the distribution of nutrients are opposite to DO, i.e. it is higher in the SCS than the North Pacific, and it is higher at the south than the north in the SCS, but this opposite distribution could reflect the same water movement trends.Based on the multi-elements analysis on the core level of the intermediate water, the results show that a seasonal variation for DO distribution exists. The water exchange through the middle and north part of the LS is more complex than that in the subsurface water. A kind of compensatory effect in the continuous water body is the main mechanism which can cause the seasonal phenomenon. The nutrients concentrations in the intermediate water have the same regional structures with the subsurface water, but the difference is that there is no obviously seasonal variation for them.Furthermore, the thesis also discusses the annual mean and seasonal mean geostrophic current under the sign of climatology and gives the dynamical mechanism for the distributions. There are the basin-scale cyclonic circulations on the core level of subsurface and intermediate water masses and the circulations change in the north of the SCS during winter monsoon. The dipole shows during summer and it has the half-year time scale characteristic in the south of the SCS. Considering the sea level anomaly simultaneously, two subbasin-scale phenomenoa exist in the west of Luzon Island and offshore region in the north of Vietnam. The time scale is about one season and the spatial scale is about several hundred kilometers.The Freshwater Fraction Method is applied to calculate the flushing time and water transport between the SCS and the North Pacific Ocean. The conservation of salt in the sea water, the salinity difference between east and west of the LS and the steady volume of freshwater input are the key points for this experiment. From the results, most of the water exchange occurs on the upper 400m, the advection dominates over diffusion in the water exchange through the LS.