南海东北部开敞性海湾余环流特征及形成机制研究

Understanding the generating mechanism and temporal and spatial variation of residual current is important to study the exchange and mixing of the seawater, diffusion of contaminant, transport of nutrients and ecological environmental protection of the bay. The residual currents at Honghai bay and Jieshi bay located at east Guangdong province in the northeast South China Sea has different characteristic from semi-diurnal bays. The variation of the residual current is faster than semi-diurnal bays. The residual currents are larger in the middle of the bay than along coast. We shall study the distribution and variation of the residual current and find the basic dynamic factors. To this end, we shall build a three dimensional unstructured grid high resolution Honghai bay and Jieshi bay model nested with a larger South China Sea model based on the deep analysis of the observations. The generating mechanism of the circulation in summer and winter in these adjacent bays will be studied. The reason why the mean sea level at east and west coast of each bay are so different will be figured out. The dynamic factors for short term variation（3~4days）of the residual current will also be found out. After the project is completed, the theory of circulation in the shallow China seas will be improved.

了解海湾余流的生成机制及其时空变化特征对研究海湾的海水交换和混合、污染物扩散和稀释、营养盐的输运及生态环境保护具有重要意义。南海东北部粤东红海湾和碣石湾的余环流，具有与半日潮开敞性海湾显著不同的特征：余流流速量级和潮流的量级相当；余流场的时间变化尺度，要远小于半日潮海湾；海湾中央余流流速大，近岸则小。为深入研究其余环流分布和变化的规律，找出形成这些特征的主要动力学因子，我们在对观测资料深入分析的基础上，用FVCOM模型，建立南海三维、斜压、无结构网格、大模型嵌套细网格、高分辨率的红海湾、碣石湾区域模式，研究这两个相邻海湾冬、夏季环流的形成机制，找出海湾东西两侧海平面显著差异之成因，同时，要研究余流短周期变化（3~4天）。在本项目完成之后，将进一步完善中国浅海海流的动力学理论。

项目基于大量的观测资料及数值模拟手段，从潮汐余流、局地风海流、大尺度环流、陆架陷波等角度对南海开敞性海湾（以红海湾为例）冬季和夏季余流的时空变化特征及产生机制进行了深入的探讨和研究。研究表明，导致红海湾余流变化的关键因子为风，这包括局地和“上游”的风，其中局地风场产生与风场方向基本一致的风海流，而上游的强风产生的陆架波会沿着南海北部沿岸自东向西传播，从而引起海湾余流变化，观测和数模均发现了两种不同类型陆架波对海湾余流造成的影响，其中低水位传播的陆架波会导致海湾余流朝东（与陆架波传播方向相反），高水位陆架波经过时则相反；观测发现在持续强风停止后，由于压强梯度力的作用，湾内会产生反向的补偿性的流动；在岬角地形的阻挡下，沿岸流的强度及与岸线的角度也会影响湾内环流形态。项目对夏季包含台风和热带低压过程的海流连续观测的资料分析研究，一定程度上填补了强海况下风和陆架波如何影响南海北部海湾余流方面研究的空白。