南海南部内潮的季节和年际变化特征及其影响因素

Internal tides (ITs) in the South China Sea (SCS) are very active with extremely large amplitudes in the world’s ocean. They could impose drastic impacts on a variety of physical, ecological and mixing process in the SCS. There have been lots of studies on the ITs in the SCS, however, until recently, they mainly restrict to the northern SCS, such as Dongsha Islands and Luzon Strait. There also exist strong ITs in the southern SCS due to the extremely complicated topography, but studies of ITs in this area remain very rare. Moreover, our recent study indicates that, the characteristics of seasonal and interannual internal tides in the southern SCS may be modulated by the nonlinear interaction. In this project, the ten-year-long current profile near Yongshu Reef are collected, jointly with the in-situ temperature, salinity and wind data, and a three dimensional numerical model is established to study: (1) the seasonal and interannual variability of the ITs and the physical mechanism, (2) the nonlinear interaction between ITs or between ITs and other internal waves, (3) the impact of typhoon and background ocean environment on the propagation and evolution of ITs. This would be important to the physical oceanography, regional oceanology and a better understanding for the characteristics, propagations, evolutions and dynamic mechanism of the ITs in the different areas of the whole SCS.

南海是全球内潮活跃的海域之一，波动振幅极大，对南海动力、生态和混合都有着重要影响。虽然关于南海内潮已经有了大量研究，但是其主要集中在南海北部东沙群岛和吕宋海峡附近海域。南海南部存在着大量岛礁，地形变化也非常复杂，因此也有着强内潮的存在，但是这一海域的内潮研究仍然非常不足。此外，基于我们最新的研究成果提出假设，南海南部内潮的季节性和年际变化有可能受到非线性作用的影响。因此，本项目将通过分析近10年的南沙永暑礁附近海域连续剖面流速、温盐和风场的实测资料，结合三维数值模拟技术，研究南海南部内潮的季节和年际变化特征及其物理机制，分析内潮与内潮以及内潮与其他内波之间的非线性相互作用的规律和机制，揭示台风和海洋各种背景场对内潮传播和演化的影响，弥补南海南部内潮研究力度的不足，从而从整体上全面、系统的把握南海不同海区内潮的特征、产生机制和演变规律。

本项目利用实测资料、再分析数据和数值模式，研究了台风诱发的近惯性内波的水平传播特征及其影响因素，以及近惯性内波与内潮的非线性相互作用过程和机制，揭示了非线性相互作用和海洋环境因素（台风、涡旋、黑潮和层结等）对内潮传播和演化的影响，从整体上全面、系统的研究了南海不同海区内潮的特征、产生机制和演化规律，完善了南海内潮的研究工作，具有重要的物理海洋学和区域海洋学意义。我们发现每个台风过境后，其诱发的近惯性内波和全日潮都会通过强非线性相互作用产生fD1波，主要由垂向非线性动量项的第一项控制，极大地受到近惯性流场垂向剪切的影响。同时发现，背景场的涡旋可以提高全日潮的垂向速度，进而增强非线性首项，从而有利于台风过后通过非线性作用将能量传递到fD1波段。此外，黑潮入侵可以增强全日潮流的垂向剪切，从而诱发O1和K1内潮的非线性相互作用，生成D2内潮，由于其与半日内潮频率相近，进而增强半日内潮，并且这种机制会对半日内潮的月变化和季节变化产生影响。其中，跨隙（Leaping）方式的黑潮入侵最有利于本研究中的观测点的非线性作用过程。最后，利用实测资料，研究了大量涡旋实例及其对应的正压潮和内潮变化特征，证明了正压潮流通过背景涡旋场时可以激发内潮的生成。