南海中尺度涡旋对近惯性运动的调制机理研究

Near-inertial motions have frequencies close to local inertial frequency and are principal component of ocean energy spectra. They share similar power input with internal tides, and show strong vertical shear. Consequently, they are key and prevalent research topic of oceanic mixing. Since near-inertial motions possess similar length scale with mesoscale eddies, they are widely modulated by mesoscale eddies. However, the modulation mechanism of near-inertial motions by mesoscale eddy still remains unclear at the moment. The South China Sea has frequent typhoon passage, strong winds, complicated topography and active circulation, thence there are active near-inertial motions and mesoscale eddies. Thus the South China Sea is an ideal sea for the research of the modulation mechanism of near-inertial motions by mesoscale eddies. The project explores the modulation mechanism of near-inertial motions by mesoscale eddies based on in-situ observation analysis and high resolution numerical model experiment. The project intends to reveal the role played by mesoscale eddies in the generation and propagation processes of near-inertial motions and the effect of mesoscale eddy type, vertical mode, intensity and horizontal length scale. The results of the project would provide new knowledge on modulation mechanism of near-inertial motions by mesoscale eddies, and are helpful to understand the oceanic mixing and evaluate oceanic mixing energy source of the South China Sea.

近惯性运动是频率接近局地惯性频率的海洋运动，是海洋能量谱的重要组成部分。近惯性运动拥有和内潮接近的能量源，而且表现出很强的垂向剪切，因此是海洋混合研究的核心和热点内容。由于近惯性运动的空间尺度同海洋中尺度涡旋接近，所以它们容易受到中尺度涡旋的影响，然而目前对中尺度涡旋调制近惯性运动机理的认识依旧不清晰。南海存在频繁的台风过境、大风过程、复杂的地形和强烈的环流，因此蕴含着活跃的近惯性运动和丰富的中尺度涡旋过程，是研究中尺度涡旋调制近惯性运动的天然试验场。本项目拟基于现场资料分析和高分辨率数值模式实验，探究南海中尺度涡旋对近惯性运动的调制机理，揭示中尺度涡旋在近惯性运动生成和传播过程中的作用，探明该过程对中尺度涡旋的类型、模态、强度和水平尺度等特征参数的依赖性。本项目可提高对南海中尺度涡旋调制近惯性运动特征与机理的认知水平，为认识南海混合过程及评估海洋混合能量源提供指导。

近惯性内波是海洋内波的重要组成部分，其空间尺度和中尺度涡旋接近，中尺度涡旋对近惯性内波的调制规律尚未明确。本项目基于再分析、遥感和现场观测资料以及高分辨率数值模式，对中尺度涡旋调制下近惯性内波的生成和传播规律进行了细致研究，主要研究成果有：首先，评估了全球风场向海洋的近惯性能通量，该能通量为0.56TW并呈现出时间和空间变化，而且中尺度涡旋、混合层厚度、风场时间分辨率对该能通量有显著影响；其次，分析了西北太平洋对慢移速强台风的响应，发现慢移速台风不能有效激发近惯性运动且台风移速是影响海洋响应的重要因素。再次，利用数值模式研究了中尺度涡旋调制下近惯性内波的生成和传播特征，发现该调制和涡旋的垂向模态密切相关；正压涡旋的调制主要表现为通过折射效应改变近惯性内波的传播方向且气旋和反气旋涡旋的作用相反；斜压涡旋调制下存在近惯性内波能量从低阶向高阶模态的转移，即产生高阶模态近惯性内波，且斜压涡旋的水平尺度是决定高阶模态近惯性内波生成的因素。本项目的科研成果完善了中尺度涡旋调制下近惯性内波生成和传播规律的认识，补充了中尺度涡旋调制内波的理论体系，可为海洋内部混合能量源的评估和海洋内波的数值预报提供参考。