南海次级中尺度能量向中尺度运动逆向级串的数值模拟研究

The oceanic currents are known to be highly turbulent involving a very broad range of scales from O(1000km) down to O(1 cm) and even smaller scales. And it can be driven by multi-scale nonlinear interactions that can transfer energy upscale or downscale. Mesoscale eddies take the most of the kinetic energy over the global ocean. It is believed that the release of the available potential energy of large scale circulation by baroclinic instability is the main reason to generate mesoscale eddies. However, recent high resolution observations and simulations revealed that the energetic submesoscale processes could transfer energy though an efficient inverse cascade by nonlinear interaction, then it has the capability to maintain or generate mesoscale eddies and modulate the large scale ocean circulation. In some selected regions, such as Kuroshio extension area, high resolution simulation results indicate that the impact of the submesoscale activities caused by the upper ocean mixed layer instability on mesoscale kinetic energy overcome all the other dynamical regimes, and leading to a strong seasonal modulation of mesoscale eddies. In South China Sea, remote sensing and simulation indicate that submesoscale activities are ubiquitous in northern, middle South China Sea and the area close to Luzon strait. There are several generation mechanisms for submesoscale activities, besides the one that mesoscale frontogenesis, mixed layer instability set up by large scale atmospheric forcing, interaction of Kuroshio and islands in Luzon Strait and nonlinear Ekman suction also generate numerous submesoscale structures. It is reasonably to make the assumption that the energy inverse cascade of submesoscale process will play a role in modulating the mesoscale field and large ocean circulations in South China Sea. In order to explore the impacts of submesoscale processes on mesoscale dynamics, we first (1) try to make a high resolution simulation in South China Sea with a 2-way nesting technology AGRIF. (2) Then we statistically assess our model results using the available observations; analyze the space-temporal variations of submesoscale activities in different parts of South China Sea (3) Using the modeled data, the energy flux will be estimated based on a wavenumber spectral analysis method. This project will construct a high resolution dataset in South China Sea for the future submesoscale related studies and help deeply understand the generation mechanism of mesoscale eddies in South China Sea.

海洋流场是包含从一千千米尺度的大洋环流，一直到厘米级的小尺度湍流的多尺度动力过程，不同尺度的非线性相互作用可以使能量在多尺度动力空间上输运。中尺度涡蕴含巨大海洋动能，一般是通过斜压不稳定释放大尺度环流蕴含的有效位能而产生。然而，高分辨数值模拟和观测发现，某些海区通过次级中尺度能量逆向级串到中尺度过程，维持甚至是形成中尺度涡，进而通过该机制实现次级中尺度动力过程对环流结构进行调控。在南海，通过黑潮与岛屿相互作用 (2) 南海季节性锋面附近以及中尺度涡的锋生（3）非线性Ekman抽吸作用产生大量的次级中尺度运动，我们有理由认为这些具有高动能的次级中尺度运动能够对南海的尺度涡造成一定的影响。该项目将利用高分辨率模式产生的三维海洋温盐、动量场，归纳南海次级中尺度运动的时空变化规律，确定次级中尺度运动成因，并最终揭示次级中尺度运动通过能量逆向级串对南海中尺度涡以及环流调控机理。

海洋流场是包含从一千千米尺度的大洋环流，一直到厘米级的小尺度湍流的多尺度动力过程，不同尺度的非线性相互作用可以使能量在多尺度动力空间上输运。中尺度涡蕴含巨大海洋动能，一般是通过斜压不稳定释放大尺度环流蕴含的有效位能而产生。我们研究发现在南海，次级中尺度运动能够为中尺度能量。通过分析南海高分辨率的模拟数据（相对涡度、动能等变量），我们发现次级中尺度具有明显的季节性分布特征，冬季次级中尺度运动活跃度（动能）远远大于夏季次级中尺度活动（大约大一个量级），而春秋二季次级中尺度运动活跃度则弱于冬季，强于夏季。通过涡度分析，我们发现南海次级中尺度涡旋主要分布于南海暖流海区，吕宋海峡、越南沿岸流附近。在南海北部南海暖流附近次级中尺度运动主要是由风驱动的次级非线性Ekman次级环流，以及沿岸锋面不稳定产生；在吕宋海峡海区，强大的黑潮与吕宋海峡内的岛屿相互作用，岛屿尾流在常年产生大量的次级中尺度涡旋；而在越南沿岸流海区，射流不稳定是该海区形成次级中尺度运动的主要原因。通过能量谱分析我们得到重要结论：次级中尺度运动能够向中尺度涡旋逆向能量级串，虽然只占整中尺度涡旋能量的5（夏季）~10（冬季）%，但是南海这一中尺度涡旋的维持机制第一次被提出。通过结合高分辨率模拟数据，我们第一次在锋面附近用卫星数据证实了非线性Ekman次级环流的存在。和我们还发现海浪对上层海洋混合、动量通量有重要调节作用并进一步影响次级中尺度运动。