西沙多岛礁尾流气旋、反气旋涡演化特征及不稳机制研究

Wake eddy turbulence induced by the island (reef) is an important mechanism for the island mass effect, therefore, it is of great significance to study the instability mechanism of island (reef) wake eddy. However, if considering the multi-island (reef) wake eddies with actual complex terrain, and with the geostrophic and stratified environment, the instability mechanism of cyclone-anticyclone is unclear to a certain extent. The Xisha Archipelago is characterized as it’s multi-island (reef). Hence, the present project, chooses cyclone-anticyclone generated by the multi island (reef) wakes in Xisha Archipelago, as the research object. We use numerical simulations, combined with satellite remote sensing and in situ data, for the scientific research of evolution and instability mechanism of cyclone-anticyclone generated in the multi-island (reef) wakes under the control of geostrophic and stratified environment. The specific contents of this study include: High resolution ocean dynamic model research for the sub mesoscale island (reef) wake, under the control of regional background circulation in Xisha Archipelago; Based on the model results of the Xisha three-dimensional high resolution ocean dynamic products, coupled with satellite and in situ observation, we plan to analyze the wake eddies’ evolution characteristics of multi-island (reef) in the Xisha Archipelago; Numerical experiments, concerned with the single island (reef) and multi-island (reef) wake eddies, under the control of geostrophic and stratified environment, are conducted; Together with the dynamic analysis, the cyclone-anticyclone instability mechanism of the multi-island (reef) wakes in Xisha Archipelago are studied. The purpose of this project is to reveal the internal dynamic mechanism of the evolution of multi-island (reef) wake eddies in geostrophic and stratified environment, and provide a basis for further exploring the ecological process of island wakes in Xisha Archipelago.

岛礁尾流涡紊动作用是岛质效应的重要驱动机制，研究其尾流涡不稳机制具有重要意义。然而，考虑实际复杂地形、以及地转、分层环境下的多岛礁尾流，其气旋、反气旋涡不稳机制尚不十分清楚。本项目选择具有独特地形的西沙多岛礁尾流气旋、反气旋涡为研究对象，采用数值模拟，结合卫星遥感、现场监测，对地转、分层环境下的多岛礁尾流气旋-反气旋涡演化特征及不稳机制进行基础科学研究。具体内容包括：研究论证能够模拟大区背景环流影响下的西沙次中尺度岛礁尾流的高分辨率海洋动力数学模型；基于模式结果——西沙高分辨率三维海洋动力要素产品以及卫星资料、现场观测，分析西沙多岛礁尾流涡的演化特征；针对可能的影响因素，设计地转、分层环境下的单岛礁、多岛礁尾流涡不稳定诊断数值实验，进行动力分析揭示西沙多岛礁尾流气旋、反气旋涡不稳机制。本项目旨在揭示地转、分层环境下的多岛礁尾流涡演化内在动力机制，为进一步探索西沙岛礁尾流生态动力过程提供基础。

岛礁尾流漩涡通常出现在岛礁的下游，此处上升流伴随着营养物质的输送。根据2009年4月14日和2014年5月30日拍摄的SAR和GF-1卫星图像，在西沙群岛永乐环礁（YA）、华光礁（HR）和玉珠礁（YR）下游发现了多个岛礁尾涡。本项目利用高分辨率数值模式和卫星资料，探讨了岛礁尾流的特征和机制。结果表明：1）气旋尾流涡（CWEs）的水平速度具有表面强化特征，最强烈的涡导致等密线抬升20～40m；而反气旋尾迹涡（AWEs）水平速度则在25～50m深度处达到最大值，并导致弱等密度面下沉。在不同来流条件下，不同半径的涡旋中，CWEs的数量往往比AWEs多，这是由于AWEs多数不稳定而瓦解，是由惯性-离心不稳定性（ICI）造成的。2）在西沙群岛，在岛礁下游侧观察到的较低SST特征表明存在由洋流引起的岛屿尾流。在YA、HR和YR的近尾流中，周期涡度的产生主要是由礁体侧边界的剪切流不稳定性引起的。这三个岛礁的反气旋涡度产生略大于气旋涡度。3）对于亚中尺度尾流，正压不稳定比斜压不稳定更为显著。在一个不稳定的强AWE剖面上，由于ICI的存在，观测到正负交替的强环形涡度，强烈扰动密度分层。但这些很难在较弱的AWEs中找到。不稳定AWEs的涡边缘速度衰减较大，核心持续涡度较大，导致核心涡度与Rossby数之比增大。而稳定的AWEs具有更长的生命周期，其Γ值几乎恒定，与稳定的CWE相似。本项目研究结果可为开展西沙珊瑚礁保护工作、西沙海洋工程环境保护评估工作等提供非常重要的动力基础。对我国认识西沙海域、保护西沙海域提供了重要技术支撑。

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