斜坡地形上涡动混合的机制研究

The most energetic oceanic motions occur on the mesoscale, with length scales between 10 and 100 km. Mixing by mesoscale eddies plays an important role in setting global distributions of temperature, salinity, CO2 and other climatically important tracers. Its effect is particularly significant over continental shelves and slopes, where tracer distributions have sharp gradients and flow fields are characterized by strong jets, meanders and eddies. In coarse resolution models commonly used in climate studies, the eddy flux is typically represented in terms of eddy diffusivity. In most flow-dependent parameterization schemes, the eddy diffusivity is constructed from the inverse time scale and length scale of the fastest-growing waves determined by classical Eady's or Phillips' model. It is clearly demonstrated by some recent studies that the sloping topography has an order one influence on eddy diffusivity. However, many features of that influence are poorly understood; neither can they be explained by the classical theory. Furthermore, few studies, if any, have been devoted to actually incorporating topographic effects into useful parameterizations. Under such circumstances, we propose to investigate the mechanism governing eddy mixing over the sloping bottom and to construct a proper parameterization for eddy diffusivity. The project will include, but not limited to, research in the following directions: to explain limitations of the classical theory, to quantify nonlinear effects on eddy diffusivity with both numerical simulations and laboratory experiments, and to seek, if possible, an analytical expression of eddy diffusivity in the context of the nonlinear quasi-geostrophy.

中尺度涡是海洋中普遍存在且能量最强的运动形式。由中尺度涡引起的物质输运通过影响物质的分布进而影响地球气候系统的变化。它的意义在陆架边缘地区更加重要，因为那里不仅有较强的涡动能，也常常见到较强的物质分布梯度。用于气候变化研究的数值模式因为无法完全分辨中尺度涡而必须借助参数化手段，即用一个涡动混合率来表示涡动输运的强度。尽管有研究表明涡动混合率在陆架区域受斜坡地形影响显著且其很多特点无法用经典理论来解释，但迄今为止少有研究关注并量化这一影响。基于此，本项研究拟通过数值手段和实验室实验来研究斜坡地形对涡动输运的影响机制，给出合理的参数化方法。在内容上包括用数值手段解释线性经典理论在预测涡动混合率上的局限，定量给出非线性效应的影响，并与实验室模拟的结果进行比对，同时在非线性准地转框架下寻求涡动混合率的解析表达。

中尺度涡旋的研究已经有逾半个多世纪的历史，但是一直进展缓慢。由中尺度涡旋导致的输运和混合过程及其与气候变化的关系问题是近一二十年学界关注的一个焦点，但由于对很多基本问题比如涡旋的结构特征和演化过程缺乏必要的认识，一直没有得到很好的解决。本项目自启动以来，围绕海洋中尺度涡旋导致的输运问题进行了一系列细致工作，创新性的成果有以下几项：1、发现了表层和次表层中尺度涡旋三维结构的全球统一分布规律：该规律只由局地的地球旋转和海水密度层结决定，并且可以用非常简单的解析函数表示。2、发现了中尺度涡旋与热盐环流的重要联系：中尺度涡旋作为新生成的深水的载体向西运动，最终汇入深层西边界流，对该流动乃至整个热盐环流系统都有重要作用。3、提出并利用观测资料验证了中尺度涡旋消衰的过程和机理：中尺度涡旋在稳定态下满足地转平衡，但在受外界扰动失稳时，会通过地转调整回到新的平衡状态；作为调整过程的一个重要结果，能量以近惯性内波的形式由涡旋向外传播，在引起中尺度涡旋能量衰减的同时导致近惯性能量在深层海洋的增强。这三项工作针对中尺度涡旋的结构、运动以及演化，解决了长期困扰学界的三个重要问题，具有较高的创新性和先进性，为进一步研究涡致输运和混合打下了良好基础，也为进一步理清中尺度涡旋与大尺度环流及气候的关系提供了前提。