Competition between mixed-layer instabilities in shallow fronts at subtropical latitudes in the ocean

海洋副热带纬度浅锋混合层不稳定性之间的竞争

• 批准号: 1558849
• 负责人: Amit Tandon
• 金额: $ 22.33万
• 依托单位: University of Massachusetts, Dartmouth
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2016
• 资助国家: 美国
• 起止时间: 2016-03-15 至 2020-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1558849&HistoricalAwards=false
• 关键词: Competition; between; mixed; layer; instabilities

Much of the flow in the ocean, down to scales of around ten kilometers, is found to be near an equilibrium where horizontal pressure gradients are balanced by an acceleration that arises from the rotation of the Earth. These flows are mostly confined to the horizontal plane and are not very efficient at mixing the warm, lighter water near the surface with the colder, denser waters below. At smaller scales of a kilometer or so, the so-called submesoscale, this balance begins to break down and vertical motions become more energetic. Bulk of the existing literature on submesoscale physics has concentrated on fronts within deep, order 100 meter wintertime mixed layers. In this study, high-resolution large-eddy simulations will be used for a systematic study of sharp fronts in shallow, order 10 meter mixed layers and the instabilities capable of extracting the available potential energy from such fronts. These fronts are often found in subtropical latitudes. These high-resolution simulations will complement our existing knowledge of frontal instabilities in deep wintertime mixed layers. The simulations will also document the effects of surface waves on the onset of instabilities. Fronts are vital to the dissipation of the large-scale oceanic kinetic energy through small-scale turbulence. An improved understanding of frontal instabilities in shallow mixed layers, the theme of this project, contributes directly to our knowledge of the large-scale circulation. Frontal mechanisms also promote the supply of nutrients from deeper layers where they are abundant to the surface layers where they are consumed by planktonic plants. The project will also enable continued outreach efforts to convey the scope of this research to general audiences at the Ocean Explorium events in New Bedford, MA and to high school students through the Massachusetts Marine Educators Association.Shallow, salinity-controlled fronts are often generated in subtropical latitudes by local precipitation or the stirring of river runoff into filaments by large mesoscale eddies. The shallow mixed layers raise important questions regarding the likelihood of occurrence of various submesoscale instabilities observed and documented previously at fronts in deep mixed layers. Earlier observations in the wintertime Gulf Stream and numerical simulations of the same suggest the absence of a preference of one over the other. One of the central tasks in this project will be to examine whether the combination of a smaller Coriolis parameter in the subtropics and shallow mixed layers lead to a preference for symmetric instability over ageostrophic baroclinic instability. Such a preference has important implications as coarse-resolution climate models currently have a parameterization for baroclinic instability in the mixed layer but none for symmetric instability. This project also includes an analysis of the effects of Stokes drift, within the Craik-Leibovich framework, on the instabilities accompanying shallow mixed-layer fronts. Together, the set of proposed simulations have the potential to greatly enhance our knowledge of submesoscale frontal mechanisms in shallow mixed layers, thus building on our current knowledge of such mechanisms in deep wintertime mixed layers.

人们发现，海洋中的大部分流量一直到大约十公里的尺度，被发现在平衡附近，在该平衡中，水平压力梯度通过地球旋转而产生的加速度平衡。这些流主要局限于水平面，并且在将表面附近的温暖，较轻的水与下面的较冷，较密的水混合在一起方面效率不高。在一个公里左右的较小尺度上，所谓的子尺度上，这种平衡开始分解，垂直运动变得更加活力。有关尺度物理学的现有文献的大部分都集中在冬季最深的冬季混合层内的深处。在这项研究中，高分辨率大型模拟将用于对浅层，订单10米的混合层和能够从此类方面提取可用势能的不稳定性的系统研究。这些前沿通常在亚热带纬度中找到。这些高分辨率模拟将补充我们在深冬季混合层中对额叶不稳定性的现有知识。模拟还将记录表面波对不稳定性发作的影响。前部对于通过小规模的湍流耗散大规模的海洋动能至关重要。对浅层混合层的额叶不稳定性的了解，该项目的主题直接有助于我们对大规模循环的了解。额叶机制还促进了较深层的养分供应，在这些层中，它们丰富地被浮游物植物消耗的表面层。该项目还将启动继续范围的努力，将这项研究的范围传达给马萨诸塞州新贝德福德的海洋探险事件，并通过马萨诸塞州海洋教育工作者协会通过马萨诸塞州海洋教育者协会向高中生传达。浅层混合层提出了有关发生在深层混合层的前部观察到的各种子尺度不稳定性的可能性的重要问题。冬季海湾流中的早期观察和相同的数值模拟表明缺乏一个偏好而不是另一个。该项目的中心任务之一将是检查亚热带和浅层混合层中较小的科里奥利参数的组合是否导致偏爱对称不稳定性而不是年龄型斜压斜利性不稳定。这种偏好具有重要的含义，因为当前的粗分辨率气候模型在混合层中具有斜压不稳定性的参数化，但没有对称不稳定性的参数化。该项目还包括对Craik-Leibovich框架内Stokes漂移影响的分析，对浅层混合层锋线的不稳定性。总之，一组提出的模拟有可能极大地增强我们对浅层混合层中尺度额叶机制的了解，从而基于我们目前对深层冬季混合层此类机制的了解。