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

人们发现，海洋中的大部分水流，小到10公里左右的尺度，都接近于一个平衡状态，在这个状态下，水平压力梯度被地球自转产生的加速度所平衡。这些水流主要局限在水平面上，不能很有效地将表面附近温暖、较轻的水与下面较冷、较稠密的水混合在一起。在一公里左右的小尺度，即所谓的亚中尺度，这种平衡开始被打破，垂直运动变得更有活力。大部分关于亚中尺度物理的现有文献都集中在深层的锋面上，大约100米的冬季混合层。在本研究中，高分辨率大涡模拟将用于系统研究浅10米级混合层的尖锐锋面以及能够从这些锋面中提取可用势能的不稳定性。这些锋面通常出现在亚热带纬度地区。这些高分辨率模拟将补充我们对冬季深层混合层锋面不稳定性的现有知识。模拟还将记录表面波对不稳定开始的影响。锋面对大尺度海洋动能通过小尺度湍流的耗散至关重要。提高对浅层混合层锋面不稳定性的理解，是本项目的主题，直接有助于我们对大尺度环流的认识。锋面机制还促进了营养物质从丰富的深层向表层的供应，在表层被浮游植物消耗。该项目还将继续开展外展工作，通过马萨诸塞州新贝德福德的海洋探索活动向普通观众和马萨诸塞州海洋教育者协会的高中生传达这项研究的范围。浅层的、盐度控制的锋面通常在副热带地区由局部降水或大型中尺度涡旋将河流径流搅拌成细丝而产生。浅层混合层提出了一个重要的问题，即以前在深层混合层锋面观测和记录的各种亚中尺度不稳定发生的可能性。早期对冬季墨西哥湾流的观测和数值模拟表明，两者之间并没有孰轻孰重之分。该项目的中心任务之一将是研究在副热带和浅混合层中较小的科里奥利参数的组合是否导致对称不稳定优于地转斜压不稳定。这种偏好具有重要意义，因为粗分辨率气候模式目前对混合层的斜压不稳定有参数化，但对对称不稳定没有参数化。该项目还包括在克雷克-莱博维奇框架内分析斯托克斯漂移对伴随浅混合层锋的不稳定性的影响。总之，这组模拟有可能极大地增强我们对浅层混合层亚中尺度锋面机制的认识，从而建立在我们目前对冬季深层混合层这种机制的认识之上。

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.