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

海洋中的大部分水流，小到十公里左右，被发现接近一个平衡点，在这个平衡点，水平压力梯度被地球自转产生的加速度平衡。这些流动大多局限在水平面上，在将靠近地表的温暖、较轻的水与下面较冷、较稠密的水混合时效率不高。在一公里左右的较小尺度，即所谓的亚中尺度，这种平衡开始被打破，垂直运动变得更有能量。现有的大部分关于亚中尺度物理的文献都集中在冬季100米深混合层内的锋面上。在这项研究中，将使用高分辨率大涡模拟来系统地研究浅层、数量级10米混合层中的锐锋以及能够从这些锋面中提取可用势能的不稳定性。这些锋面经常出现在亚热带纬度地区。这些高分辨率的模拟将补充我们现有的关于深冬混合层锋面不稳定性的知识。这些模拟还将记录表面波对不稳定性开始的影响。锋面对于大尺度海洋动能通过小尺度湍流的耗散是至关重要的。对浅混合层锋面不稳定性的更好的理解是这个项目的主题，这直接有助于我们对大尺度环流的了解。锋面机制还促进从深层向表层供应营养物质，在那里营养物质丰富，被浮游植物消耗。该项目还将继续开展推广工作，在马萨诸塞州新贝德福德的海洋探索活动中向普通观众以及通过马萨诸塞州海洋教育者协会向高中生传达这项研究的范围。在亚热带纬度地区，由于当地降水或大的中尺度涡流将河流径流搅动成细丝，往往会产生浅薄的盐度控制锋面。浅混合层对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的发生的可能性提出了重要的问题。早先对冬季墨西哥湾流的观测和数值模拟表明，并不存在偏爱其中一个的现象。这个项目的中心任务之一将是研究副热带较小的科里奥利参数和浅混合层的结合是否导致对称不稳定优先于非地转斜压不稳定。这种偏好具有重要的意义，因为目前粗分辨率气候模式对混合层中的斜压不稳定有一个参数化，但对对称不稳定没有一个。该项目还分析了Craik-Leibovich框架内的斯托克斯漂移对伴随浅混合层锋面的不稳定性的影响。总之，这套拟议的模拟有可能极大地增强我们对浅混合层亚中尺度锋面机制的了解，从而建立在我们目前对深冬混合层这种机制的知识基础上。

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.