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

海洋中的大部分流动，小到大约10公里的尺度，被发现接近平衡，其中水平压力梯度被地球旋转产生的加速度所平衡。这些水流大多局限在水平面上，不能很有效地将表面附近温暖、较轻的水与下面较冷、密度较大的沃茨混合。在一公里左右的较小尺度上，即所谓的次中尺度，这种平衡开始被打破，垂直运动变得更加活跃。大部分现有文献的次中尺度物理学集中在锋面内深，订单100米冬季混合层。在这项研究中，高分辨率的大涡模拟将用于系统研究的尖锐的前在浅，顺序10米混合层和不稳定性能够提取可用的势能从这样的战线。这些锋面经常出现在亚热带地区。这些高分辨率的模拟将补充我们现有的知识锋面不稳定性在深冬混合层。模拟还将记录表面波对不稳定性发生的影响。锋面是通过小尺度湍流耗散大尺度海洋动能的关键。加深对浅层混合层锋面不稳定性的理解，这一项目的主题，直接有助于我们对大尺度环流的认识。锋面机制还促进营养物质从较深的层供应到表层，在较深的层中营养物质丰富，在表层中营养物质被营养植物消耗。该项目还将使继续推广工作，传达这项研究的范围，以一般观众在海洋探索活动在新贝德福德，马萨诸塞州和高中学生通过马萨诸塞州海洋教育工作者协会。浅，盐度控制锋往往产生在亚热带地区的当地降水或搅拌河流径流成细丝的大型中尺度漩涡。浅混合层提出了重要的问题，发生的可能性，各种次中尺度不稳定性观察和记录以前在深混合层的前锋。在冬季墨西哥湾流的早期观测和数值模拟表明，没有一个偏好的其他。在这个项目中的中心任务之一将是检查是否在副热带和浅混合层中的一个较小的科里奥利参数的组合导致的非地转斜压不稳定的对称不稳定的偏好。这种偏好具有重要的意义，因为粗分辨率气候模式目前对混合层中的斜压不稳定性有参数化，但对对称不稳定性没有参数化。该项目还包括分析斯托克斯漂移的影响，在Craik-Leibovich框架内，对伴随浅混合层锋的不稳定性。总之，一套拟议的模拟有可能大大提高我们的知识浅混合层的次中尺度锋面机制，从而建立在我们目前的知识，这种机制在深冬季混合层。

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.