Submesoscale instabilities in the ocean bottom boundary layer: A new pathway for energy dissipation

物理海洋学中一个长期存在的问题是，输入到海洋环流的能量最终是如何消散的，这一问题对我们对海洋环流的基本理解和开发准确的海洋模型都具有深远的影响。最近有人提出，底层不稳定和地形尾迹是湍流混合的一个重要来源，然而，这些不稳定的动力学和能量学在很大程度上仍未被探索。在相对较小的尺度上，地球的自转不再是被称为亚中尺度的主导因素，海洋底层的不稳定会改变流动中能量的提取和耗散，因此可能会改变海底湍流的经典概念。这项研究将使用理论和理想化的高分辨率数值模式来确定近底部、地形上方和下游的亚中尺度不稳定性如何影响湍流混合。这些不稳定性被认为是底部附近的常见特征，但在这种情况下相对于表层来说，人们对此知之甚少。同样，这些过程目前在大多数大规模海洋模式中都不存在，这突出表明需要对它们的影响和对控制参数的依赖进行量化了解。这一认识是诸如如何在深海倾覆环流中将稠密的水带到地表，以及营养物质和其他生物地球化学示踪剂在底层和内层之间的交换等问题的核心。这项工作的结果将提高我们对地形动力学的理解和最终的参数化，特别是这些边界层过程如何改变更大尺度的海洋环流和能量学。该项目还将培训一名博士后研究人员，并支持建立一个初级教员的初步研究小组。初步结果表明，底部边界层中的亚中尺度不稳定性为耗散动能开辟了一条新的途径，并进一步修改了地形附近浮力的湍流混合，据信这是在深海颠覆环流中将稠密水带回地面的关键因素。这些结果表明，关于流动-地形相互作用如何产生湍流的经典概念是不完整的，需要考虑亚中尺度不稳定性的作用。本项目将研究沿地形流动产生的斜压、对称和离心力不稳定的能量学。大涡模拟(LES)将被用来研究这些不稳定性如何改变涡动和平均动能和势能之间的能量分配和通量，以及动能的不可逆耗散和浮力的混合。可变地形的雷诺平均模拟也将与LES一起使用，以确定上游边界层的演变如何影响地形尾迹的能量学。这项工作将提供对物理过程和参数相关性的更完整的描述，这些参数决定了亚中尺度不稳定性如何影响海洋环流的能量。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

One of the long standing questions in physical oceanography is how the energy input to the ocean circulation is eventually dissipated, a problem with far-reaching implications for both our basic understanding of ocean circulation and for developing accurate ocean models. Recently it has been suggested that instabilities in the bottom layer and topographic wakes provide an important source of turbulent mixing, however the dynamics and energetics of these instabilities remain largely unexplored. At relatively small scales, where the rotation of the Earth is no longer the dominant factor, called submesoscales, instabilities in the ocean bottom layer modify the extraction, and dissipation of energy from the flow, and hence may alter classic conceptions of bottom turbulence. This research will use theory and idealized high-resolution numerical models to determine how submesoscale instabilities near the bottom, over and downstream of topography, modify turbulent mixing. These instabilities are believed to be common feature near the bottom, but are poorly understood in this setting relative to the surface layers. Likewise, these processes are currently absent in most large-scale ocean models, underscoring the need to develop a quantitative understanding of their impact and dependence on controlling parameters. This understanding is central to problems such as how dense water is brought to the surface in the abyssal overturning circulation, and the exchange of nutrients and other biogeochemical tracers between the bottom and interior layers. Results of this work will improve our understanding, and eventual parameterization, of dynamics along topography, and in particular how these boundary layer processes modify the larger-scale ocean circulation and energetics. This project will also enable the training of a postdoctoral researcher, and support the establishment of the initial research group of a junior faculty member.Preliminary results suggest that submesoscale instabilities in the bottom boundary layer (BBL) open a new pathway for dissipating kinetic energy, and further modify the turbulent mixing of buoyancy near topography, believed to be a key aspect of how dense water is brought back to the surface in the abyssal overturning circulation. These results suggest that classic conceptions of how flow-topography interaction generates turbulence are incomplete, and the role of submesoscale instabilities needs to be considered. This project will investigate the energetics of baroclinic, symmetric, and centrifugal instabilities generated by flow along topography. Large-Eddy Simulation (LES) will be used to examine how these instabilities modify the partitioning and flux of energy between eddy and mean kinetic and potential energy terms, and the irreversible dissipation of kinetic energy and mixing of buoyancy. Reynolds-averaged simulations with variable topography will also be used, in conjunction with the LES, to determine how the upstream BBL evolution influences the energetics of topographic wakes. This work will provide a more complete description of the physical processes, and parameter dependencies, which determine how submesoscale instabilities affect the energetics of the ocean circulation.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.