Submesoscale instabilities and the forward energy cascade in seamount wakes

热量、淡水、溶解气体和其他示踪剂的能量耗散和混合模式影响着它们在全球海洋中的运输。这些模式还影响深海运动的深部分支的向上运动(上升流)。在全球海洋中，海山可能是具有明显浮力的水的显著混合和能量耗散的地点，但最近的结果表明，用于混合和消散的能量在很大程度上取决于在数百米至数千米之间发展的过程(亚中尺度不稳定性)。这些过程可能对限制海山在海洋能量学和运动中的作用至关重要。拟议的研究将通过高分辨率的数学模拟来研究流过海山的动力学，以考察海山在全球垂直运动中所起的作用，即翻转环流。大气环流模型目前只近似于这些过程的有限子集，不能正确地反映从数千米到米的能量转移的影响。该项目将在广泛的现实配置中调查这一问题，以研究混合和能量耗散的模式如何根据环境和海山特征而变化。这一知识将为未来的近似提供信息，这些近似最终可能会将这些小规模影响包括在全球和气候模拟中。这些数字工具将在开源社区包“Ocean anigans”中随时可用，该包促进结果的再现性，并将这些工具提供给其他项目和其他研究人员。该项目支持早期职业研究人员的博士后工作。亚中尺度不稳定性从平衡流场中提取能量，并产生前向叶栅到湍流混合。目前还不知道湍流尾迹中这种前向级联的能量路径，也不知道能量通量的参数依赖性和促成向湍流转变的次要不稳定性的性质。这里提出的工作将解决三个潜在的变革性问题：Q1：在亚中尺度尾迹中，什么过程控制着前向能量级联？问题2：次中尺度不稳定性如何影响与海山相遇的水流的整体(沿地形和尾流)耗散和混合？问3：在环流模式中，围绕海山的次中尺度能量级联的能量学表现如何？这些问题将在理想化的数值域中解决，并使用Oceanigans进行湍流解析的非静力大涡模拟(LES)，这是一个开源的灵活的流体动力学求解器，使用基于MITgcm的有限体积离散化。LES湍流闭合有两个选项可用，项目PI将调查次网格尺度的模式相关性。大涡模拟方法与雷诺平均方法的不同之处在于，大涡模拟方法允许分解能量的相关尺度，包括厘米尺度湍流在正向能量级联中的重要作用。还将使用基于区域海洋模拟系统(ROMS)公式的GCM Croco进行少量补充RANS模拟，以评估湍流闭合表示地形尾迹湍流的精度。CROCO使用地形跟踪西格玛坐标和高阶压力梯度力方案，以最大限度地减少陡峭地形地貌的误差，并提供各种BBL湍流闭合，使其非常适合研究流动-地形相互作用问题。该奖项反映了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。

Patterns of energy dissipation and of mixing of heat, freshwater, dissolved gases and other tracers affect their transport in the global ocean. These patterns also affect upward motions (upwelling) in the deep branches of the deep-ocean motions. Seamounts can be sites of prominent mixing of waters with distinct buoyancy and of energy dissipation in the global ocean, but recent results suggest that the amount of energy that goes to mixing and dissipation depends heavily on processes that develop at scales between hundreds and thousands of meters (submesoscale instabilities). These processes may be critical in constraining the role of seamounts in ocean energetics and motion. The proposed research will investigate the dynamics of flows past seamounts with high-resolution mathematical simulations, to examine the role seamounts play in the global vertical motions, that is, the overturning circulation. General circulation models currently approximate only a limited subset of these processes and do not correctly represent the effects of the transfer of energy from thousands of meters to meters. This project will investigate the problem in a wide range of realistic configurations to examine how the patterns of mixing and energy dissipation change, depending on environmental and seamount characteristics. This knowledge will inform future approximations that can eventually include these small-scale impacts in global and climate simulations. The numerical tools will be readily available in the open-source community package 'Oceananigans,' which promotes reproducibility of results and provides these tools to other projects and other researchers. The project supports the postdoctoral work of an early career researcher. Submesoscale instabilities extract energy from the balanced flow field and generate a forward cascade to turbulent mixing. The energy pathways for this forward cascade in turbulent wakes are not currently known, nor is the parameter dependence of the energy flux and the nature of the secondary instabilities that mediate the transition to turbulence. The work proposed here will address three potentially transformative questions: Q1: What processes control the forward energy cascade in submesoscale wakes? Q2: How do submesoscale instabilities impact the integrated (along topography and in the wake) dissipation and mixing for flow encountering seamounts? Q3: How well are the energetics of the submesoscale-mediated energy cascade around seamounts represented in General Circulation Models? These questions will be addressed in an idealized numerical domain with turbulence-resolving nonhydrostatic Large-Eddy Simulations (LES) using Oceananigans, which is an open-source, flexible fluid dynamics solver that uses a finite volume discretization based on that of MITgcm. Two options for LES turbulence closures are available, with which the project PIs will investigate the subgrid-scale model dependence. The LES approach differs from Reynolds-Averaged approaches (RANS - which most other recent work on related topics has used) in that LES allows the relevant scales of the energy to be resolved, including the vital role of centimeter-scale turbulence in the forward energy cascade. A small number of complementary RANS simulations will also be performed to evaluate how accurately turbulence closures represent topographic wake turbulence using the GCM CROCO, which is based on a formulation of the Regional Oceanic Modeling System (ROMS). CROCO uses a topography-following sigma- coordinate with a higher order pressure gradient force scheme to minimize errors at steep topographic features and has a variety of available BBL turbulence closures, making it well-suited for investigations of flow-topography interaction problems.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.