Submesoscale instabilities and the forward energy cascade in seamount wakes

能量耗散和热量、淡水、溶解气体和其他示踪物混合的模式影响到它们在全球海洋中的迁移。这些模式也影响到深海运动深层分支的上升运动（涌升）。海山可能是全球海洋中具有独特浮力的沃茨的显著混合和能量耗散的场所，但最近的结果表明，用于混合和耗散的能量在很大程度上取决于在数百米至数千米尺度上发展的过程（次中尺度不稳定性）。这些过程可能对限制海山在海洋动力学和运动中的作用至关重要。拟议的研究将利用高分辨率数学模拟来研究流经海山的水流动态，以考察海山在全球垂直运动即翻转环流中的作用。目前的大气环流模型只近似这些过程的一个有限的子集，并不能正确地表示能量从数千米到米的转移的影响。该项目将在各种实际配置中调查这一问题，以研究混合和能量耗散模式如何根据环境和海山特征而变化。这些知识将为未来的近似计算提供信息，最终可以将这些小规模影响纳入全球和气候模拟。这些数值工具将在开源社区包“Oceananigans”中提供，该软件包促进了结果的可重复性，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，并产生湍流混合的前向级联。目前还不知道湍流尾流中这一前向叶栅的能量路径，也不知道能流的参数依赖性和介导向湍流过渡的二次不稳定性的性质。这里提出的工作将解决三个潜在的变革性问题：Q1：什么过程控制的前向能量级联亚中尺度尾流？问题2：亚中尺度不稳定性如何影响气流遇到海山时的整体（沿着地形和尾流）耗散和混合？问题3：在大气环流模型中，海隆周围次中尺度介导的能量级联的能量学表现得如何？这些问题将在一个理想化的数值域中解决，其中使用Oceanigans的连续性解决非流体静力大涡模拟（LES），Oceanigans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湍流闭合有两种选择，项目PI将利用这两种选择研究亚网格尺度模型依赖性。大涡模拟方法与雷诺平均方法（RANS -最近大多数相关主题的工作都使用了RANS）的不同之处在于，大涡模拟允许解决能量的相关尺度，包括厘米尺度湍流在前向能量级联中的重要作用。还将进行少量的补充RANS模拟，以评估湍流闭合如何准确地代表地形尾流湍流使用GCM CROCO，这是基于区域海洋模拟系统（ROMS）的配方。CROCO采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特征的误差，并具有各种可用的BBL湍流闭合，使其非常适合于流动-地形相互作用问题的研究。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值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.