GEOMETRIC: Geometry and Energetics of Ocean Mesoscale Eddies and Their Representation in Climate models

在10-100公里的空间尺度上，海洋环流由一个充满活力的中尺度涡动场控制，类似于大气中的天气系统。在用于长期气候预测的海洋模式中，这些涡旋没有得到解决，或者充其量没有得到充分解决。因此，有必要将缺失的中尺度涡对大尺度环流的影响参数化。绝大多数数值海洋环流模式采用Gent和McWilliams的“涡参数化”，其作用是使密度面变平，模拟势能的释放，以促进中尺度涡的增长。在这种涡流参数化的一个关键参数是“涡流扩散率”，这是至关重要的，因为它在设置全球海洋环流，分层和热含量，全球环流的调整时间尺度，以及潜在的大气CO2中起着主导作用。在这个项目中，我们将实现一个新的封闭的根特和麦克威廉姆斯涡扩散率，来自第一原理，这只取决于海洋分层，涡能和一个无量纲参数，小于或等于1。如果涡流能量是已知的，那么就没有明确指定任何附加尺寸参数的自由，例如涡流扩散率。出于这个原因，我们认为，现有的方法参数化海洋气候模型中的涡旋是根本性的缺陷。我们的新方法需要解决一个方程的深度积分涡流能量。这是一项重大挑战，将构成本项目的一个主要组成部分。然而，我们相信，解决涡流能量是容易的，因为我们有一些了解的关键物理成分。这些关键成分包括通过大尺度流动的不稳定性产生的涡动能量的来源、涡动的向西传播、涡动能量的扩散、西部边界“涡动墓地”中涡动能量的耗散以及通过底阻力和背风波产生的涡动能量的耗散。一旦纳入一致的涡流能量预算，我们的新的涡流参数化导致三个非常理想的结果，作为重要的概念证明：1。它再现了正确的尺寸增长率的涡流在一个简单的模型的不稳定性的大气和海洋流动，有一个精确的数学解决方案。假设对涡流能量有完美的了解，它再现了从完全湍流不稳定性的高分辨率计算机模拟中诊断出的涡流扩散率。3.它预测和解释的物理“涡流饱和”，南极绕极流的大小表面风强迫显着的不敏感性，以及长期存在的挑战和已知的缺陷电流涡流parameterisations。工作计划将包括四个相互关联的工作包：1.在NEMO海洋模型中实施和验证新的涡流参数化框架，NERC和英国气象局沿着其他欧洲合作伙伴使用。2.参数化涡动能量收支的发展和改进。3.量化新参数化对英国地球系统模型中海洋吸收热量和被动示踪剂的影响，用于英国对政府间气候变化专门委员会（IPCC）气候预测的贡献。4.项目管理，确保工作全面及时地交付。

The ocean circulation is dominated by an energetic mesoscale eddy field on spatial scales of 10-100 km, analogous to weather systems in the atmosphere. These eddies are unresolved, or at best inadequately resolved, in the ocean models used for long-range climate projections. Thus it is necessary to parameterise the impacts of the missing mesoscale eddies on the large-scale circulation. The vast majority of numerical ocean circulation models employ the Gent and McWilliams "eddy parameterisation" which acts to flatten density surfaces, mimicking the release of potential energy to fuel the growth of the mesoscale eddies. A key parameter in this eddy parameterisation is the "eddy diffusivity", which is critical as it plays a leading order role in setting global ocean circulation, stratification and heat content, the adjustment time scale of the global circulation, and potentially atmospheric CO2. In this project, we will implement a new closure for the Gent and McWilliams eddy diffusivity, derived from first principles, which depends only on the ocean stratification, the eddy energy and a non-dimensional parameter that is less than or equal to 1. If the eddy energy is known, then there is no freedom to specify explicitly any additional dimensional parameters, such as an eddy diffusivity. For this reason, we argue that existing approaches to parameterising eddies in ocean climate models are fundamentally flawed. Our new approach requires solving an equation for the depth-integrated eddy energy. This is a significant challenge and will form a major component of the present project. However, we believe that solving for the eddy energy is tractable as we have some understanding of the key physical ingredients. These key ingredients include the source of eddy energy through instability of the large-scale flow, westward propagation of eddies, diffusion of eddy energy, dissipation of eddy energy in western boundary "eddy graveyards", and dissipation of eddy energy through bottom drag and lee wave generation. Once a consistent eddy energy budget is incorporated, our new eddy parameterisation leads to three highly desirable results, which serve as important proofs of concept: 1. It reproduces the correct dimensional growth rate for eddies in a simple model of instability of atmospheric and oceanic flows for which there is an exact mathematical solution.2. Assuming perfect knowledge of the eddy energy, it reproduces the eddy diffusivity diagnosed from high-resolution computer simulations of fully turbulent instabilities. 3. It predicts and explains the physics of "eddy saturation", the remarkable insensitivity of the size of the Antarctic Circumpolar Current to surface wind forcing, and a long standing challenge and known deficiency of current eddy parameterisations. The work plan will consist of four inter-related work packages: 1. Implementation and validation of the new eddy parameterisation framework in the NEMO ocean model, used by NERC and the UK Met Office, along with other European partners. 2. Development and refinement of the parameterised eddy energy budget. 3. Quantifying the impact of the new parameterisation on the oceanic uptake of heat and passive tracers in the UK Earth System Model, used for the UK contribution to the Intergovernmental Panel for Climate Change (IPCC) climate projections. 4. Project management, to ensure that the work is delivered fully and in a timely manner.