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，海洋环流由一个高能中尺度涡旋场主导，类似于大气中的天气系统。在用于长期气候预测的海洋模型中，这些漩涡是无法解决的，或者充其量是无法充分解决的。因此，有必要对缺失的中尺度涡旋对大尺度环流的影响进行参数化。绝大多数海洋环流数值模型采用了根特和麦克威廉姆斯的“涡流参数化”方法，这种方法可以使密度表面变平，模拟势能的释放，从而促进中尺度涡流的增长。涡旋参数化中的一个关键参数是“涡旋扩散率”，它在确定全球海洋环流、分层和热含量、全球环流调整时间尺度以及潜在的大气CO2方面起着主导作用。在这个项目中，我们将对Gent和McWilliams涡旋扩散系数实施一个新的闭合，该闭合来自第一性原理，它仅取决于海洋分层、涡旋能量和小于或等于1的无量纲参数。如果涡旋能量是已知的，那么就不能自由地明确指定任何额外的维度参数，例如涡旋扩散率。因此，我们认为，在海洋气候模型中参数化涡旋的现有方法存在根本性缺陷。我们的新方法需要解一个深度积分涡流能量的方程。这是一项重大挑战，将成为本项目的主要组成部分。然而，我们相信，求解涡旋能量是容易处理的，因为我们有一些关键的物理成分的理解。这些关键要素包括：大尺度流的不稳定性为涡旋能量的来源；涡旋向西传播；涡旋能量的扩散；西部边界“涡旋墓地”涡旋能量的耗散；一旦一个一致的涡流能量预算被纳入，我们的新涡流参数化导致三个非常理想的结果，这是重要的概念证明：它在大气和海洋流动不稳定的一个简单模型中再现了涡旋的正确尺寸增长率，该模型有一个精确的数学解。假设完全了解涡旋能量，它再现了从高分辨率计算机模拟完全湍流不稳定性诊断出的涡旋扩散率。3. 它预测并解释了“涡旋饱和”的物理学，南极环极流的大小对地面风强迫的显著不敏感，以及当前涡旋参数化的长期挑战和已知缺陷。工作计划将包括四个相互关联的工作包：在NEMO海洋模型中实施和验证新的涡流参数化框架，由NERC和英国气象局以及其他欧洲合作伙伴使用。2. 参数化涡流能量收支的发展和改进。3. 量化英国地球系统模式中新的参数化对海洋吸收热量和被动示踪剂的影响，该模式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，确保工作及时、全面地完成。

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.