NSFGEO-NERC: Collaborative Research: Properties and Mechanisms of the Multiscale Eddy-Induced Diffusion

传统上，海洋流动被分解为两个主要部分，“大尺度”和“中尺度/小尺度”（“涡旋”），部分原因是大多数数值海洋模式对涡旋分量的解析不足。涡流通量必须参数化，最常用的方法是利用通量梯度与涡流扩散系数（张量）的关系。这种关系的简单性很吸引人，但主要的挑战在于找到合适的扩散张量，扩散张量随地理位置和深度（即非均匀性）和时间而变化，并且是方向相关的（即各向异性），这一点由观测和基于gcm的估计证明。本研究的总体目标是探索中纬度地区非均匀和各向异性涡旋诱导输运的性质，并研究它们对示踪剂分布的重要性。主要的假设是，涡旋诱导输运可以通过涡旋扩散张量的时空图简洁地量化，并且这种复杂性可以系统地还原为其最基本的性质。具体而言，我们将客观地计算和分析相应的涡流扩散张量映射，并探讨结果对时空尺度的依赖关系。然后，该扩散率图将用于生成未完全解决涡流驱动示踪剂平流的流中的示踪剂分布，并且将使用相关度量来量化所得到的技能。我们将采用漩涡解析数值模拟的层次结构，真实的海洋漂流者轨迹和广泛的尺度感知流动分解，以及几种用于估计和解释漩涡扩散张量的新方法，以实现基本理解和实际目的。这项研究将充分利用美国和英国研究小组成员之间现有的密集和高效合作的协同作用。

Oceanic flows have been traditionally decomposed into two main components, "large-scale" and "mesoscale/small-scale" ("eddy"), which is in part motivated by the fact that most numerical ocean models under-resolve the eddy component. The eddy fluxes then have to be parameterized, and the most common approach is to use the flux-gradient relation with the eddy diffusivity (tensor) coefficient. The simplicity of this relation is appealing, but the main challenge lies in finding the appropriate diffusivity tensor, which varies with geographical location and depth (i.e., inhomogeneous) and time, and is direction-dependent (i.e., anisotropic), as evidenced by observation- and GCM-based estimates. The overarching goal of the proposed study is to explore properties of the inhomogeneous and anisotropic eddy-induced transport at mid-latitudes, and to examine their importance for tracer distribution. The main hypotheses are that the eddy-induced transport can be succinctly quantified by a spatio-temporal map of the eddy diffusivity tensor, and that this complexity can be systematically reduced to its most essential properties. Specifically, we will objectively calculate and analyze the corresponding eddy-diffusivity tensor maps and explore the dependence in the results on the spatial and temporal scales. This diffusivity map will then be used to produce tracer distributions in flows that do not fully resolve the eddy-driven tracer advection, and the resulting skill will be quantified using relevant metrics. We will employ a hierarchy of eddy-resolving numerical simulations, real-ocean drifter trajectories and a wide range of scale-aware flow decompositions, as well as several novel methods for estimating and interpreting the eddy diffusivity tensors for both fundamental understanding and practical purposes. The study will capitalize on the synergy of existing intensive and efficient collaboration between the US and UK members of the research group.