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组成部分，即“大尺度”和“中尺度/小尺度”(“涡流”)，部分原因是大多数海洋数值模式未能充分分辨涡流成分。然后，涡旋通量必须被参数化，最常见的方法是使用涡旋扩散率(张量)系数的通量梯度关系。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系数张量，它随地理位置和深度(即不均匀)和时间而变化，并且是方向相关的(即各向异性)，如基于观测和基于GCM的估计所证明的。拟议研究的主要目标是探索中纬度地区非均匀和各向异性涡旋引起的输送的特性，并检查它们对示踪剂分布的重要性。主要的假设是，涡流输运可以用涡旋扩散张量的时空映射简洁地量化，并且这种复杂性可以系统地降低到其最本质的性质。具体来说，我们将客观地计算和分析相应的涡旋扩散张量图，并探索结果对空间和时间尺度的依赖性。然后，这张扩散系数图将被用来在不能完全解决涡动驱动的示踪剂平流的流动中产生示踪剂分布，由此产生的技能将使用相关的度量标准进行量化。我们将使用一系列涡旋分辨数值模拟、真实海洋漂流轨迹和广泛的尺度感知流动分解，以及几种新的方法来估计和解释涡旋扩散率张量，以达到基本理解和实际目的。这项研究将利用该研究小组美国和英国成员之间现有的密集和高效合作的协同效应。

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.