Clustering of Floating Tracers in the Ocean

该项目的主要动机是海洋塑料污染以及了解和缓解它的必要性。海洋环流的特点是复杂的中尺度和亚中尺度涡旋运动(即涡流)，其作用尺度从几公里到数百公里，时间尺度从几天到几个月--这些现象提供了涉及许多基本和实际方面的活跃研究范围。漩涡影响了许多示踪剂和漂浮在海洋表面的物质：塑料污染(包括垃圾和微塑料)、石油泄漏、马尾藻、浮游生物、冰和残骸碎片。我们主要关注的是塑料污染，这是最大的环境威胁之一。漂浮物在大尺度海洋环流、涡旋和小尺度流动特征的作用下，以一种复杂而又鲜为人知的方式不断地移动、重新分布和聚集。在这里，通过集群，我们指的是物质属性在结构化和时间演变的空间模式中积累和聚集的过程。这一过程实际上很重要，因为不同的漂浮和中性浮力示踪剂，包括营养物和生物量，聚集在一起的方式不同--这对它们的相互作用产生了深远的影响；例如，污染聚集与海洋生物之间的正相关加强了对后者的不利影响。在该项目中，我们的目标是了解集群的一些基本原理。对漂浮示踪剂的分布和运输路径的准确预测有许多实际需要，但也涉及到许多理论方面，这构成了该项目的研究设计和目标。这些方面可归结为以下几个方面：(1)大多数海洋塑料和石油是漂浮的，塑料也是由有限大小的颗粒组成--这导致这些示踪剂具有显著的非被动和非拉格朗日性质。因此，它们在被动示踪剂浓度或拉格朗日粒子系综方面的共同近似存在系统误差和偏差。该项目将估计后者，并探索更准确的浮动示踪剂近似值。(2)大部分示踪物的时空变异性是由中尺度和亚中尺度涡度场驱动的，而在大气环流模式和野外观测中，这些涡旋场通常是未分辨或欠分辨的。因此，由于缺乏(固有的昂贵的)现场观测，以及在操作大气环流模式时暴力分辨中、亚中尺度涡旋所需的巨大计算负担，阻碍了对浮动示踪剂的预测理解的进展。这使得对所涉及的机制和示踪剂行为的理论理解处于研究的前沿，如该项目所概述的那样。(3)云团过程在很大程度上是由多尺度的弱表面速度散度引起的，这种散度很难观测和模拟，因为它是由不同的物理过程诱发和控制的，贡献很少，相互反馈也很复杂。该项目旨在以一种系统的方式，对基本物理过程的集群性贡献进行分类。(4)关于(1)-(3)，该项目还旨在开发有效的集群分析方法，以及用于实际应用的集群数学模型。预计该项目的理论结果将对现有和即将到来的观测计划以及许多处理海洋污染和海洋中物质运输的研究人员有用。已经考虑到与相关工作组和任务的联系，并获得了协调全球海洋塑料污染研究的埃里克·范·塞比勒的支持。与EPSRC的战略主题和研究领域保持一致：流体动力学、数学物理、非线性系统、与环境变化共存。

The Project is mostly motivated by marine plastic pollution and the need to understand and mitigate it. Ocean circulation is characterized by complicated mesoscale and submesoscale eddying motions (i.e., eddies) acting on length scales from a few to hundreds of kilometres, and on timescales from days to many months - these phenomena feed an active research scope involving many fundamental and practical aspects. The eddies affect many tracers and substances floating on the ocean surface: plastic pollution (including litter and microplastic), oil spills, sargassum, plankton, ice and wreckage debris. Our main focus is on the plastic pollution, which is one of the largest environmental threats. In a complicated and poorly understood way, the floating materials are constantly moved around, redistributed and clustered by the large-scale ocean circulation, eddies and smaller-scale flow features. Here, by clustering we refer to the process of accumulation and aggregation of material properties in structured and temporally evolving spatial patterns. This process is practically important, because different floating and neutrally buoyant tracers, including nutrients and biomass, cluster differently - this has profound effect on their interactions; e.g., positive correlations between clusters of pollution and marine life have enhanced adverse effect on the latter. In the Project we aim to understand some fundamentals of the clustering. There are many practical needs for accurate predictions of distributions and transport pathways of the floating tracers, but there are also many theoretical aspects involved, which shape up the research design and aims of the Project. These aspects can be grouped as the following: (1) Most of the marine plastic and oil is floating, and the plastic is also made up of finite-size particles - this results in significant non-passive and non-Lagrangian properties of these tracers. As a result, their common approximations in terms of passive-tracer concentrations or ensembles of Lagrangian particles have systematic errors and biases. The Project will estimate the latter and explore more accurate approximations for floating tracers. (2) Most of the spatio-temporal variability of the tracers is driven by the mesoscale and sub-mesoscale eddy fields, which are routinely either unresolved or underresolved in general circulation models and field observations. Thus, progress in predictive understanding of floating tracers is hindered by the lack of (inherently expensive) in situ observations and by the enormous computational burden required for brute-force resolution of mesoscale and submesoscale eddies in operating general circulation models. This puts theoretical understanding of the involved mechanisms and tracer behaviours at the research frontline, as outlined in the Project. (3) To a large degree the clustering process is induced by the multiscale and weak, surface velocity divergence, that is difficult to observe and model, because it is induced and controlled by different physical processes with poorly understood contributions and complicated mutual feedbacks. In a systematic way the Project aims to sort out clustering contributions of the underlying physical processes. (4) In connection with (1)-(3), the Project also aims to develop efficient methodologies for clustering analyses, as well as mathematical models of clustering for practical applications. It is expected that theoretical results of the Project will be useful for the existing and upcoming observational programs, as well as for many researchers dealing with marine pollution and material transport in the ocean. Links to relevant working groups and missions have been thought out, and support from Erik van Sebille, who coordinates the global research on marine plastic pollution, has been secured. Alignment to EPSRC's strategic theme and research areas: fluid dynamics, mathematical physics, nonlinear systems, living with environmental change.