Clustering of Floating Tracers in the Ocean

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.