On Clustering of Floating Tracers

There are many floating tracers and substances on the ocean surface: plastic pollution (including litter and microplastic), oil spills, sargassum, plankton, ice, and wreckage debris. In a complicated and poorly understood way, the floating tracers are constantly moved around, redistributed and clustered by the large-scale ocean circulation, mesoscale eddies, submesoscale motions, and smaller-scale flow features. Here, by clustering we refer to the process of accumulation and aggregation of tracers in structured and temporally evolving spatial patterns. Density of a floating tracer evolves in non-Lagrangian way, because of the additional compressibility effect induced by the velocity divergence, but the Lagrangian description can be modified to remain useful. All our results are obtained within the modified Lagrangian framework.

First, we explain such concepts as clustering mass and area, as well as the processes of exponential (complete) and fragmentation clusterings. Second, we discuss clustering phenomenology in random kinematic velocity fields, its important aspects, and the involved assumptions and methods. Clusterings of floating and passive tracers are systematically compared. Finally, we extend the results by considering hybrid velocity fields, characterized by dynamically constrained and realistic mesoscale component and by randomly modelled submesoscale component.

We argue that our study paves the way for physically sound, scientifically systematic and methodologically equipped exploration of the floating-tracer clustering phenomena in progressively more complicated flows.