Comment on egusphere-2023-1624

Abstract. Although the movement and aggregation of microplastics at the ocean surface has been well studied, less is known about the subsurface. Within the Maxey-Riley framework governing the movement of small spheres with high drag in fluid, aggregation of buoyant particles is encouraged in vorticity-dominated regions. We explore this process in an idealized model of a three-dimensional eddy with an azimuthal and overturning circulation. In the axially symmetric state, particles that do not accumulate at the top boundary are attracted to a closed contour consisting of periodic orbits. Such a contour exists when drag on the particle is sufficiently strong. For small slightly-buoyant particles, this contour is located close to the periodic fluid trajectory. If the symmetric flow is perturbed by a symmetry-breaking disturbance, additional attractors arise near periodic orbits of fluid particles within the resonance zones created by the disturbance. Disturbances with periodic time dependence produce even more attractors, with a shape and location that recurs periodically, and which are composed of quasiperiodic orbits of rigid particles. Not all such contours attract, and particles released in the vicinity may instead be attracted to a nearby attractor. Examples are presented along with mappings of the respective basins of attraction.