Maxey-Riley advection leads to enhanced spatial variability of buoyant macroplastic on the north-west European shelf seas

Macroplastics (plastic objects > 5 cm) make up most of the mass of plastic in the ocean. Most plastic enters the ocean in the form of macroplastic to only later fragment into microplastic. Thus, cleaning up macroplastic is potentially an effective way to prevent microplastic pollution in the ocean. However, the distribution of macroplastic varies widely in space and time, especially in coastal regions where most macroplastic enters the ocean. What processes cause this large variability is not yet understood.

In our study, we investigate how the “inertia” of macroplastic affects their trajectories. For this, we use Lagrangian analysis. Most commonly in Lagrangian analysis, plastic particles are advected with the fluid flow, sometimes with an additional windage term and added vertical velocity due to particles' buoyancy. However, due to the finite size and positive buoyancy of macroplastics, this simplified approach does not fully describe their movement: instead, their motion is governed by the Maxey-Riley equations. These equations describe the motion of particles in a fluid as a result of the forces working on these particles. In this work we include the effects of inertia, viscous drag, added mass and the Coriolis acceleration. We implemented the Maxey-Riley equations in OceanParcels, allowing simulation of the trajectories of a large number of particles in surface 2D ocean flows. Using this implementation, we study the Maxey-Riley effects on the trajectories of the particles. Here, we focus on gaining understanding under what conditions the trajectories and accumulation patterns of buoyant inertial particles deviate from tracer particles, where we investigate both the role of the characteristics of the ocean flow and particle properties (i.e., size and buoyancy).

As most macroplastic enters the ocean in coastal areas, we chose to study the effect of their finite size and positive buoyancy on their trajectories in the north-west European shelf. We find that the Coriolis forces in the Maxey-Riley equations affect the surface 2D accumulation pattern of macroplastic. Compared to the accumulation patterns of tracer particles, we find enhanced accumulation in specific areas under specific conditions. Thus, the large observed spatial variability of macroplastic might partly be explained by the Coriolis effect coupling to the finite size and positive buoyancy of the particles.