Abstract
The dispersal of buoyant particles in the ocean mixed layer is influenced by a variety of physical factors including wind, waves, and turbulence. Microplastics observations are often made at the free surface, which is strongly forced by surface gravity waves. Many studies using numerical simulations to examine how turbulence and wave effects (e.g. breaking waves, Langmuir circulation) control buoyant particle dispersal at the ocean surface, however, are not wave phase-resolving. Therefore, the effects of an unsteady free surface due to surface gravity waves remain unknown in this context. Using numerical models and analytical techniques, we quantify the effects of a nonbreaking, monochromatic, progressive wave train on the equilibrium vertical and horizontal distributions of buoyant particles. We find that waves result in non-uniform horizontal distributions of particles with more particles under the wave crests than the troughs. We also find that the waves can stretch or compress the equilibrium vertical distribution. Finally, we consider the effects of waves on the sampling of microplastics with a towed net, and we show that waves have the ability to lower the measured concentrations relative to nets sampling without the influence of waves.
Highlights
Plastic pollution in the ocean is ubiquitous
Concentrations are a function of the local conditions: on a calm day, the buoyant plastic can aggregate at the surface, but strong winds can disperse plastic throughout the mixed layer
Both the numerical simulations and the analytical model demonstrate how the unsteady free surface associated with surface gravity waves fundamentally alters the equilibrium vertical and horizontal distributions of buoyant particles
Summary
Plastic pollution in the ocean is ubiquitous. It is found across ocean basins, in the deep-sea, and along our coasts. Concentrations are a function of the local conditions: on a calm day, the buoyant plastic can aggregate at the surface, but strong winds can disperse plastic throughout the mixed layer To address this variability, large-eddy simulations of buoyant particles at the ocean surface have been conducted to show how microplastics can redistribute due to turbulent mixing by wind, currents, breaking waves, and Langmuir circulation (Brunner et al, 2015; Kukulka and Brunner, 2015; Liang et al, 2018). The wind-mixing model assumes a balance between the upward buoyancy flux and the downward turbulent flux of particles (Kukulka et al, 2012) This is represented as a horizontally-averaged concentration of microplastics c that is a function of vertical position z. Setting the buoyancy flux equal to the turbulence flux of particles gives:
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