The Role of the Unsteady Surface Wave‐Driven Ekman–Stokes Flow in the Accumulation of Floating Marine Litter

• Published in: Journal of geophysical research. Oceans 2022-06, Vol.127 (6), p.n/a
• Main Authors: Cunningham, H. J., Higgins, C., Bremer, T. S.
• Format: Article
• Language: English
• Subjects: Accumulation; Boundary layers; Convolution; Coriolis force; Distribution; Drift; Driftnets; Earth rotation; Ekman‐Stokes flow; Equator; Floating; Flow; Geophysics; Gyres; Kernels; Lagrangian modeling; Litter; Litters (births); Marine debris; Marine pollution; microplastic accumulation; Microplastics; Modelling; Oceans; Particle tracking; Particle transport; Plastic debris; Plastic pollution; Polar environments; Polar regions; Sediment transport; Simulation; Stokes drift; Stokes flow; Surface currents; Surface water waves; Surface waves; Viscosity
• ISSN: 2169-9275; 2169-9291
• DOI: 10.1029/2021JC018106

Recently, a number of authors have used global particle tracking simulations to identify the effect that different surface currents have on marine litter accumulation, including the role of surface waves through the Stokes drift. However, in the upper‐ocean boundary layer and in the presence of the Coriolis force, a wave‐driven Eulerian flow forms that must be superimposed onto the Stokes drift in order to obtain the correct Lagrangian velocity. Taking into account both the Coriolis–Stokes force and the surface wave stress, Higgins et al. (2020), https://doi.org/10.1029/2020GL089189 derived an expression for this unsteady wave‐driven Eulerian‐mean flow in the form of a convolution between the Stokes drift and the so‐called Ekman–Stokes kernel. In this paper, we apply this Ekman–Stokes kernel to generate a 12‐year global hindcast of the wave‐driven Eulerian current and show that its inclusion in particle tracking simulations has a significant effect on the distribution of small floating marine litter, such as microplastics. Using Lagrangian simulations, we find that the wave‐driven Eulerian current is sensitive to the value of viscosity but generally opposes the dispersive behavior of the Stokes drift, reducing the amount of cross‐Equator particle transport and transport to the polar regions, resulting in closer agreement between modeled and observed microplastic distributions. Plain Language Summary Though marine litter is found to accumulate near the surface in each of the subtropical ocean gyres, its transport pathways and accumulation behaviors are poorly understood. Particle tracking simulations have been used to model the distribution of marine litter. In recent studies, the effects of wave‐driven transport have been considered by simply superimposing the Stokes drift (net drift induced by surface waves) onto other non‐wave‐driven surface currents. However, due to the Earth's rotation and turbulent behavior in the upper‐ocean boundary layer, the Stokes drift also generates an additional wave‐driven Eulerian‐mean flow, known as the Ekman–Stokes flow. This wave‐driven Eulerian flow must be superimposed on the Stokes drift to form the correct wave‐driven Lagrangian current. Using the convolution expression derived by Higgins et al. (2020), https://doi.org/10.1029/2020GL089189, we develop a 12‐year global hindcast for the unsteady wave‐driven Eulerian current and subsequently perform Lagrangian particle tracking simulations to model the accumulation of sm