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

Abstract

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.