Dispersion of buoyant Lagrangian particles in the wave-driven ocean surface boundary layer

"Upper ocean turbulence, generated by wind and wave forcing, directly controls air-sea exchange processes and the dispersion of material within the ocean surface boundary layer (OSBL). This study investigates the dispersion and transport of buoyant material, such as seaweed, phytoplankton, oil, and plastics, within the OSBL for varying buoyant rise velocities and wave conditions. Wave conditions studied include: shear turbulence, breaking wave (BW) effects, and Langmuir turbulence (LT). Breaking surface gravity waves transfer turbulent kinetic energy (TKE) into the ocean and result in enhanced TKE dissipation rates and mixing within a near-surface region. LT, captured by the Craik-Leibovich vortex force and other wave terms, results from interactions between the wave-driven Stokes drift and the turbulent current. LT is characterized by counter-rotating, near-surface vortices, which are a key for horizontal organization and submergence of buoyant particles. To model buoyant tracers in the turbulent OSBL, we employ a Lagrangian approach by tracking buoyant particles within a simulated OSBL flow field. The flow simulations are based on a large eddy simulation (LES) model coupled to a Lagrangian stochastic model, which captures particle velocities not resolved by the LES. Particle clouds are released at different vertical and horizontal positions and their dispersion characteristics quantified with probability density functions (e.g., concentration profiles) and the mean squared distance of particle pairs. In particular, we determine horizontal turbulent dispersion coefficients for dispersion times much larger than turbulent integral times. The initial dispersion of particle clouds depends on the local TKE dissipation rate and is nearly independent of buoyant rise velocity, consistent with the expected behavior for the inertial subrange. Enhanced TKE levels due to BW substantially increase initial dispersion rates. For longer time scales, both mean currents and turbulent eddies critically drive dispersion of buoyant particles within the OSBL. For small buoyant rise velocities, particle concentrations are transported vertically by turbulent eddies in all cases. Under shear turbulence conditions, sheared mean currents differentially advect particle clouds with respect to depth, resulting in large turbulent diffusion coefficients for cases without LT. In contrast, enhanced vertical mixing due to Langmuir turbulence homogenizes currents with respect to depth, decreasing shear dispersion and, consequentially, turbulent diffusion coefficients for small buoyant rise velocities. When buoyant rise velocity is increased, small-scale shear and breaking wave turbulence are unable to efficiently submerge particle concentrations. This results in surface trapping of highly buoyant particles and significantly reduces shear dispersion and turbulent diffusion coefficients. Large Langmuir circulations, however, are still able to submerge highly buoyant particle concentrations, increasing horizontal dispersion. Results of this study indicate that dispersion of particles is highly dependent on both buoyant rise velocity and wave conditions. Therefore, both buoyant rise velocity and wave effects must be considered when modeling the transport of buoyant material within the OSBL."