Effects of Turbulence on the Transport of Poistively Buoyant Particles in the Ocean Mixed Layer

Positively buoyant particles such as microplastic have large residence times in the ocean mixed layer (OML), and their large-scale horizontal transport is strongly influenced by the physical processes within this thin, turbulent layer. We introduce a general framework to predict the fate of positively buoyant materials in the OML under a broad spectrum of oceanic mixed layer regimes produced by various combinations of wind, waves, and surface cooling. The framework is based on the estimation of a turbulence velocity scale for the vertical mixing of buoyant materials under different combinations of wind shear, Stokes drift, and surface buoyancy flux. By combining this velocity scale with the particle's terminal rise velocity and an eddy diffusivity parameterization, an analytical prediction for the vertical profile of material concentration can be obtained. This prediction is shown to be a reasonably accurate and general representation for oceanic mixed layer regimes, and it allows simple estimates of important parameters that govern the large-scale transport, such as the depth of the center of mass and the horizontal speed and direction of material transport. In agreement with large-eddy simulations of particle transport in the OML, theoretical results highlight the strong sensitivity of large-scale transport to the balance between vertical turbulent mixing and particle buoyancy.