| Popis: |
The majority of buoyant, marine microplastics are missing from the ocean surface, however, their presence has been observed throughout the ocean water-column and in seafloor sediments. The accumulation of microorganisms on the microplastics' surface, termed biofouling, has been proposed as a mechanism contributing substantially to their removal from the ocean surface by changing the particles' buoyancy. Mathematical models outlining the biofouling processes on microplastics have been implemented in General Ocean Circulation models (OGCMs) in an effort to predict their long-time transport in the ocean. The resolution of OGCMs is extremely course and sub-grid turbulent motions so far have either been neglected or parameterised via stochastic perturbations through an eddy diffusivity term. Accurately capturing the sub-grid turbulent motions, and quantifying the uncertainty they introduce to particle trajectories, is critical to establishing predictive maps of biofouled microplastic transport in the ocean. As an intuitive first step using representative models of oceanic, turbulent motions to define a more accurate parameterisation, we use the Maxey-Riley-Gatignol equations for the motion of small particles and analyse the trajectories of both individual and many particles with increasing, negative buoyancy in steady and unsteady deterministic, Taylor-Green vortex flow. We quantify the uncertainty these vortical flows introduce across parameter ranges of interest to microplastic and oceanic turbulence, finding that coherent turbulent structures, such as Langmuir circulations, could lead to errors in vertical position prediction on the order of kilometres. Comparatively, a stochastic parameterisation is shown to significantly under-approximate the dispersion of particles from the vortical flow, suggesting that the current standard using an eddy diffusivity term to capture turbulent mixing may be too simplistic. |
