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The role of convective vortices in accumulating buoyant material in the ocean

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Buoyant material such as microplastics tend to accumulate near the ocean surface in regions with convergent surface currents where they can be harmful to marine life. In this talk, we investigate the small-scale transport and accumulation of buoyant material by small-scale turbulence in the ocean mixed layer under combined wind and convective forcing using large-eddy simulations. We model non-inertial buoyant particles with a combination of buoyant tracers and Lagrangian surface particles, which allows us to explore a wide range of buoyancies. Surface cooling drives convection, and under this regime persistent convective vortices form which trap buoyant material, leading to large concentrations. Despite their small size, the convective vortices that form exhibit a bias towards cyclonic vorticity. We find that the average time a particle spends inside a convective vortex is long enough for vortex stretching to amplify planetary vorticity and that subsequently particle vorticity increases exponentially. For sufficiently weak winds, convective vortices survive but become less effective at clustering material as the wind stress increases. Under strong wind forcing, convective vortices are no longer visible, but some particle clustering occurs in downwelling regions associated with longitudinal wind rolls.

This talk is part of the Seminars for the Centre for Environmental and Industrial Flows series.

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