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Evidence for the Influence of Surface Heat Fluxes on Turbulent Mixing of Microplastic Marine Debris
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This oceanographic study found that microplastic concentrations at the ocean surface increase during daytime heating and decrease at night when temperature-driven turbulence pulls floating plastics deeper. The findings help explain why surface measurements of ocean plastic may underestimate total plastic loads in the water column.
Abstract Buoyant microplastic marine debris (MPMD) is a pollutant in the ocean surface boundary layer (OSBL) that is submerged by wave-driven turbulent transport processes. This study analyzes observed MPMD surface concentrations in the Atlantic and Pacific Oceans to reveal a significant increase in concentrations during surface heating and a decrease during surface cooling. Turbulence-resolving large-eddy simulations of the OSBL for an idealized diurnal heating cycle suggest that turbulent downward fluxes of buoyant tracers are enhanced at night, facilitating deep submergence of plastics, and suppressed in heating conditions, resulting in surface-trapped MPMD. Simulations agree better with observations if enhanced mixing due to wave-driven Langmuir turbulence (LT) is included. The simulated time-dependent OSBL response results in hysteresis effects so that surface concentrations depend also on the phase of the diurnal heating cycle. The results demonstrate the controlling influence of surface heat fluxes and LT on turbulent transport in the OSBL and on vertical distributions of buoyant marine particles.
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When a large batch of papers lands in the Atlas, we read through it and send a short write-up of what stood out.