Vertical distribution of weakly inertial, quasi-neutrally buoyant particles in a convective ocean mixed layer

Microplastic pollution is one of the major threats to ocean health. However, the processes governing the transport and redistribution of microplastics remain poorly understood due to the interaction of multiple physical mechanisms at different scales  We investigate the vertical transport and concentration of quasi-neutrally buoyant microplastics by direct numerical simulations of small inertial particles in an inhomogeneous turbulent flow. An idealized two-dimensional convective mixed-layer model reproduces some relevant features of the upper ocean: at the surface, a well-mixed region where temperature and density are nearly homogeneous, and a lower region of weak mixing and gravity waves with strong temperature and density gradients. The dynamics of these inertial particles in both regions are analyzed using a simplified model derived from the Maxey-Riley-Gatignol equation. The model assumes particle density equal to a reference fluid density at a given depth, with density variations only affecting buoyancy (i.e., the Boussinesq approximation). Our results show that temperature differences along Lagrangian paths determine whether particles settle at specific depths or remain near the surface. The observed vertical concentration profiles in the thermocline are explained using a discrete particle framework based on a stochastically forced wave–driven relaxation model. Particle accumulation occurs preferentially near specific depths where internal gravity wave signatures are detected through oscillations of the local isopycnal structure. In the proposed description, these wave-induced fluctuations imprint a structured modulation of the concentration profile, while turbulent fluctuations are represented as a white-noise forcing that accounts for particle spreading around the accumulation depths. The relative importance of wave-driven relaxation and turbulent diffusion varies with depth, reflecting the anisotropic and inhomogeneous nature of the stratified flow. This approach consistently reveals that, while gravity has a pivotal role on particle transport and accumulation, the fluid’s eddy diffusivity can also have non-negligible effects on the spreading of particles, depending on the physical properties of the latter.