Turbulent dispersion in the Ocean Surface Boundary Layer

The Ocean Surface Boundary Layer is where the atmosphere and the ocean interact and where the main exchanges of energy and matter take place. This layer is of considerable interest from an environmental point of view, as it is the area richest in flora and fauna, but in pollutants. In particular, among these, the study of the dispersion of plastic fragments with dimensions smaller than 5 mm defined as microplastics is crucial. In fact, in recent years, the growing consumption of single-use plastics, the low recycling rate and the improper disposal of waste have resulted in a serious threat to the marine ecosystem and to human health. Moreover, considering that there is a large mismatch between the estimates of the amount of plastic that enters the oceans and the total amount of plastic sampled at sea surface, for this particular pollutant, the study cannot be limited to the surface, but must also be extended to the entire water column. The study of microplastics dispersion in the mixed layer is tackled by the scientific community with numerical models, laboratory experiments and in situ measurement campaigns. Lagrangian models are very commonly used for this purpose, because they can explicitly take into account the different scales of turbulence and they are very often combined with parametric laws directly related to the travel time of the particles. The aim of this work is to develop a methodology to deduce the marine turbulence, usable for calculating the dispersion of pollutants in the marine environment; in particular, an innovative parameterization of the vertical turbulent diffusivity have been developed with the aim to consider the sub-grid turbulence occurring in the mixed layer, which is not completely resolved by oceanographic models. This similarity law is based on characteristic scales calculated from meteorological and operational oceanographical models. This parametrization of the vertical turbulence has been implemented in a 3D Lagrangian dispersion model (Wiener3D v6). The model has been validated by comparison with microplastics sampling data deriving from marine monitoring campaigns. With this Lagrangian model, long-term simulations have been conducted in a domain comprising the Tyrrhenian Sea. The simulations allowed investigating the role of mean currents, sub-grid turbulence and, along the vertical direction, a rising/settling velocity in the microplastics displacements. Thus, the model allowed the analysis of the vertical distribution of microplastics along the water column as well as of beaches or seafloor zones subjected to accumulation. The results showed that vertical turbulent diffusivity is an essential factor for the vertical dispersion of microplastics within the mixed layer. Finally, it is observed that the settling/rising velocity assumes a key role in determining the accumulation of pollutants on the seafloor or on the surface, respectively.