Including the effects of subsurface currents on buoyant particles in Lagrangian particle tracking models: Model development and its application to the study of riverborne plastics over the Louisiana/Texas shelf

We report on the development and implementation of a novel physics-based algorithm that includes the effects of subsurface current on buoyant particles in a Lagrangian particle tracking model, specifically the Larval TRANSport Lagrangian (LTRANS) model. In the upper ocean, the direction and the magnitude of horizontal current change with water depth due to local effects such as the Ekman balance and large-scale influences such as the thermal wind balance. The subsurface current alters both the advection and the dispersion of weakly buoyant particles that could be suspended in the ocean surface boundary layer by turbulence. The transport and fate of riverborne plastics from the Mississippi and Atchafalaya Rivers were examined using a hindcast model solution for the Louisiana and Texas Continental Shelf (LATEX shelf). The model results show that after leaving the river outlets, plastic particles travel on average in a southwestward direction over the LATEX shelf for up to four months, transiting from a few hundred kilometers to more than a thousand kilometers. The pathways and the fate of particles change as a function of the particle size, with smaller particles traveling slower. Also, less buoyant particles transit longer distances over more offshore regions and remain longer on the LATEX shelf. Particle pathways also display a strong seasonal cycle associate with the seasonal cycle of circulation on the LATEX shelf. The model results further reveal that more than 50% of the plastic particles delivered by the Mississippi and Atchafalaya Rivers reach the land boundary of the LATEX shelf, and more than 18% of those reach the land boundary west of Louisiana including Texas and portions of Mexico. This modeling framework can be used to study the transport of other positively buoyant particulate materials in the ocean, such as spilled oil and plankton.