The impact of windage on the transport and accumulation of marine litter: A Lagrangian analysis in the Bay of Biscay

The southeastern Bay of Biscay (SE-BoB) is known as an accumulation zone for marine litter. Coastal Current Convergence Structures (CCS), which are linked to vertical movements at the edges of river plumes, estuarine fronts, or other frontal processes, have been observed in this area to act as aggregators of floating marine litter (FML). Observed at the ocean surface, CCS often form narrow filament-like streaks with variable loads of FML. Understanding the role of CCS in the transport and aggregation of FML is crucial for quantifying and mitigating pollution. Within the Lagrangian framework, Finite Size Lyapunov Exponents (FSLE) and Finite Domain Lagrangian Divergence (FDLD) provide an effective way to identify and analyze CCSs in the area using velocity field data from the EuskOOS High-Frequency Radar network. Backwards-in-time FSLE and FDLD unveil Lagrangian Coherent Structures (LCS), which act as convergent transport barriers, a proxy of CCS where larger concentration of FML of different buoyancy is likely to occur. Since FML is buoyant and its motion can be driven also by windage, additional computations are conducted considering the effect of the wind drag force on FML trajectories and Lagrangian quantities. Escaping and beaching rate of particle clusters, delineating areas of different dynamic regimes, are also assessed considering different windage coefficients. Our results suggest that the wind has a significant influence on the FML distribution and accumulation paths in the study area, causing the aggregation of materials in areas not coinciding with CCS driven only by currents. Different locations of barriers to transport are obtained, depending on the wind regime and windage considered. Below 2–3% windage, aggregation structures are shifted in the direction of currents and winds. Above 3% windage, beaching and escaping rates are increasing and aggregation structures tend to merge. • Wind has a significant influence on FML distribution and accumulation paths. • Along-shore anisotropic dispersion is observed in the SE-BoB. • Coastal Coherent Convergent Structures are drastically modified by windage. • Beaching events increase due to the effect of windage.