Modeling the convergence of buoyant debris in the Indian Ocean: Insights from surface drifter trajectories and Lagrangian simulations

While it is known that plastics floating on the ocean surface converge at the center of the five subtropical gyres, this convergence remains poorly documented in the Indian Ocean, particularly due to a lack of in situ measurements. In this study, we use the trajectories of six surface drifters, active between December 2024 and May 2025 and likely representative of the behavior of floating plastic debris, to develop an optimized current product that best reproduces their observed drift. This product is constructed as a weighted sum of three components: general circulation, tidal currents, and Stokes drift. The inclusion of Stokes drift appears essential for minimizing errors. Particle convergence is then studied using this optimized product in Lagrangian simulations performed over the Southern Indian Ocean. These simulations reveal a convergence zone located in the western part of the basin, fed by two main pathways via the Agulhas Return Current and the South Equatorial Current, and characterized by a particularly intense escape route via the Agulhas Current. The interannual variability of the convergence zone is also discussed.