Modeling Indian Ocean circulation to study marine debris dispersion: insights into high-resolution and Stokes drift effects with Symphonie 3.6.6

Abstract. The Indian Ocean basin faces significant anthropogenic pressure due to its connection to over 2.2 billion people through river basins. Indian Ocean dynamics are characterized by strong regional and seasonal variability driven by the monsoon system and intense eddy activity. To address the issue of land-sea transfers and marine debris dispersion in this complex ocean, we developed a new circulation modeling configuration using the hydrodynamic model SYMPHONIE. Our configuration introduces a unique telescopic grid covering the entire basin, enabling the study of sub-basin connectivity while resolving meso and submesoscale processes in the coastal region, from the Mozambique Channel to the Bay of Bengal, at a resolution of 1 to 3 km. Additionally, we integrate the recently released high-resolution GloFAS river discharge dataset to force the physical simulations with daily freshwater inputs. Three annual experiments are conducted, alternatively considering Stokes drift forcing and different grid resolutions. Comparisons of temperature, salinity and sea level with in situ and satellite data show the good performance of the simulations and the ability of the high resolution model to accurately capture the spatial and temporal variability of surface dynamics and water masses over the Indian Ocean. We further analyze energy budgets and perform Lagrangian experiments to illustrate the critical role of resolution and Stokes drift in shaping the circulation and the resulting marine debris dispersion patterns. The effect of energy levels is particularly significant on trajectory statistics such as average travel distances and preferred spread direction. Notably, Stokes drift has a significant seasonal effect in the Arabian Sea during the southwest monsoon, while current field resolution strongly influences trajectories in the Mozambique Channel. Our results provide a robust modeling framework for studying Indian Ocean dynamics and exploring their effect on marine connectivity and the transport of matter, including pollutants, larvae or organic matter.