Data-driven enhancement of ocean surface forcing for accurate floating debris transport modelling in the East/Japan Sea

Abstract. Floating debris transport in marginal seas is controlled by a combination of geostrophic currents, wind-driven currents, wave-induced drift, and direct wind forcing, but the relative importance of these processes remains difficult to quantify without in-situ trajectory observations. This study evaluates near-surface forcing combinations for particle tracking in the East/Japan Sea using 33 GPS-tracked surface drifters deployed off the Korean coast in November 2021. The drifters revealed a clear cross-basin transport pathway toward the Japanese coastline and an early bifurcation near a hyperbolic-type Lagrangian structure. We used a particle tracking model to test combinations of geostrophic current, Ekman current, Stokes drift, and windage against the observed drifter trajectories using MAE and NCLS metrics. For undrogued DX drifters, the best performance was obtained by combining geostrophic current, Stokes drift, and windage. For drogued DO drifters, which sampled a deeper effective depth, the inclusion of all four forcing components provided the best agreement with observations. The optimized forcing combinations outperformed benchmark simulations driven by the GOFS analysis and GLORYS reanalysis in this specific event, primarily because they better reproduced the Lagrangian structure associated with the early trajectory bifurcation. Seasonal simulations showed that winter winds enhance eastward transport and beaching along Japan, whereas weaker summer winds allow mesoscale eddies to broaden particle dispersion. The present framework represents the fluid-dynamical component of near-surface particle transport and does not explicitly include plastic-specific fate processes such as fragmentation, biofouling-induced buoyancy changes, aggregation, settling, or resuspension.