Since 2011, massive strandings of pelagic Sargassum have become a recurrent environmental hazard across the tropical Atlantic and Caribbean archipelago, creating an urgent need for reliable short-term drift forecasts to support coastal risk management. This study evaluates key sources of uncertainty in operational Sargassum drift forecasting by analyzing the sensitivity of Lagrangian simulations to the representation of floating material and to environmental forcing fields. The analysis uses two complementary observational datasets: trajectories of four GPS-tracked Sargassum mats deployed near Puerto Rico and thirteen 24 h displacement vectors derived from sequential Sentinel-3 satellite detections across the tropical North Atlantic. Drift simulations were performed with the MOTHY model under multiple configurations, testing two material parameterizations, different atmospheric forcings, and several ocean circulation products and vertical current integration strategies. The results indicate that the best agreement with observed trajectories is obtained for partially immersed structures, highlighting the importance of balancing wind exposure and hydrodynamic drag. Sensitivity experiments further show that ocean circulation forcing dominates trajectory skill, while higher-resolution atmospheric forcing provides limited improvement under offshore conditions. Overall, the study confirms the importance of accurately representing upper-ocean transport processes and provides observational support for several operational choices implemented in the Météo-France Sargassum forecasting system.

Marine oil spill management requires rapid and accurate forecasts to mitigate environmental and operational risks. Created in 2002 during the Prestige spill and formally established in 2006, the French Drift Committee (CODER) unites expertise from CEDRE, Météo-France, Ifremer and Shom to refine oil drift modeling and response strategies. CODER’s operational scope combines multi-model forecasting, including MOTHY and OILMAP, integrated with real-time observations from drifting buoys, satellite/aerial observations and high-resolution environmental data, such as Copernicus Marine Environment Monitoring Service (CMEMS) current forecasts and Météo-France’s ARPEGE and AROME wind models. Annual exercises, such as RAMOGEPOL 2025, provide a controlled environment to test model performance in diverse hydrodynamic conditions and recalibrate simulations using real-time buoy data. During this exercise, the French Navy’s Anti-Pollution Practical Expertise Center (CEPPOL) deployed buoys in the Mediterranean Sea, off the coast of Saint-Tropez, France, in both the Northern Current and wind-driven coastal waters, enabling CODER to assess model performance and refine simulations by comparing buoy trajectories with model outputs. This process enhances the accuracy of drift predictions, improves the understanding of local hydrodynamics, and identifies models’ strengths and limitations. By systematically evaluating and adjusting models using operational data, CODER strengthens the reliability of drift forecasts, ensuring more effective and adaptive responses to marine pollution incidents.