Sargassum Seasonal Forecast Modelling

The historical Seasonal Sargassum Forecast relies on the system described by Jouanno et al. (2023), operated by at the Laboratoire d’Etude en Géophysique et Océanographie Spatiale (LEGOS) and developed within ANR FORESEA and TOSCA SAREDA projects. This system relies on a mechanistic model of Sargassum populations, the NEMO-Sarg 1.0 model (Jouanno et al. 2021,2023, 2025), which integrates both transport and a physiology model of the macroalgae. The ensemble version of the Sargassum model at 1/4° horizontal resolution has been implemented and allows for the production of 7-month forecasts every month, initialized from Sargassum areal coverage near-real time estimates from the Moderate Resolution Imaging Spectroradiometer MODIS (Berline & Descloitres, 2021). The operational Seasonal Sargassum Forecasts (September 2025 – on going) are produced globally by Mercator Ocean International, building on the same mechanistic NEMO-Sarg 1.0 configuration (Jouanno et al., 2021, 2023, 2025, see details in 2.1). Sargassum biomass is initialized each month using near-real-time estimates derived from OLCI satellite observations (DOI: 10.24400/527896/a01-2022.007), providing an up-to-date representation of surface concentrations. This global operational framework thus complements the IRD regional setup and extends forecasting capability across the entire Atlantic basin. More details on both processing are available in the user manual.