Seasonal and interannual drivers of Sargassum inundations in the Northern Gulf of Guinea

Sargassum strandings have become recurrent along the northern Gulf of Guinea (n-GoG) and are reported in the literature as having significant societal impacts, particularly on fisheries. However, persistent cloud cover limits satellite monitoring, complicating efforts to study the phenomenon. We combine satellite-derived observations, NEMO-Sarg simulations of transport, growth and stranding, and Lagrangian trajectories to identify the seasonal pathways and interannual controls of coastal arrivals during 2010–2024. Both observations and the model reveal a semiannual cycle, with coastal maxima in March–May and September–November. In both seasons, biomass is supplied mainly from the eastern tropical Atlantic (e-TA) rather than by local growth. Spring events are linked to biomass retained off Guinea and Sierra Leone during winter, together with an additional lower-latitude pool, whereas autumn events result from a larger upstream accumulation near the Intertropical Convergence Zone (ITCZ). Transport occurs through the North Equatorial Countercurrent (NECC) and Guinea Current (GC), with an advection time of two to three months between the e-TA and the n-GoG. After passing south of Cape Palmas, southerly winds drive Sargassum shoreward and help maintain it north of the Equator. Without windage and Stokes drift, Sargassum remains embedded in the Guinea Current, spreads farther across the Gulf and partly recirculates through the South Equatorial Current, potentially favouring wider proliferation. Stranding removes nearshore biomass and limits its persistence. Interannual variability depends on both upstream biomass supply and its position relative to Cape Palmas, especially in autumn. Both are linked to the Atlantic Meridional Mode, with negative phases shifting biomass southward and enhancing eastward transport into the n-GoG. These results provide a process-based framework for interpreting sparse observations and improving seasonal risk assessment and coastal preparedness in West Africa.