Seasonal and Interannual Drivers of Sargassum Inundations in the Northern Gulf of Guinea

Abstract. 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 frequently hampers satellite detection in this region, complicating efforts to monitor and study the phenomenon. To overcome this limitation, we combine remote-sensing observations with process-oriented numerical simulations that represent Sargassum transport, growth, and stranding. This approach provides new insights into the seasonal cycle and the interannual variability of coastal arrivals since 2011. Both observations and the model reveal a semiannual cycle of arrivals along the n-GoG, with peaks in March-April-May and September-October-November. Our results suggest that arrivals in the n-GoG depend primarily on the advection of Sargassum from the eastern Tropical Atlantic (e-TA), whereas local growth appears to play only a minor role.  Indeed, a prior build-up of biomass preconditions n-GoG Sargassum intrusion events: off Sierra Leone for spring events, and within the ITCZ for autumn events. In both seasons, transport toward West Africa occurs via the North Equatorial Counter Current (NECC) and the Guinea Current (GC), and is particularly strong in autumn, with an advection time of about three months between the e-TA and the n-GoG coasts. After crossing Cape Palmas, Sargassum is driven shoreward by the prevailing southerly winds, which both promote coastal accumulation and maintain Sargassum north of the Equator. Without this wind-driven transport, Sargassum would remain embedded in the GC, spread more broadly across the Gulf of Guinea (GoG), and could recirculate southward through the South Equatorial Current (SEC). Coastal stranding acts as an additional regulator of the regional distribution by removing biomass from the open ocean and limiting its persistence near the coast. At interannual timescales, variability is controlled first by the amount of Sargassum biomass in the e-TA and second by their latitudinal position and likelihood of crossing Cape Palmas, especially in autumn. Both factors appear to be linked to the Atlantic Meridional Mode (AMM), with negative phases favoring entry into the GoG.