Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System

The holopelagic macroalgae sargassum has proliferated across the tropical Atlantic since 2011, of consequence for coastal populations from West Africa to Caribbean with limited early warning major beaching events. As part an interdisciplinary project, ‘Teleconnected SARgassum risks Atlantic: building capacity TRansformational Adaptation in and Africa’ (SARTRAC), ensemble forecast system, SARTRAC-EFS, is providing seasonal predictions drift. An eddy-resolving ocean model hindcast provides winds currents necessary generate members. Ensemble forecasts are then obtained different combinations ‘windage’, fractional influence on mats, situ rates growth, mortality, sinking. Forecasts north south Jamaica evaluated satellite-observed distributions, associated events specific years heavy inundation, 2015 2018-20. These evaluated, lead times up 180 days. subject leading modes climate variability, particular Meridional Mode (AMM). More accurate a given year members similar spring AMM-index. This most clearly evident during negative AMM phases 2018, when positive sea surface temperature anomalies anomalously weak trade were established northern tropics. On this evidence, SARTRAC-EFS potentially useful high prevalence. Extended drift off Africa, extensive cloud cover limits availability satellite data needed full application evaluation region, although experimental coast Ghana found highly sensitive windage that strong onshore boreal summer. Alongside other systems, warnings inundation at timescale.