Forecasting sargassum across the tropical Atlantic with SARTRAC-EFS

The emergence and spread of sargassum seaweed in the tropical Atlantic is a major environmental challenge for many communities of the islands and coastal states of this region. Observed in satellite images, several groups have worked to predict the movement of sargassum, subject to ocean currents and surface winds. We add a little bit of biology too!

Through the SARTRAC project , we developed an ‘Ensemble Forecast System’ (EFS), hence SARTRAC-EFS. We use SARTRAC-EFS to forecast the drift of sargassum on timescales of 3-6 months, and its arrival offshore at selected locations. Sargassum is represented with many thousand ‘particles’, in proportion to how much sargassum is ‘seen’ by the satellite. Individual particles are then moved with sampled ocean currents, and optionally winds, for up to 6 months. Each particle follows a unique ‘trajectory’. We average all the trajectories together to obtain predicted distributions and local forecasts.

A full description of SARTRAC-EFS can be found in:

Marsh R, Addo KA, Jayson-Quashigah P-N, Oxenford HA, Maxam A, Anderson R, Skliris N, Dash J, Tompkins EL. Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System. Front. Mar. Sci. 2021; 8:722524. doi: 10.3389/fmars.2021.722524

Reflecting on the challenges of this forecasting, we also evaluate such approaches in:

Marsh R, Oxenford HA, Cox S-AL, Johnson DR, Bellamy J. Forecasting seasonal sargassum events across the tropical Atlantic: Overview and challenges. Front. Mar. Sci. 2022; 9:914501. doi: 10.3389/fmars.2022.914501

Here, we provide some examples from SARTRAC-EFS, with predicted distributions in the Caribbean and West African regions of the tropical Atlantic, along with corresponding forecasts around Jamaica and along the coast of Ghana.

First choose to look at 30-day snapshots of sargassum distributions (percentage cover) in the Caribbean or West African region. For the Caribbean region, you will see distributions up to 180 days from the initial satellite-observed distribution. For the west African region, these distributions are only reliable up to 90 days. In both cases, we show distributions subject to ocean currents alone, and subject to ocean currents plus the additional effects of wind acting on exposed sargassum (sometimes called ‘windage’).