Seasonal Predictions of Holopelagic Sargassum Across the Tropical Atlantic Accounting for Uncertainty in Drivers and Processes: The SARTRAC Ensemble Forecast System
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Date
2021
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Abstract
The holopelagic macroalgae sargassum has proliferated across the tropical Atlantic
since 2011, of consequence for coastal populations from West Africa to the Caribbean
with limited early warning of major beaching events. As part of an interdisciplinary
project, ‘Teleconnected SARgassum risks across the Atlantic: building capacity for
TRansformational Adaptation in the Caribbean and West Africa’ (SARTRAC), an
ensemble forecast system, SARTRAC-EFS, is providing seasonal predictions of
sargassum drift. An eddy-resolving ocean model hindcast provides the winds and
currents necessary to generate ensemble members. Ensemble forecasts are then
obtained for different combinations of ‘windage’, the fractional influence of winds on
sargassum mats, and in situ rates of growth, mortality, and sinking. Forecasts for north
and south of Jamaica are evaluated with satellite-observed distributions, associated
with beaching events in specific years of heavy inundation, 2015 and 2018-20. These
seasonal forecasts are evaluated, on lead times of up to 180 days. Forecasts are
subject to leading modes of tropical climate variability, in particular the Atlantic Meridional
Mode (AMM). More accurate forecasts for a given year are obtained with ensemble
members from hindcast years with a similar spring AMM-index. This is most clearly
evident during negative AMM phases in spring of 2015 and 2018, when positive sea
surface temperature anomalies and anomalously weak trade winds were established
across the northern tropics. On this evidence, SARTRAC-EFS is potentially useful in
providing early warning of high sargassum prevalence. Extended to sargassum drift
off West Africa, extensive cloud cover limits availability of the satellite data needed for
full application and evaluation of SARTRAC-EFS in this region, although experimental forecasts off the coast of Ghana are found highly sensitive to the windage that is
associated with strong onshore winds during boreal summer. Alongside other forecast
systems, SARTRAC-EFS is providing useful early warnings of sargassum inundation at
seasonal timescale.
Description
Research Article
Keywords
sargassum, currents, windage, seasonal, prediction, Jamaica, Ghana