Abstract

During 2011, and again in 2014-2015, a number of Caribbean islands as well as west Africa and equatorial Brazil experienced the most widespread coastal accumulations of Sargassum seaweed ever reported for these regions. The resulting socioeconomic impacts spurred interest in the likelihood of similar events in the future, and in the prediction of such events for future planning and mitigation strategies. Several diverse hypotheses were put forward by the scientific community as to the source of the Sargassum and why it seemed to be blooming in such large quantities in areas where it had never been observed before, most significantly in the western tropical North Atlantic Ocean east of Barbados. Using a combination of NCEP/NCAR reanalysis fields of winds and surface currents, satellite-tracked surface drifter data from NOAA’s Global Drifter Center, and high-resolution surface currents from the global HYCOM numerical model, we find that the key to understanding the typical abundance and distribution patterns of Sargassum as well as its anomalous presence during 2011 and 2014-15 in areas outside of its usual location in the Sargasso Sea and the Gulf of Mexico lies in the effect of direct wind forcing on the sea surface by “windage”, Stommel shear, Stokes drift, or a combination thereof. These wind effects, which are not explicitly accounted for in the NCEP reanalysis or HYCOM fields and which do not play a major role in the movement of drogued surface drifters, are nevertheless very important to take into consideration. They explain not only the typical Sargassum distribution, but also the role of the extreme large-scale wind anomalies that occurred during the winters of 2010 and 2013 in dramatically (but perhaps temporarily) altering the usual Sargassum pattern.

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