The establishment of a pelagic Sargassum population in the tropical Atlantic: Biological consequences of a basin-scale long distance dispersal event

• Published in: Progress in oceanography 2020-03, Vol.182, p.102269, Article 102269
• Main Authors: Johns, Elizabeth M., Lumpkin, Rick, Putman, Nathan F., Smith, Ryan H., Muller-Karger, Frank E., T. Rueda-Roa, Digna, Hu, Chuanmin, Wang, Mengqiu, Brooks, Maureen T., Gramer, Lewis J., Werner, Francisco E.
• Format: Article
• Language: English
• Subjects: Caribbean Sea; Ekman pumping; Inter-Tropical Convergence Zone (ITCZ); Mixed layer depth; North Atlantic Ocean; North Atlantic Oscillation (NAO); Sargasso Sea; Sargassum; Tropical Atlantic; Windage
• ISSN: 0079-6611; 1873-4472
• DOI: 10.1016/j.pocean.2020.102269

•Sargassum was exported to the tropical Atlantic during the 2009–2010 NAO anomaly.•Windage is required to reproduce the observed Sargassum distributions.•Exceeding a biosphere tipping point may have led to a tropical Atlantic Sargasso Sea.•Sargassum is aggregated seasonally by Inter‐Tropical Convergence Zone (ITCZ) winds.•Growth in the central tropical Atlantic is enhanced by vertical mixing dynamics. Starting in 2011, coastal areas of the Caribbean Sea and tropical Atlantic Ocean began to experience extraordinary yearly accumulations of pelagic Sargassum brown alga. Historical reports place large quantities of Sargassum only in the North Atlantic (mostly in the Gulf of Mexico and the Sargasso Sea). Accumulations of Sargassum in the tropical Atlantic have continued. We used a numerical particle-tracking system, wind and current reanalysis data, drifting buoy trajectories, and satellite imagery to determine the origin of the Sargassum that is now found persistently in the tropical Atlantic. Our analyses suggest that during the extreme negative phase of the winter 2009–2010 North Atlantic Oscillation (NAO), unusually strong and southward-shifted westerly winds explain the transport of Sargassum from the Sargasso Sea (∼20–40°N, 80–20°W) into the far eastern North Atlantic. Our hindcast Sargassum distributions agree with surface current simulations with the inclusion of “windage”. Windage is the additional, wind-induced drift of material floating at the free surface resulting from direct wind forcing on the sea surface, as well as on floating or partially-submerged objects. In our simulations, windage is included as an added vector (speed and direction) to the model-computed surface ocean currents equivalent to 1% of surface wind velocities. Lagrangian analysis of the regional circulation suggests that (1) part of the Sargassum subsequently drifted to the southwest in the North Equatorial Current (NEC) and entered the central tropical Atlantic, arriving in the Caribbean by the spring of 2011, with (2) another portion continuing southward along the coast of Africa in the Canary Current, eventually joining the seasonally-varying system of tropical Atlantic currents and thereby delivering a large Sargassum population to the tropical Atlantic. Since then, Sargassum patches aggregate from March to September in massive windrows along the Inter-Tropical Convergence Zone (ITCZ) under the action of converging winds. The windrows follow the ITCZ in its seasonal northwar