Abstract 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 northward migration in the central tropical Atlantic. They are stretched across the central tropical Atlantic as the ITCZ crosses the latitude of the seasonal formation of the North Equatorial Counter Current (NECC). These patches and windrows are exposed to high sunlight and open-ocean upward flux of nutrients due to eddy and wind-driven mixing in the central tropical Atlantic. During the northern spring and summer, as the Sargassum drifts farther north with the ITCZ, large portions of the population are advected into the eastern Caribbean Sea. Some of these patches remain dispersed as the ITCZ migrates southward, and re-aggregate into new windrows as the ITCZ intensifies the following March-April. If wind mixing is strong and the mixed layer is deeper than about 50–60 m in the southern tropical Atlantic at this time, the Sargassum will bloom and form a massive windrow. Otherwise, the bloom will be inhibited. The extreme 2009–2010 NAO wind anomaly could be considered as triggering a biosphere “tipping point” that caused important ocean-scale ecosystem changes in the tropical Atlantic, with significant recurrent social and economic consequences. Understanding whether this new expanded geographic range of massive Sargassum blooms is temporary or whether it will revert to its pre-2009 distribution requires sustained monitoring and research.

中文翻译：

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在热带大西洋建立远洋马尾藻种群：盆地尺度长距离扩散事件的生物学后果

摘要 从 2011 年开始，加勒比海和热带大西洋沿岸地区开始出现中上层马尾藻褐藻异常的逐年积累。历史报告仅在北大西洋（主要在墨西哥湾和马尾藻海）发现了大量马尾藻。热带大西洋中马尾藻的积累仍在继续。我们使用数值粒子跟踪系统、风和洋流再分析数据、漂流浮标轨迹和卫星图像来确定现在在热带大西洋中持续发现的马尾藻的起源。我们的分析表明，在 2009-2010 年冬季北大西洋涛动 (NAO) 的极端负相期间，异常强劲且向南移动的西风解释了马尾藻从马尾藻海（~20-40°N，80-20°W）进入远东的北大西洋。我们的后测马尾藻分布与包含“风阻”的表面电流模拟一致。风阻是由风直接作用在海面以及漂浮或部分淹没的物体上引起的漂浮在自由表面上的物质的额外的风引起的漂移。在我们的模拟中，风阻作为附加矢量（速度和方向）包含在模型计算的表面洋流中，相当于表面风速的 1%。对区域环流的拉格朗日分析表明 (1) 部分马尾藻随后随北赤道流 (NEC) 向西南方向漂移，进入热带大西洋中部，于 2011 年春季抵达加勒比地区，(2) 另一部分在加那利洋流中沿非洲海岸继续向南，最终加入季节性变化的热带大西洋洋流系统，从而将大量马尾藻种群输送到热带大西洋。从那以后，在汇聚风的作用下，马尾藻斑块从 3 月到 9 月聚集在热带间辐合带 (ITCZ) 沿线的大量成堆中。在热带大西洋中部的季节性向北迁移中，这些草堆跟随 ITCZ。当 ITCZ 穿过北赤道逆流 (NECC) 季节性形成的纬度时，它们横跨热带大西洋中部。由于热带大西洋中部的涡流和风驱动混合，这些斑块和草堆暴露在高阳光和公海向上的营养物质流中。在北部春季和夏季，随着马尾藻随 ITCZ 向北漂流，大部分人口平流进入东加勒比海。随着 ITCZ 向南迁移，其中一些斑块仍然分散，并在接下来的 3 月至 4 月 ITCZ 加剧时重新聚集成新的小块。如果此时在热带大西洋南部混合风强且混合层深约50-60 m，马尾藻将开花并形成巨大的草丛。否则会抑制开花。2009-2010 年的极端 NAO 风异常可被视为触发了生物圈“临界点”，导致热带大西洋发生了重要的海洋尺度生态系统变化，并具有显着的反复出现的社会和经济后果。