Causes and solutions for the Great Atlantic Sargassum Belt

自2011年以来，加勒比海海滩上大量海藻（各种马尾藻）的出现已成为一个年度问题，例如在巴巴多斯和特立尼达和多巴哥。少量的马尾藻到达海岸是有益的——给植物施肥，加强海岸线，为当地物种提供食物等，但最近的大规模登陆破坏了当地的生态和经济，很难清理。在海岸上，马尾藻在分解时释放出含硫的气味，不仅阻塞了海滩，阻碍了游泳者的游泳。清除这些垃圾既耗时又昂贵，还会破坏海滩。来袭的木筏使海草和珊瑚礁窒息，而当地渔民则难以下水，巨大的海草木筏阻塞了他们的发动机和渔具。上岸筑巢的海龟种群也面临着风险。筑巢的地方可能会被马尾藻堵塞，或者被清理工作破坏，海龟可能会被缠住而死亡。马尾藻的新来源似乎位于热带大西洋中部（大大西洋马尾藻带，GASB，例如Wang et al., 2019）。已经提出了一些可能的原因，例如，由于森林砍伐和农业肥料使用增加，亚马逊流域的营养物质排放量增加，以及由于全球变暖和海水生化成分造成的海面温度（SST）变化。卫星图像（如MODIS）观测到马尾藻从巴西延伸到西非（Wang et al., 2019），然后由圭亚那洋流沿小安的列斯岛弧向西北方向输送并进入加勒比海。该项目是一个令人兴奋的机会，可以参与海洋科学的跨学科研究，探索物理学、生物地球化学和生态学之间的联系，以解决管理海洋的实际问题。它将使你掌握遥感和建模方面的重要技能，以及大数据管理，并有机会学习人工智能和机器学习。在这里，我们建议通过三维斜压流体动力环流的区域数值模型（使用嵌入全球NEMO模型的区域NEMO模型：Wilson等人，2019）来研究马尾藻的输送，该模型由风波引起的Stokes漂移调制（来自WAVEWATCH III模型，以下简称WW3，见Bricheno和Wolf， 2018）。该模型可以捕获三维环流（由潮汐、风、河流淡水排放和热通量驱动），以及表面温度和盐度，使我们能够探索源条件的可变性，以及大西洋和加勒比海内的运输和分散。这也可能为有害海藻的发生提供有用的预测工具，并有助于了解可能适用于其管理的各种选择的机制和影响。工具和方法将使用的方法包括：检查MODIS卫星数据（检查马尾藻在空间和时间上分布的程度和变异性）检查其他卫星数据集，例如Sentinel-1和Sentinel-2的卫星测高数据，包括海平面和波浪、海温（来自AVHRR）和盐度（来自SMOS），以验证三维斜压水动力-波浪耦合模型运行加勒比海的NEMO-WW3耦合模型；利用来自全球NEMO和WW3模型的边界强迫、模型诊断、验证和情景测试。包裹（或其他）粒子跟踪方法将应用于跟踪马尾藻通过模型的扩散。

BackgroundSince 2011 the appearance of large amounts of seaweed (various species of Sargassum) on the beaches of the Caribbean Sea has become an annual problem e.g. in Barbados and Trinidad and Tobago. Small amounts of Sargassum reaching shores has beneficial effects - fertilising plants that strengthen shorelines, providing food for local species, etc., but the recent massive landings damage local ecology and economies and is challenging to clear up.On shore, as well as blocking beaches and discouraging swimmers, the Sargassum releases sulphurous odours as it decomposes. Removal is time-consuming, expensive and can damage the beaches. Incoming rafts smother sea grasses and coral reefs, while local fishermen struggle to get into the water, with the huge rafts of seaweed blocking their engines and fishing gear. There is also the risk to the sea turtle population, which comes ashore to nest. Nesting sites can be blocked by the Sargassum or damaged by removal work - and the turtles may become entangled and die.The new source of Sargassum appears to be in the tropical central Atlantic (the Great Atlantic Sargassum Belt, GASB e.g. Wang et al., 2019). Some possible causes have been suggested, such as the increase in discharge of nutrients from the Amazon, due to deforestation and increased use of agricultural fertiliser, as well as changes in sea surface temperature (SST) due to global warming and the biochemical composition of the seawater. The Sargassum has been observed in satellite images (e.g. MODIS) to extend from Brazil to West Africa (Wang et al., 2019), and it is then transported NW by the Guiana Current along the Lesser Antilles Island arc and into the Caribbean Sea.The ProjectThis project is an exciting opportunity to get involved in interdisciplinary research in marine science, exploring the connections between physics, biogeochemistry and ecology, to address a real problem in managing our oceans. It will equip you with important skills in remote sensing and modelling, as well as management of Big Data, with opportunities to learn about AI and machine learning.Here we propose to examine the transport of Sargassum by a regional numerical model of the 3D baroclinic hydrodynamic circulation (using a regional NEMO model embedded in the global NEMO model: Wilson et al., 2019), modulated by the Stokes' drift caused by wind waves (from the WAVEWATCH III model, hereafter referred to as WW3, see Bricheno and Wolf, 2018). The model can capture the 3D circulation (driven by tides, winds, freshwater discharge from rivers and heat fluxes), as well as surface temperature and salinity, allowing us to explore the variability of the source conditions and transport and dispersion within the Atlantic Ocean and the Caribbean Sea. This may also provide a useful predictive tool for occurrence of the nuisance seaweed and help understand the mechanisms and impacts of options that may be applied to its management.Tools and MethodsThe methods to be used include:Examination of satellite data from MODIS (to examine extent and variability of Sargassum distribution in space and time)Examination of other satellite datasets e.g. satellite altimetry data from Sentinel-1 and Sentinel-2 for sea level and waves, SST (from AVHRR) and salinity (from SMOS), to validate the 3D baroclinic hydrodynamic-wave coupled modelRunning the coupled NEMO-WW3 model of Caribbean Sea, with boundary forcing from global NEMO and WW3 models, model diagnostics, validation and scenario testingThe PARCELS (or other) particle-tracking methodology will be applied to track to dispersion of the Sargassum through the model.