The Gulf Stream control of the North Atlantic carbon sink

我们都知道，在我们的一生中，大气中的二氧化碳含量一直在上升，导致全球变暖。海洋通过吸收和储存25%的排放碳，在减缓大气上升方面发挥了重要作用。然而，随着海洋变暖并变得更酸，海洋将在多大程度上继续以这种方式行动尚不清楚，正如我们在未来20-30年内将碳排放减少到净零排放时的未来反应一样。解决这个问题的一个关键区域是北大西洋，它对海洋碳吸收尤为重要。这种碳汇既包括自然碳的吸收（由于地表冷却和生物吸收），也包括人为碳的吸收（由于大气中二氧化碳的增加）。一种普遍的观点是，由于表面变暖降低了溶解度并增加了分层，从而抑制了向海洋表面提供营养物质和碳，因此这种碳汇将在未来减弱。然而，这种观点采取了局部视角，并没有考虑到环流在全球海洋中重新分配营养物质和碳的影响。我们希望提出并检验另一种观点，即环流通过设定北大西洋表层水的营养物质和碳的供应，在决定碳汇方面起着核心作用。特别是，有一种现象——西部边界流或墨西哥湾流——对这个问题至关重要。我们知道墨西哥湾暖流对高纬度地区提供热量很重要，导致欧洲气候变暖。然而，它在推动碳吸收方面的作用仍然很少被探索。地面观测表明，墨西哥湾流下游的碳吸收率有所提高。这种吸收发生在墨西哥湾流从地表以下携带的较老的水向下游转移到地表时。这些古老的水域营养丰富，人为碳排放减少。当这些水露出地表时，它们决定了地表的营养物质和碳浓度，从而控制了从大气中吸收的碳。在北大西洋，这种营养物质和碳“流”驱动了多少碳吸收，这取决于多种气候敏感过程，包括流的密度范围、墨西哥湾流的输送，以及沿其路径发生的一系列物理和生物地球化学过程。我们将使用观测和模型来全面理解这一关键现象，区分从墨西哥湾流开始到进入北大西洋的性质通量转变的几种不同机制。我们将对墨西哥湾流提供的营养物质和碳的变化进行新的测量，从佛罗里达海峡一直到下游的碳吸收热点，距离超过2000英里。我们将在佛罗里达海峡使用系泊来确定墨西哥湾流开始时的营养和碳特性。我们将部署BioArgo浮标和滑翔机，揭示营养物质和碳是如何从低纬度输送到高纬度的，并通过物理运输、混合和生物循环的影响记录它们的下游演变。我们的工作计划位于两个正在进行的大西洋经向翻转环流观测阵列之间，26北纬的RAPID和拉布拉多和苏格兰之间的OSNAP，这些阵列将我们的观测置于更广泛的背景下。我们将通过循环模型实验来验证我们的想法，包括评估北大西洋碳汇对物理过程的敏感性。最后，我们将评估碳汇如何在气候模式预测中变化，并确定模式的响应是否有正确的原因。如果我们要了解和可靠地预测碳汇的未来演变，特别是考虑到海洋对净零排放的不确定反应，那么解开海洋碳汇的这些控制是至关重要的。

We are all aware that atmospheric CO2 has risen over our lifetimes leading to global warming. The ocean has played an important role in moderating that atmospheric rise by taking up and storing 25% of the emitted carbon. However, the extent to which the ocean will continue to act in this manner as the ocean warms and becomes more acidic is unclear, as is the response in the future as we reduce carbon emissions to net zero in the next 20-30 years. A key region in addressing this question is the North Atlantic, which is disproportionately important for ocean carbon uptake. This carbon sink involves both the uptake of natural carbon (due to surface cooling and biological uptake) and of anthropogenic carbon (due to the rise in atmospheric CO2). A prevailing view is that this carbon sink will weaken in the future as surface warming decreases solubility and increases stratification, which inhibits the supply of nutrients and carbon to the surface ocean. However, this viewpoint takes a local perspective and does not account for the effect of the circulation in redistributing nutrients and carbon over the global ocean.We wish to propose and test the alternative viewpoint that the circulation plays a central role in determining the carbon sink, by setting the supply of nutrients and carbon to the surface waters of the North Atlantic. In particular, there is a phenomenon - the western boundary current or Gulf Stream - that is crucial for this problem. We know that the Gulf Stream is important for supplying heat to higher latitudes, leading to a warmer European climate. However, its role in driving carbon uptake remains little explored. Surface observations show that there are elevated rates of carbon uptake downstream of the Gulf Stream. This uptake occurs as older waters carried below the surface by the Gulf Stream are transferred downstream to the surface. These older waters are rich in nutrients and depleted in anthropogenic carbon. When these waters outcrop to the surface, they determine the surface nutrient and carbon concentrations, and so control the carbon uptake from the atmosphere. How much carbon uptake is driven by this nutrient and carbon 'stream' in the North Atlantic depends on multiple climate-sensitive processes, including the density range of the stream, the Gulf Stream transport, and a suite of physical and biogeochemical processes occurring along its path. We will use observations and models to comprehensively understand this pivotal phenomenon, distinguishing between several different mechanisms that transform the fluxes of properties at the beginning of the Gulf Stream to those entering the North Atlantic. We will make new measurements of how the Gulf Stream supply of nutrients and carbon varies all the way from Florida Straits to a carbon uptake hotspot downstream, a distance of over 2000 miles. We will employ moorings in Florida Straits to determine the nutrient and carbon properties at the start of the Gulf Stream. We will deploy a fleet of BioArgo floats and gliders to reveal how nutrients and carbon are conveyed from low to high latitudes, documenting their downstream evolution through the effects of physical transport, mixing and biological cycling. Our work programme sits between two ongoing observing arrays of the Atlantic meridional overturning circulation, RAPID at 26N and OSNAP between Labrador and Scotland, and these arrays place our observations in a wider context. We will test our ideas using experiments in circulation models, including assessing the sensitivity of the North Atlantic carbon sink to physical processes. Finally, we will evaluate how the carbon sink varies in climate model projections and establish whether the models' responses occur for the right reasons. Unravelling these controls of the ocean carbon sink is crucial if we are to understand and credibly predict the future evolution of the carbon sink, especially given the uncertain ocean response to net zero emissions.