The Gulf Stream control of the North Atlantic carbon sink

我们都知道，在我们的一生中，大气中的二氧化碳已经上升，导致全球变暖。海洋通过吸收和储存25%的碳排放，在减缓大气层上升方面发挥了重要作用。然而，随着海洋变暖和变酸，海洋将在多大程度上继续以这种方式发挥作用尚不清楚，随着我们在未来20-30年内将碳排放量减少到净零，未来的反应也是如此。解决这一问题的一个关键区域是北大西洋，它对海洋碳吸收的重要性不成比例。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CO2的增加）。一种普遍的看法是，随着表面变暖降低溶解度并增加分层，这种碳汇将在未来减弱，这抑制了向海洋表面供应营养物质和碳。然而，这种观点采取了局部的角度来看，并没有考虑到在全球海洋的营养物质和碳的再分配的环流的影响，我们希望提出和测试的替代观点，即环流在确定碳汇中发挥着核心作用，通过设置营养物质和碳的供应到北大西洋的表面沃茨。特别是，有一种现象-西部边界流或墨西哥湾流-对这一问题至关重要。我们知道墨西哥湾流对于向高纬度地区提供热量非常重要，导致欧洲气候变暖。然而，其在推动碳吸收方面的作用仍然很少探索。地面观测表明，墨西哥湾流下游的碳吸收率较高。当墨西哥湾流将地表以下的旧沃茨水转移到下游至地表时，就会发生这种吸收。这些较老的沃茨富含营养物质，并耗尽了人为碳。当这些沃茨露出地表时，它们决定了地表营养物质和碳的浓度，从而控制了从大气中吸收碳。在北大西洋，有多少碳吸收是由这种营养和碳“流”驱动的，这取决于多种气候敏感的过程，包括流的密度范围，墨西哥湾流输送，以及沿着其路径发生的一系列物理和地球化学过程。我们将使用观测和模型来全面了解这一关键现象，区分几种不同的机制，这些机制将墨西哥湾流开始时的特性通量转换为进入北大西洋的特性通量。我们将对墨西哥湾流从佛罗里达海峡到下游2000多英里的碳吸收热点的营养和碳供应如何变化进行新的测量。我们将在佛罗里达海峡使用系泊设备，以确定墨西哥湾流开始时的营养和碳特性。我们将部署一支BioArgo浮标和滑翔机舰队，以揭示营养物质和碳是如何从低纬度输送到高纬度的，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。我们的工作方案位于两个正在进行的观测阵列之间的大西洋纬向翻转环流，快速在26 N和OSNAP拉布拉多和苏格兰之间，这些阵列将我们的意见在更广泛的背景下。我们将使用循环模型实验来测试我们的想法，包括评估北大西洋碳汇对物理过程的敏感性。最后，我们将评估碳汇在气候模型预测中的变化，并确定模型的响应是否出于正确的原因。如果我们要理解和合理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，解开这些对海洋碳汇的控制至关重要。

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.