Role of the Overturning Circulation in Carbon Accumulation (ROCCA)

人类活动导致大气中二氧化碳含量急剧增加，但海洋吸收了大约四分之一的人为碳，从而减缓了二氧化碳的增长。在全球范围内，北大西洋储存了最多的Canth，这是由于当地从大气中吸收二氧化碳，以及大规模的洋流，特别是大西洋经向翻转环流（AMOC）将Canth高处的水输送到北部地区，在那里它们冷却，密度增加，下沉到远离大气的深处。模式预测，尽管大气二氧化碳持续增加，但未来几十年碳汇的规模将减少，因为地表变暖增加了分层，降低了二氧化碳的溶解度，AMOC的减弱减缓了稠密水向深处的输送。然而，在通量峰值和下降时间方面存在较大的模型差异。同样的模型显示了海洋碳运输的大范围，通常与AMOC表示有关。因此，无论是现在还是将来，海气通量和海洋输送到北大西洋北部积累之间的平衡都没有得到很好的约束，而且存在很大的不确定性。以前的观察性研究试图量化这些过程对Canth积累的贡献，以协助模型验证和验证。然而，目前还不可能直接测量人为的空气-海洋二氧化碳通量——它们在化学上与“正常的”、非人为来源的二氧化碳通量相同。虽然它们可以通过跨洋盆地十年重复巡航间接计算出来，但这种方法存在很大的不确定性。因此，不可能限制为什么通量（或碳输送）在较短的时间尺度上变化，或者它们如何与AMOC相互作用。为此，我们需要经常估计海洋运输，并经常估计海洋中碳浓度增加的速度。这个项目就是要做到这一点。首先，我们将根据RAPID（10天）和OSNAP（每月）系泊阵列的输出，生成横跨北大西洋亚热带和亚极地边界的Canth输送的新的高分辨率估计。在RAPID，我们将把我们在2021年发布的2004-2013年时间序列延伸到2024年，该序列确定了一个稳定的、向北的北太平洋输送，该输送在所有时间尺度（周、月、季、年、年际）上都是高度可变的，并且与AMOC高度相关。我们将在西部边界的佛罗里达海峡收集新的分季节水样。流经海峡的水代表了绝大多数上层、向北流动的翻转环流，但我们目前在那里的输运计算中没有考虑到水团特征（化学或其他）的任何变化，因此没有完全描述AMOC：碳耦合。我们将在OSNAP上生成2014-2022年的碳输运时间序列，确定它如何与AMOC和RAPID碳输运共变。我们将使用基于船舶和自主平台数据的新型公开数据集来跟踪不断变化的内部（人为）碳信号。结合起来，我们将形成北大西洋的预算，在南部和北部边界运输，并在内陆发展集中。余量将表示通过地表进入（或离开）的热量——海气通量。海气通量和海洋环流对区域碳积累的贡献将被确定，从而更好地了解它们如何与AMOC一起储存碳。在将观测值与模拟输出进行比较之前，将在地球系统模式中对计算方案、其组成部分和输送/海气通量/AMOC关系进行测试。我们的发现将有助于提高气候模型的准确性，这对于预测气候变化的影响至关重要。

Human activities have caused atmospheric CO2 levels to increase dramatically, but their growth has been slowed by the oceans absorbing approximately one quarter of this anthropogenic carbon (Canth). Globally, the North Atlantic Ocean stores the highest quantities of Canth, due to local CO2 uptake from the atmosphere, and large-scale ocean currents, particularly the Atlantic Meridional Overturning Circulation (AMOC) delivering waters high in Canth to northern locations where they cool, get denser and sink to great depths away from contact with the atmosphere.Models project that the size of this carbon sink will reduce in the coming decades despite continued atmospheric CO2 increases, as surface warming increases stratification, decreases CO2 solubility, and AMOC weakening slows the transport of dense waters to depth. However there is substantial model spread regarding flux peak, and decline timing. The same models show a large range in ocean carbon transports, often related to AMOC representation. The balance between air- sea fluxes and ocean transports to North Atlantic Canth accumulation is thus not well constrained both now and into the future, and subject to large uncertainties.Previous observational studies have attempted to quantify the contributions of these processes to Canth accumulation in order to assist with model verification and validation. However, it is not currently possible to directly measure anthropogenic air-sea CO2 fluxes - they are chemically identical to those with 'normal', non-human-derived CO2. And while they can be calculated indirectly from trans-ocean basin decadal repeat cruises, this approach is subject to large uncertainties. It is thus impossible to constrain why fluxes (or carbon transports) vary on shorter timescales, or how they interact with the AMOC. For this we require frequent estimates of ocean transports combined with frequent estimates of how quickly carbon concentrations are increasing in the ocean.This project will look to do precisely that. Firstly, we will generate new high-resolution estimates of Canth transports across the subtropical and subpolar boundaries of the North Atlantic, relying on the outputs from the RAPID (10day) and OSNAP (monthly) mooring arrays. At RAPID, we will extend to 2024 the 2004-2013 time-series we published in 2021 and that identified a stable, northward Canth transport that was highly variable over all time scales (weekly, monthly, seasonally, annually, interannually), and highly correlated to the AMOC.We will collect new sub-seasonal water samples in Florida Straits, at the western boundary. The waters that flow through the Straits represent the vast majority of the upper, northward-flowing part of the overturning circulation but we don't currently account for any variability in water mass characteristics (chemical or otherwise) in the transport calculation there, so are not fully characterising the AMOC:carbon coupling.We'll generate a novel Canth transports time-series for 2014-2022 at the OSNAP, identifying how it co-varies with AMOC, and RAPID carbon transports. We'll track the changing interior (anthropogenic) carbon signal using novel, publicly-available datasets based on ship and autonomous platform data. Combined, we'll form a North Atlantic budget with transports at the southern and northern boundaries, and evolving concentrations in the interior. The residual will represent Canth entering (or leaving) through the surface - the air-sea flux.The contributions of air-sea fluxes and ocean circulation to regional carbon accumulation will be determined, better understanding how, with AMOC, they work together to store carbon. The calculation scheme, its components and transport/air-sea flux/AMOC relationships will be tested in earth system models, before observations are compared to simulation outputs. Our findings will help improve the accuracy of climate models, which is crucial for predicting the effects of climate change.