Exploring AMOC controls on the North Atlantic carbon sink using novel inverse and data-constrained models (EXPLANATIONS)

The global ocean currently absorbs a quarter of the carbon dioxide (CO2) released into the atmosphere due to human activities, slowing the rate of climate change. A disproportionate amount of this anthropogenic carbon (Canth) accumulates in the North Atlantic. A component of the global ocean circulation known as the Atlantic Meridional Overturning Circulation (AMOC) plays a key role in how the ocean carbon uptake (the carbon 'sink') varies due to interactions between physics, biology, and chemistry. However, disagreement among model simulations of the system limits confidence in climate projections. There is also uncertainty in the relative contributions to net ocean carbon uptake of anthropogenic carbon and 'natural' carbon that already existed in the preindustrial environment, and how these contributions may change in the future. In recent years, instruments moored in the North Atlantic have made the first direct continuous observations of the AMOC in programmes call 'RAPID' and 'OSNAP', providing new information about the regional circulation. Furthermore, observations of carbon in the ocean interior allow us to determine how carbon is accumulating in the basin. The combination of the availability of observational data and a climatically important regional circulation make the North Atlantic an ideal place to study the underlying mechanisms controlling ocean carbon uptake transport and storage, to help us improve our models.

In this project, we will employ cutting-edge methods to characterise the North Atlantic Ocean CO2 sink and interior carbon reservoir. First, we will use a novel inverse method, which combines observations with simple physical principles, to estimate the uptake, transport, and storage of carbon, using a combination of different observational datasets, including those from the RAPID and OSNAP programmes. Second, we will use a model that simulates the ocean physical, chemical, and biological systems, while incorporating available observations in a way that maximises the realism of the simulation, to explore the role of the circulation in the North Atlantic carbon sink. For the first time, we will incorporate OSNAP and RAPID observations into this model, named 'ECCO-Darwin', and separate natural and anthropogenic contributions to the region's changing carbon inventory. Third, we will use the state-of-the-art ocean ecosystem model 'NEMO-PlankTOM' to estimate the impacts of rapid changes in the ocean circulation on the carbon sink, and to determine the strength of any interaction between the carbon sink and the ocean's biological ecosystem. Finally, we will compare outputs from climate models with our results to identify common features in those models that best represent North Atlantic carbon uptake, transport, and storage, as a means of improving our projections of future climate change. In summary, these combined methods will provide an improved understanding of how the AMOC and North Atlantic Ocean are sequestering anthropogenic CO2.