Role of the Overturning Circulation in Carbon Accumulation

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 (NA) 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.

Nevertheless, the balance of contributions to NA Canth accumulation from air-sea CO₂ fluxes and ocean transports is not well constrained. 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 balance between air-sea fluxes and ocean transports to North Atlantic Canth accumulation is therefore not well constrained both now and into the future, and subject to large uncertainties. ROCCA will address these fundamental knowledge gaps. This will be through: - generating new high-resolution estimates of Canth transports across the subtropical and subpolar boundaries of the North Atlantic, relying on the outputs from the RAPID and OSNAP transport mooring arrays, to identify how they co-vary with AMOC, - determining the regional Canth accumulation rate from assorted data products and data-assimilating model outputs, - calculating contributions from local air-sea fluxes, including those derived from contemporary surface CO₂ measurements, and anthropogenic CO₂ fluxes derived from transport divergence estimates, - evaluating CMIP6 models for how carbon is accumulating in the North Atlantic, constraining model performance and uncertainty in future projections. Our findings will help improve the accuracy of climate models, which is crucial for predicting the effects of climate change, forming new understanding of how the AMOC and the carbon system interlink.