North Atlantic Carbon Uptake and Variability: The Gulf Stream's Role in Air-Sea CO2 Flux and Storage

The North Atlantic Ocean contributes approximately 30% of the global ocean carbon uptake. This region plays a vital role in anthropogenic carbon uptake and hosts a significant natural carbon cycle driven by physical and biogeochemical processes. This study focuses on understanding the inter-annual variability of air-sea CO₂ fluxes, anthropogenic carbon storage, and the role of the Gulf Stream in transporting water masses with low anthropogenic carbon concentrations into the subpolar North Atlantic. We present the development and application of our forward and adjoint ocean carbon cycle and biogeochemistry models within the Estimating the Circulation and Climate of the Ocean (ECCOv4) framework (ECCOv4r2-Dissolved Inorganic Carbon (DIC)). The ECCOv4r2-DIC simulation overall captures the inter-annual variability and decadal trends of ocean carbon uptake in the subpolar North Atlantic. The adjoint model for ocean biogeochemistry is a powerful tool that enables us to investigate the sensitivity of ocean carbon uptake to physical and biogeochemical factors under dynamic ocean conditions. Preliminary results from the adjoint biogeochemistry sensitivity simulations indicate that subpolar North Atlantic carbon storage is highly sensitive to dissolved inorganic carbon (DIC) in the Gulf Stream region on inter-annual timescales (e.g., lag of -4 years). This finding suggests that remote advective carbon transport significantly influences inter-annual carbon variability in the subpolar North Atlantic Ocean.