Temporal variability of zonal circulation and anthropogenic carbon in the Western North Atlantic Ocean

Abstract

The North Atlantic Subtropical Gyre (NASG) plays a key role in large-scale ocean circulation and climate regulation, particularly through its connection to the Atlantic Meridional Overturning Circulation (AMOC). This thesis investigates the physical and biogeochemical dynamics of the western NASG, with a focus on zonal circulation, heat and freshwater transport, and anthropogenic carbon (Canth) trends. To achieve this, the study utilizes hydrographic data from the A20 and A22 sections, collected in 1997, 2003, 2012, and 2021, combined with inverse box modeling techniques. A key methodological advancement is the application of a three-box inverse model, which improves upon single-box approaches by incorporating additional observational constraints, thereby enhancing estimates of mass transport and variability at decadal scales. These results are further compared with numerical models, including GLORYS, ECCO, and MOM, to evaluate their ability to reproduce observed transport patterns. Findings reveal that while individual currents such as the Gulf Stream (GS) and Deep Western Boundary Current (DWBC) exhibit seasonal and interannual variability, their long-term average transports remain consistent with previous estimates, supporting the robustness of the inverse box model methodology. The GS is identified as the dominant carrier of heat and freshwater poleward, while the DWBC serves as the primary conduit for southward export of dense water, forming the lower limb of the AMOC. The estimated heat fluxes indicate a net ocean-to- atmosphere exchange at the A20 section, with variability across different survey years reflecting the influence of atmospheric forcing. Freshwater fluxes suggest an overall increase in precipitation and runoff relative to evaporation over the decades, a trend corroborated by reanalysis products. In addition to circulation, the thesis examines the role of the western NASG in Canth uptake and transport. Results indicate that the GS and North Atlantic Current play a crucial role in advecting Canth-rich waters from subtropical to subpolar regions, contributing to the long-term sequestration of atmospheric CO₂. Rising CO₂ levels worsen ocean acidification—evidenced by significantly fewer carbonate ions and a shoaling aragonite horizon— making high-resolution monitoring essential for future predictions.