Anthropogenic Carbon Dynamics: Concentrations, Transports, and Trends in the Western North Atlantic Subtropical Gyre

Abstract

The ocean acts as a major carbon sink, absorbing anthropogenic CO₂ and mitigating climate change. The North Atlantic Ocean, particularly the western North Atlantic Subtropical Gyre (NASG), plays a crucial role in this process, yet the mechanisms governing the transport of anthropogenic carbon (C_anth) remain incompletely understood. In this study, we quantify C_anth transport across the western NASG using hydrographic observations from repeat meridional sections (A20 and A22) and an inverse box model approach. We show that C_anth transport has increased over the past four decades, primarily in surface and intermediate layers, driven by rising atmospheric CO₂ concentrations rather than significant changes in mass transport. The strongest C_anth transport occurs in the Subtropical Underwater (STUW) and Subtropical Mode Water (STMW) layers, following the circulation patterns of the Gulf Stream (GS) and its recirculation. While deep and bottom layers, such as Labrador Sea Water, Iceland-Scotland Overflow Water, and Denmark Strait Overflow Water, exhibit weaker and more variable transport, the overall trend reflects increasing C_anth sequestration across the region. Long-term atmospheric CO₂ trends from four global monitoring stations are compared with C_anth levels in the western NASG, showing that both have steadily increased since 1970. Slight variations in these trends imply that factors such as ocean circulation, temperature fluctuations, and water mass dynamics could affect the region's efficiency in CO₂ uptake.