The evolution of anthropogenic carbon accumulation in the North Atlantic Ocean: the influence of physical and chemical drivers over the last three decades

Abstract. The North Atlantic (NA) plays a central role in global carbon uptake and storage, yet recent studies suggest that the efficiency of this sink – defined as its capacity to absorb and store anthropogenic CO₂ (C_Anth) relative to rising atmospheric concentrations – may be weakening, raising concerns about its future capacity to offset anthropogenic emissions. Climate model ensembles project that the NA’s anthropogenic carbon uptake may peak mid-century and decline thereafter, partly due to a projected slowdown of the AtlanticMeridional Overturning Circulation (AMOC), which could reduce the rate of NA C_Anth accumulation. In this context, quantifying and understanding recent C_Anth accumulation variability and its drivers is essential for understanding its future trajectory.

Here, we use quality-controlled hydrographic observations to examine temporal and spatial patterns of C_Anth concentration change across the NA between 1993 and 2021. C_Anth estimates are combined with the ARMOR3D gridded physical data product to quantify C_Anth inventories and interannual variability in C_Anth storage, determine water mass volume changes, assess the drivers of observed changes, and evaluate the role of key oceanic processes such as circulation and ventilation. We analyse these results across 13 NA subregions representing distinct biogeochemical regimes and 7 density intervals representing the NA water masses. A basin-wide C_Anth inventory increase of 0.39 ± 0.02 PgC a⁻¹ over the study period is observed, with broadly linear increases across subregions and density layers, but with pronounced latitudinal contrasts and depth-dependent variability. Larger trends are evident in near-surface waters at low latitudes and enhanced C_Anth accumulation in intermediate densities at higher latitudes. Substantial interannual variability is also identified, with deviations from linear C_Anth inventory growth majoritively dominated by water-mass volume variability (driven by circulation and ventilation changes) rather than C_Anth concentrations. While we do find interannual variability in C_Anth concentrations across the NA, it is most pronounced in the Nordic and subpolar regions where changes in ventilation and deep convection drive coherent anomalies across multiple density layers. Subtropical and tropical regions exhibit more episodic variability largely confined to upper and intermediate density layers, consistent with changes in stratification and lateral redistribution. Overall, this interannual signal has a smaller impact on the basin-wide C_Anth inventory. Indeed, over the period considered, an inventory increase rate of 1.72 % a⁻¹ was identified, with the fractional contribution of each region exhibiting small but still measurable multi-year fluctuations. We conclude that temporal changes in circulation and ventilation modulate not only interannual C_Anth anomalies but also the regional partitioning of long-term storage. The North Atlantic Oscillation (NAO) emerges as the primary driver of interannual variability, imprinting strong, spatially consistent signals on C_Anth inventories. In contrast, while no direct correlation with AMOC variability is. discerned, accumulated AMOC anomalies reveal delayed responses of C_Anth inventories to NAO forcing, consistent with the ocean’s overturning dynamics and reflective of extended periods of AMOC imprint. These findings highlight that while NAO directly modulates interannual C_Anth variability, ocean circulation integrates its influence over longer timescales. Hence, reliable projections of the future North Atlantic carbon sink require accounting for the dual control of atmospheric forcing and ocean circulation, whereby NAO drives rapid variability while the overturning circulation integrates and propagates this signal over longer timescales.