North Atlantic CO2 sink variability revealed by the Go-Ship A25-OVIDE section

Abstract

<p>About 30% of the carbon dioxide derived from human activities (C<sub>ANTH</sub>) has been absorbed by the ocean (DeVries, 2014; Gruber et al., 2019; Friedlingstein et al., 2021), with the North Atlantic (NA) being one of the largest C<sub>ANTH </sub>sinks per unit area (Khatiwala et al., 2013; Sabine et al., 2004). In the NA, oceanic C<sub>ANTH</sub> uptake strongly relies on the meridional overturning circulation and the associated regional winter deep convection. In fact, the formation and deep spreading of Labrador Sea Water stands as a critical C<sub>ANTH</sub> gateway to intermediate and abyssal depths. The NA C<sub>ANTH </sub>uptake has fluctuated over the years according to changes in the North Atlantic Oscillation. Biennial observation of the marine carbonate system along the Go-Ship A25-OVIDE section has allowed us assessing the decadal and interannual variability of the C<sub>ANTH</sub> storage in the subpolar NA from 2002 to 2021. In this study, we investigate 1) the trend of C<sub>ANTH</sub> and 2) the relationship between the C<sub>ANTH</sub> saturation, the apparent oxygen utilization, and the ventilation of the water masses between the A25-OVIDE section and the Greenland-Iceland-Scotland sills during 2002-2021. We divided the A25-OVIDE section into three main basins (Irminger, Iceland, and Eastern NA). Our results show that the Irminger Basin presents a more homogenous C<sub>ANTH</sub> profile and higher C<sub>ANTH </sub>saturation values at depth than the other two basins, which is related to the pronounced convective activity in the Irminger Basin. In contrast, the Eastern NA Basin has higher C<sub>ANTH</sub> values at the surface due to its higher surface temperature, but its deep water masses show the lowest C<sub>ANTH</sub> values since they are the less ventilated in the section. Our analysis also reveals that, overall, the NA C<sub>ANTH</sub> storage has increased during 2002-2021, but varied according to the ventilation changes. While the Eastern NA water masses experienced a relatively constant, although shallower, average ventilation, the Irminger and Iceland Basins underwent a less steady C<sub>ANTH</sub> uptake pattern characterized by alternating periods of strong and weak C<sub>ANTH</sub> storage.</p>