FY 2026

Progressive oxygenation of the North Atlantic subpolar gyre

Koelling, J., A.J. Fassbender, A.R. Gray, G.C. Johnson, J.D. Sharp, and D. Carroll

J. Geophys. Res., 130(11), e2024JC022157, doi: 10.1029/2024JC022157, View open access article at AGU/Wiley (external link) (2025)

The subpolar North Atlantic (SPNA) is one of the few regions where the deep ocean is in direct contact with the atmosphere, making it a key location for interior ocean ventilation through gas exchange. We use a novel observation-based data product to analyze large-scale patterns of the air-sea flux of oxygen, finding a mean annual flux of 48.1 ± 14.6 Tmol year ^–1 from the atmosphere into the ocean integrated over the SPNA (45°N–65°N). An analysis of a fully-closed oxygen budget from the data-assimilative ECCO-Darwin ocean biogeochemistry model suggests that the net uptake is counteracted by oxygen removal through ocean circulation and mixing. Over an annual cycle, a SPNA oxygen uptake of 63.6 ± 13.8 Tmol at densities greater than 26.7 kg m^–3 drives a wintertime oxygen increase in corresponding mode and deep water layers. 87% of this net annual uptake occurs in the density range of subpolar mode water (SPMW), 26.7 kg m^–3 ≤ δ_θ < 27.63 kg m^–3, in the upper branch of the Atlantic Meridional Overturning Circulation (AMOC). Our results demonstrate that oxygen is injected during mode water formation throughout the subpolar gyre's cyclonic pathway from the North Atlantic Current toward the Labrador Sea. Along this path, SPMW becomes progressively denser and more oxygenated, and is ultimately transformed into Labrador Sea Water which exports the accumulated oxygen to the global ocean in the lower branch of the AMOC.

Plain Language Summary. At high latitudes, cold dense water takes up oxygen from the atmosphere and sinks to the deep ocean in a process known as ocean ventilation. These waters then flow toward other ocean regions, carrying life-sustaining oxygen to deep-sea organisms in much of the world's oceans. The subpolar North Atlantic (SPNA) is one of the regions where this “breathing” of the ocean occurs, but the details of this process are not well understood, which may be a major reason why climate simulations have consistently underestimated the amount of oxygen the ocean has lost in recent decades. Here, we use a new ocean oxygen data product to study patterns of ventilation in the North Atlantic in unprecedented detail. We find that only a small percentage of the total oxygen uptake takes place in the Labrador Sea where oxygenated waters sink to depth. Instead, oxygen is injected from the atmosphere into the ocean throughout the SPNA and accumulates in the waters that are carried toward the Labrador Sea by ocean currents. These insights have important implications for our understanding of how oxygen enters the deep ocean and may help improve predictions of future oxygen loss due to climate change.