North Atlantic deoxygenation despite strong subpolar oxygen uptake

The North Atlantic is one of the most well-ventilated regions in the global ocean, but oxygen levels there have begun declining in recent decades. We use a gridded data product to analyze drivers of 0-2000m oxygen content changes for the subtropical (15-45N) and subpolar (45-65N) North Atlantic from 2004 to 2023. Oxygen content remained persistently high in the subpolar basin, with an increase after 2014 owing to particularly intense uptake from the atmosphere, while subtropical oxygen content has decreased steadily since 2010. An analysis of concurrent oxygen and salinity changes shows that subtropical oxygen loss is not consistent with changes in oxygen concentrations in water mass source regions. Instead, it can be explained by a change in the relative contribution of different water masses, with a decreasing influence of Subpolar Mode Water and Labrador Sea Water. These changes reflect two contrasting responses to the predominantly positive phase of the North Atlantic Oscillation (NAO) since 2012. Stronger winds and intense water mass formation increased oxygen uptake at high latitudes, raising the subpolar oxygen content. At the same time, changes in circulation decreased the southward penetration of these oxygen-rich waters from the subpolar into the subtropical basin. The oxygen loss in the subtropics outweighs the increase in the subpolar basin, resulting in an overall decline in the upper 2000 m of the whole North Atlantic (15-65N) of 104 Tmol O 2 (1 Tmol = 10 12 mol) from 2004 to 2023.