Changes of ventilation in the North Atlantic over the past three decades - a climate change signal

The meridional overturning circulation in the North Atlantic supplies oxygen to a large part of the world ocean's interior via the formation of mode waters and North Atlantic Deep. Whether human activities have altered this ventilation system remains uncertain. To assess the temporal changes of ocean ventilation in the North Atlantic, we calculated the "age" of seawaters, that is the duration since its last contact with the ocean surface, from both observed and climate models simulated chlorofluorocarbon-12 and sulfur hexafluoride concentrations. Results suggest enhanced ventilation in the intermediate waters and slowed-down ventilation in the deep waters over the past three decades. We propose such ventilation change is a climate change signal because (i) observed ventilation evolution pattern, although likely influenced by the major driver of natural variability in the region, the North Atlantic Oscillation, consistently emerges in historical simulations across different Earth System models, each representing different states of natural climate variability,  (ii) the pattern intensifies with ongoing climate change in model projections under a high-emission scenario, indicating it is an anthropogenically forced signal, and (iii) observed and simulated ventilation changes in the North Atlantic seem to be part of a broader global trend, with enhanced upper-ocean ventilation, and slowed deep-ocean ventilation also in other ocean basins.