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

The North Atlantic Meridional Overturning Circulation (AMOC) ventilates a large part of the world ocean via the formation of mode waters and North Atlantic Deep Water. Whether human activities have altered this ventilation system remains uncertain. To assess the temporal variations of ocean ventilation in the North Atlantic, we calculated the "age" of seawater, that is the duration since its last contact with the ocean surface, from both observed and simulated chlorofluorocarbon-12 and sulfur hexafluoride concentrations. Results suggest enhanced ventilation in the intermediate waters and slowing-down ventilation in the deep waters over the past three decades. We propose such ventilation change is a climate change signal because (i) the observed ventilation evolution pattern consistently emerges in historical simulations across different Earth System models where each model might be in a different phase 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 slowing deep-ocean ventilation also in other ocean basins.