OVIDE-A25, a Biennial Hydrographic Transect across the North Atlantic Subpolar Gyre since 2002: Overview of the Main Scientific Findings about the Variability of the Meridional Overturning Circulation and its Impact on the CO2 Physical Pump

The meridional overturning circulation (MOC) transports heat from the subtropics to high latitudes and hence plays an important role in the Earth's climate. A region crucial for the MOC is the northern North Atlantic, where waters transported northwards in the MOC upper limb gain density and eventually sink into the southward flowing lower limb. The variability of the subpolar gyre circulation, the MOC and heat transport was quantified from a joint analysis of hydrographic and velocity data from eight repeats of the Greenland to Portugal OVIDE section. The obtained circulation patterns revealed remarkable transport changes in the whole water column and evidenced large variations in the magnitude of the MOC computed in density coordinates (MOCσ). The extent and timescales of the MOCσ variability in 1993-2014 were then evaluated using a monthly MOCσ index built upon altimetry and ARGO data at the OVIDE section location. The MOCσ index, validated by the good agreement with the in situ estimates, shows a large variability on monthly to decadal time scales. The heat transport estimated from the repeated hydrographic OVIDE sections is linearly related to the MOCσ intensity. The uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean is also strongly impacted by the variability of the MOCσ. We found that the uptake of anthropogenic carbon dioxide occurred almost exclusively in the subtropical gyre. In contrast, natural carbon dioxide uptake dominated in the subpolar gyre. We attributed the weakening of total carbon dioxide uptake observed between 1997 and 2006 in the subpolar North Atlantic to a reduction in the natural component. We also showed that the transitory slowdown of the MOCσ was largely responsible for this phenomenon, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic carbon dioxide storage in subpolar waters.