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Vous êtes ici : Accueil / Actualités / Séminaires / Seminaires Septembre 2018-Aout 2019 / Jeudi 17 Janvier - 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 Atlantic Meridional Overturning Circulation and its impact on the CO2

Jeudi 17 Janvier - 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 Atlantic Meridional Overturning Circulation and its impact on the CO2

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Pascale Lherminier

Laboratoire d'Océanographie Physique et Spatiale (LOPS, Brest)

 

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 Atlantic Meridional Overturning Circulation and its impact on the CO2 physical pump

 

Abstract: Important processes regulate our climate in the northern North Atlantic: this is presently the place where waters transported northwards in the upper limb gain density and eventually sink into the southward flowing lower limb, driving the Meridional Overturning Circulation, and where the anthropogenic carbon storage rate is the highest. The variability of the subpolar gyre circulation, the MOC and heat transport were quantified from an inversion of hydrographic and velocity data from nine repeats of the Greenland to Portugal OVIDE-A25 section covering 1997-2016. 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 heat transport estimated from the repeated hydrographic OVIDE sections was found linearly related to the MOC strength. The extent and timescales of the MOCσ variability in 1993–2016 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 that will be analyzed. 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  the transitory slowdown of the MOCσ, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic carbon dioxide storage in subpolar waters. Since 2014, we observed a drastic change in the subpolar gyre, with an enhanced MOC and deep convection reaching 1500m in the Irminger and Labrador seas. As a consequence, the acidification signal penetrated deeper and is susceptible to affect the deep coral reefs in the North Atlantic.

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