Variability of the transport of anthropogenic CO2 at the Greenland--Portugal OVIDE section: controlling mechanisms.

The interannual to decadal variability of the transport of anthropogenic carbon dioxide (Cant) across the Subpolar North Atlantic (SPNA) is investigated, using data of the OVIDE high resolution transoceanic section, from Greenland to Portugal, occupied six times from 1997 to 2010. The transport of Cant across this section, T_Cant hereafter, is northward, with a mean value of 254±29 kmols^-1 over the 1997-2010 period. The T_Cant presents a high interannual variability, masking any trend different from 0 for this period. In order to understand the mechanisms controlling the variability of the T_Cant across the SPNA, we propose a new method that quantifies the transport of Cant caused by the diapycnal and isopycnal circulation. The diapycnal component yields a large northward transport of Cant (400±29 kmols^-1) which is partially compensated by a southward transport of Cant caused by the isopycnal component (-171±11 kmols^-1), mainly localized in the Irminger Sea. Most importantly, the diapycnal component is found to be the main driver of the variability of the T_Cant across the SPNA. Both the Meridional Overturning Circulation (MOC) and the Cant increase in the water column have an important effect on the variability of the diapycnal component and of the T_Cant itself. Based on this analysis, we propose a simplified estimator for the variability of the T_Cant based on the intensity of the MOC and on the difference of Cant between the upper and lower limb of the MOC (ΔCant). This estimator shows a good consistency with the diapycnal component of the T_Cant, and help to disentangle the effect of the variability of both the circulation and the Cant increase on the T_Cant variability. We find that ΔCant keeps increasing over the past decade, and it is very likely that the continuous Cant increase in the water masses will cause an increase in the T_Cant across the SPNA at long time scale. Nevertheless, at the time scale analyzed here (1997-2010), the MOC is controlling the T_Cant variability, blurring the expected T_Cant increase. Extrapolating the observed ÄCant increase rate and considering the predicted slow-down of 25% of the MOC, the T_Cant across the SPNA is expected to increase by 430 kmols^-1 during the 21st century. Consequently, an increase in the storage rate of Cant in the SPNA could be envisaged. [ABSTRACT FROM AUTHOR]