Abstract
Abstract. The interannual to decadal variability in the transport of anthropogenic CO2 (Cant) across the subpolar North Atlantic (SPNA) is investigated, using summer data of the FOUREX and OVIDE high-resolution transoceanic sections, from Greenland to Portugal, occupied six times from 1997 to 2010. The transport of Cant across this section, Tcant hereafter, is northward, with a mean value of 254 ± 29 kmol s−1 over the 1997–2010 period. We find that Tcant undergoes interannual variability, masking any trend different from 0 for this period. In order to understand the mechanisms controlling the variability of Tcant 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 kmol s−1) that is partially compensated by a southward transport of Cant caused by the isopycnal component (−171 ± 11 kmol s−1), mainly localized in the Irminger Sea. Most importantly, the diapycnal component is found to be the main driver of the variability of Tcant across the SPNA. Both the Meridional Overturning Circulation (computed in density coordinates, MOCσ) and the Cant increase in the water column have an important effect on the variability of the diapycnal component and of Tcant itself. Based on this analysis, we propose a simplified estimator for the variability of Tcant based on the intensity of the MOCσ and on the difference of Cant between the upper and lower limb of the MOCσ (ΔCant). This estimator shows a good consistency with the diapycnal component of Tcant, and help to disentangle the effect of the variability of both the circulation and the Cant increase on the Tcant 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 Tcant across the SPNA at long timescale. Nevertheless, at the timescale analyzed here (1997–2010), the MOCσ controls the Tcant variability, blurring any Tcant trend. Extrapolating the observed ΔCant increase rate and considering the predicted slow-down of 25% of the MOCσ, Tcant across the SPNA is expected to increase by 430 kmol s−1 during the 21st century. Consequently, an increase in the storage rate of Cant in the SPNA could be envisaged.
Highlights
The ocean acts as an important sink for the CO2 emitted by human activities
At gyre scale, the structures intersecting the section are a cyclonic circulation in the Irminger Sea, a cyclonic circulation in the Iceland Basin, the North Atlantic Current (NAC) flowing directly northward east of Eriador Seamount, and lastly, an anticyclonic circulation dominating the West European Basin
Note that a positive Tcant value means a northward transport of Cant while a negative value points to a southward transport
Summary
The ocean acts as an important sink for the CO2 emitted by human activities. It has stored approximately one third of the total anthropogenic CO2 (Cant hereafter) emissions since the beginning of the industrial era (Sabine et al, 2004). Pérez et al (2013) have inferred that Cant is the main component of the air–sea CO2 fluxes at mid-latitude in the North Atlantic, while the natural component is the dominant one in the subpolar region They detected a decrease in the storage rate of Cant between 1997 and 2006 in the subpolar region that was related to the reduction in the intensity of the Meridional Overturning Circulation (computed in density coordinates, MOCσ ). At gyre scale, the structures intersecting the section are a cyclonic circulation in the Irminger Sea, a cyclonic circulation in the Iceland Basin, the North Atlantic Current (NAC) flowing directly northward east of Eriador Seamount, and lastly, an anticyclonic circulation dominating the West European Basin Beside this gyre-scale circulation, the MOCσ is an important feature of the circulation across the OVIDE section. 1765-1995 1765–1995 1990s–2000s 1992 1992–1998 2002–2006 (referred to 2004) 1765–1995 1990s–2000s
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