ALBERT

Description: 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, TCant hereafter, is northward, with a mean value of 254 ± 29 kmol s–1 over the 1997–2010 period. The TCant 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 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) which 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 the TCant 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 TCant itself. Based on this analysis, we propose a simplified estimator for the variability of the TCant 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 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 the TCant across the SPNA at long time scale. Nevertheless, at the time scale analyzed here (1997–2010), the MOC is controlling the TCant variability, blurring the expected TCant increase. Extrapolating the observed ΔCant increase rate and considering the predicted slow-down of 25% of the MOC, the 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.

Description: A high-quality inorganic carbon system database spanning over three decades (1981–2006) and comprising 13 cruises has allowed applying the φCT° method and coming up with accurate estimates of the anthropogenic CO2 (Cant) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like Cant, into deeper ocean layers and confer this region an added oceanographic interest from the point of view of air-sea CO2 exchanges. Commonly, a tendency for decreasing Cant storage rates towards the deep layers has been observed. In the Iberian Basin, the deep waters (North Atlantic Deep Water) have low Cant values and negligible Cant storage rates, while the North Atlantic Central Water in the upper layers shows the largest Cant concentrations and capacity to increase its storage on a yearly basis (1.13±0.14 μmol kg−1 yr−1). This unmatched Cant storage capacity of the warm upper limb of the Meridional Overturning Circulation weakens towards the Irminger basin (0.68±0.06 μmol kg−1 yr−1) due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather homogeneous Cant storage rates (between 0.33 and 0.45 μmol kg−1 yr−1), whereas in the Iceland basin these layers seem to have been less affected by Cant. The Cant storage rates in the study region during the 1991–1997 high NAO (North Atlantic Oscillation) phase are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO2has taken place during the present decade. The changes in deep-water ventilation together with a detrimental renewal of the main water masses are likely the main driving processes causing this weakening of the North Atlantic CO2sink.

Description: 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.

Description: of the global combined atmosphere-ocean heat flux and so is important for the mean climate of the Atlantic sector of the Northern Hemisphere. This meridional heat flux is accomplished by both the Atlantic Meridional Overturning Circulation (AMOC) and by basin-wide horizontal gyre circulations. In the North Atlantic subtropical latitudes the AMOC dominates the meridional heat flux, while in subpolar latitudes and in the subtropical South Atlantic the gyre circulations are also important. Climate models suggest the AMOC will slow over the coming decades as the earth warms, causing widespread cooling in the Northern hemisphere and additional sea-level rise. Monitoring systems for selected components of the AMOC have been in place in some areas for decades, nevertheless the present observational network provides only a partial view of the AMOC, and does not unambiguously resolve the full variability of the circulation. Additional observations, building on existing measurements, are required to more completely quantify the Atlantic meridional heat transport. A basin-wide monitoring array along 26.5°N has been continuously measuring the strength and vertical structure of the AMOC and meridional heat transport since March 31, 2004. The array has demonstrated its ability to observe the AMOC variability at that latitude and also a variety of surprising variability that will require substantially longer time series to understand fully. Here we propose monitoring the Atlantic meridional heat transport throughout the Atlantic at selected critical latitudes that have already been identified as regions of interest for the study of deep water formation and the strength of the subpolar gyre, transport variability of the Deep Western Boundary Current (DWBC) as well as the upper limb of the AMOC, and inter-ocean and intrabasin exchanges with the ultimate goal of determining regional and global controls for the AMOC in the North and South Atlantic Oceans. These new arrays will continuously measure the full depth, basin-wide or choke-point circulation and heat transport at a number of latitudes, to establish the dynamics and variability at each latitude and then their meridional connectivity. Modeling studies indicate that adaptations of the 26.5°N type of array may provide successful AMOC monitoring at other latitudes. However, further analysis and the development of new technologies will be needed to optimize cost effective systems for providing long term monitoring and data recovery at climate time scales. These arrays will provide benchmark observations of the AMOC that are fundamental for assimilation, initialization, and the verification of coupled hindcast/forecast climate models.

Description: A high-quality inorganic carbon system database, spanning over three decades (1981–2006) and comprising of 13 cruises, has allowed the applying of the φC°T method and coming up with estimates of the anthropogenic CO2 (Cant) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like Cant, into deeper ocean layers and grants this region an added oceanographic interest from the point of view of air-sea CO2 exchanges. Generally, a tendency for decreasing Cant storage rates towards the deep layers has been observed. In the Iberian Basin, the North Atlantic Deep Water has low Cant concentrations and negligible storage rates, while the North Atlantic Central Water in the upper layers shows the largest Cant values and the largest annual increase of its average concentration (1.13 ± 0.14 μmol kg−1 yr−1). This unmatched rate of change in the Cant concentration of the warm upper limb of the Meridional Overturning Circulation decreases towards the Irminger basin (0.68 ± 0.06 μmol kg−1 yr−1) due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather similar Cant concentration rates of increase (between 0.33 and 0.45 μmol kg−1 yr−1), whereas in the Iceland basin these layers seem to have been less affected by Cant. Overall, the Cant storage rates in the North Atlantic subpolar gyre during the first half of the 1990s, when a high North Atlantic Oscillation (NAO) phase was dominant, are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO2 has taken place during the present decade. The changes in deep-water ventilation are the main driving processes causing this weakening of the North Atlantic CO2 sink.