Uptake of atmospheric carbon dioxide in the subpolar North Atlantic Ocean declined rapidly between 1990 and 2006. This reduction in carbon dioxide uptake was related to warming at the sea surface, which-according to model simulations-coincided with a reduction in the Atlantic meridional overturning circulation. The extent to which the slowdown of this circulation system-which transports warm surface waters to the northern high latitudes, and cool deep waters south-contributed to the reduction in carbon uptake has remained uncertain. Here, we use data on the oceanic transport of volume, heat and carbon dioxide to track carbon dioxide uptake in the subtropical and subpolar regions of the North Atlantic Ocean over the past two decades. We separate anthropogenic carbon from natural carbon by assuming that the latter corresponds to a pre-industrial atmosphere, whereas the remaining is anthropogenic. We find that the uptake of anthropogenic carbon dioxide-released by human activities-occurred almost exclusively in the subtropical gyre. In contrast, natural carbon dioxide uptake-which results from natural Earth system processes-dominated in the subpolar gyre. We attribute the weakening of contemporary carbon dioxide uptake in the subpolar North Atlantic to a reduction in the natural component. We show that the slowdown of the meridional overturning circulation was largely responsible for the reduction in carbon uptake, through a reduction of oceanic heat loss to the atmosphere, and for the concomitant decline in anthropogenic CO 2 storage in subpolar waters. 2. However, air-sea CO 2 uptake in the North Atlantic is not necessarily predominantly anthropogenic 3, 4. In fact, air-sea CO 2 fluxes in the North Atlantic result from anthropogenic forcing and progressive northward cooling of the upper limb of the meridional overturning circulation
Abstract. Five of the most recent observational methods to estimate anthropogenic CO 2 (C ant ) are applied to a highquality dataset from five representative sections of the Atlantic Ocean extending from the Arctic to the Antarctic. Between latitudes 60 • N-40 • S all methods give similar spatial distributions and magnitude of C ant . However, discrepancies are found in some regions, in particular in the Southern Ocean and Nordic Seas. The differences in the Southern Ocean have a significant impact on the anthropogenic carbon inventories. The calculated total inventories of C ant for the Atlantic referred to 1994 vary from 48 to 67 Pg (10 15 g) of carbon, with an average of 54±8 Pg C, which is higher than previous estimates. These results, both the detailed C ant distributions and extrapolated inventories, will help to evaluate biogeochemical ocean models and coupled climate-carbon models.
Regional air‐sea fluxes, ocean transport, and storage of anthropogenic carbon () are quantified. Observation‐based data from the ocean interior are assimilated into the Bern3D dynamic ocean model using an Ensemble Kalman Filter. Global uptake of is estimated to be 131 ± 18 GtC over the period 1770 to 2000. Uncertainties from systematic biases in the reconstruction of are assessed by assimilating data from four global and six Atlantic reconstructions and found to be comparable or larger than uncertainties from ocean transport. Aggregated fluxes for the southern high‐latitude, tropical and midlatitude, and northern high‐latitude ocean agree within 0.11 GtC a−1 for the two reconstructions with the highest skill score, whereas regional uptake rates are up to a factor of three different. Results indicate that uptake and regional partitioning of anthropogenic carbon in the Southern Ocean remains uncertain.
Abstract.A high-quality inorganic carbon system database, spanning over three decades and comprising of 13 cruises, has allowed the applying of the ϕC • T method and coming up with estimates of the anthropogenic CO 2 (C ant ) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like C ant , into deeper ocean layers and grants this region an added oceanographic interest from the point of view of air-sea CO 2 exchanges. Generally, a tendency for decreasing C ant storage rates towards the deep layers has been observed. In the Iberian Basin, the North Atlantic Deep Water has low C ant concentrations and negligible storage rates, while the North Atlantic Central Water in the upper layers shows the largest C ant 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 C ant 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 C ant 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 C ant . Overall, the C ant 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 CO 2 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 CO 2 sink.
Abstract. The anthropogenic CO 2 (C ant ) estimates from cruises spanning more than two decades in the Irminger Sea area of the North Atlantic Subpolar Gyre reveal a large variability in the C ant storage rates. During the early 1990's, the C ant storage rates (2.3±0.6 mol C m −2 yr −1 ) doubled the average rate for 1981-2006 (1.1±0.1 mol C m −2 yr −1 ), whilst a remarkable drop to almost half that average followed from 1997 onwards. The C ant storage evolution runs parallel to chlorofluorocarbon-12 inventories and is in good agreement with C ant uptake rates of increase calculated from sea surface pCO 2 measurements. The contribution of the Labrador Seawater to the total inventory of C ant in the Irminger basin dropped from 66% in the early 1990s to 49% in the early 2000s. The North Atlantic Oscillation shift from a positive to a negative phase in 1996 led to a reduction of air-sea heat loss in the Labrador Sea. The consequent convection weakening accompanied by an increase in stratification has lowered the efficiency of the northern North Atlantic CO 2 sink.
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