International audienceWe discuss the distributions and transports of the main water masses in the North Atlantic Subpolar Gyre (NASPG) for the mean of the period 2002–2010 (OVIDE sections 2002–2010 every other year), as well as the inter-annual variability of the water mass structure from 1997 (4x and METEOR sections) to 2010. The water mass structure of the NASPG, quantitatively assessed by means of an Optimum MultiParameter analysis (with 14 water masses), was combined with the velocity fields resulting from previous studies using inverse models to obtain the water mass volume transports. We also evaluate the relative contribution to the Atlantic Meridional Overturning Circulation (AMOC) of the main water masses characterizing the NASPG, identifying the water masses that contribute to the AMOC variability. The reduction of the magnitude of the upper limb of the AMOC between 1997 and the 2000s is associated with the reduction in the northward transport of the Central Waters. This reduction of the northward flow of the AMOC is partially compensated by the reduction of the southward flow of the lower limb of the AMOC, associated with the decrease in the transports of Polar Intermediate Water and Subpolar Mode Water (SPMW) in the Irminger Basin. We also decompose the flow over the Reykjanes Ridge from the East North Atlantic Basin to the Irminger Basin (9.4 ± 4.7 Sv) into the contributions of the Central Waters (2.1 ± 1.8 Sv), Labrador Sea Water (LSW, 2.4 ± 2.0 Sv), Subarctic Intermediate Water (SAIW, 4.0 ± 0.5 Sv) and Iceland–Scotland Overflow Water (ISOW, 0.9 ± 0.9 Sv). Once LSW and ISOW cross over the Reykjanes Ridge, favoured by the strong mixing around it, they leave the Irminger Basin through the deep-to-bottom levels. The results also give insights into the water mass transformations within the NASPG, such as the contribution of the Central Waters and SAIW to the formation of the different varieties of SPMW due to air–sea interaction

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Global ocean acidification is caused primarily by the ocean's uptake of CO 2 as a consequence of increasing atmospheric CO 2 levels. We present observations of the oceanic decrease in pH at the basin scale (50°S-36°N) for the Atlantic Ocean over two decades . Changes in pH associated with the uptake of anthropogenic CO 2 (ΔpHCant) and with variations caused by biological activity and ocean circulation (ΔpHNat) are evaluated for different water masses. Output from an Institut Pierre Simon Laplace climate model is used to place the results into a longer-term perspective and to elucidate the mechanisms responsible for pH change. The largest decreases in pH (ΔpH) were observed in central, mode, and intermediate waters, T he uptake of anthropogenic CO 2 (Cant) by the ocean has lowered seawater pH since preindustrial times. This largescale and long-term change is referred to as "ocean acidification," a process that has led to changes in seawater carbonate chemistry (1-3) with impacts on the chemical speciation of seawater and biogeochemical cycles. A predominant effect is the decrease of carbonate ions in seawater that impacts calcareous marine organisms (4-6). The uptake of Cant is the main cause for the gradual reduction of seawater pH, but biological, physical, and chemical "natural" changes in the ocean, such as changes in the oxidation of organic matter, impact pH as well (7,8).Several studies have focused on ocean acidification in the last decade. Model-based studies have examined pH changes on a global scale (2, 9), and observation-based studies have focused on time-series stations (10-13) and specific regions such as the North Pacific (7, 14) and North Atlantic (8). However, investigations of basin-wide pH changes throughout the water column from direct measurements are sparse (15), in large part because of a dearth of quality referenced pH measurements.In this study, we present the first (to our knowledge) measurement-based changes in pH along meridional lines in the Atlantic from 50°S-36°N using observations from three cruises: OACES/ CO 2 (1993), CITHER-II (1994), and FICARAM-XV (2013) (Fig. 1A and SI Appendix, Table S1). We investigate the total change in pH and refer to it as acidification. The change in pH is separated into anthropogenic (ΔpHCant) and natural (ΔpHNat) components and is evaluated for the water masses along the section over two decades . Both components are related to Cant uptake and to processes such as the remineralization of organic matter and changes in water mass. The observed pH changes are compared with outputs from an Institut Pierre Simon Laplace (IPSL) climate model to place the observations into context and to elucidate the mechanisms controlling anthropogenic and natural pH changes, which are clearly discerned in climate models. ResultsMore than 4,000 pH measurements referenced to the seawater scale (SWS) at 25°C in the Atlantic Ocean (50°S-36°N) were examined to estimate the changes in oceanic pH between 1993 and 2013. We used pH measurements referenced to the SWS at 25°C thr...

Abstract. The GEOVIDE cruise was carried out in the subpolar North Atlantic (SPNA) along the OVIDE section and across the Labrador Sea in May-June 2014. It was planned to clarify the distribution of the trace elements and their isotopes in the SPNA as part of the GEOTRACES international program. This paper focuses on the state of the circulation and distribution of thermohaline properties during the cruise. In terms of circulation, the comparison with the 2002-2012 mean state shows a more intense Irminger Current and also a weaker North Atlantic Current, with a transfer of volume transport from its northern to its central branch. However, those anomalies are compatible with the variability already observed along the OVIDE section in the 2000s. In terms of properties, the surface waters of the eastern SPNA were much colder and fresher than the averages over 2002-2012. In spite of negative temperature anomalies in the surface waters, the heat transport across the OVIDE section estimated at 0.56 ± 0.06 PW was the largest measured since 2002. This relatively large value is related to the relatively strong Meridional Overturning Circulation measured across the OVIDE section during GEOVIDE (18.7 ± 3.0 Sv). By analyzing the air-sea heat and freshwater fluxes over the eastern SPNA in relation to the heat and freshwater content changes observed during 2013 and 2014, we concluded that on a short timescale these changes were mainly driven by air-sea heat and freshwater fluxes rather than by ocean circulation.

Abstract. The subpolar region in the North Atlantic is a major sink for anthropogenic carbon. While the storage rates show large interannual variability related to atmospheric forcing, less is known about variability in the natural dissolved inorganic carbon (DIC) and the combined impact of variations in the two components on the total DIC inventories. Here, data from 15 cruises in the Irminger Sea covering the 24-year period between 1991 and 2015 were used to determine changes in total DIC and its natural and anthropogenic components. Based on the results of an extended optimum multiparameter analysis (eOMP), the inventory changes are discussed in relation to the distribution and evolution of the main water masses. The inventory of DIC increased by 1.43 ± 0.17 mol m −2 yr −1 over the period, mainly driven by the increase in anthropogenic carbon (1.84 ± 0.16 mol m −2 yr −1 ) but partially offset by a loss of natural DIC (−0.57 ± 0.22 mol m −2 yr −1 ). Changes in the carbon storage rate can be driven by concentration changes in the water column, for example due to the ageing of water masses, or by changes in the distribution of water masses with different concentrations either by local formation or advection. A decomposition of the trends into their main drivers showed that variations in natural DIC inventories are mainly driven by changes in the layer thickness of the main water masses, while anthropogenic carbon is most affected by concentration changes. The storage rates of anthropogenic carbon are sensitive to data selection, while changes in DIC inventory show a robust signal on short timescales associated with the strength of convection.