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

International audienceAnnual and seasonal mean circulations in the Azores–Gibraltar Strait region (North-Eastern Atlantic) are described based on climatological data. An inverse box model is applied to obtain absolute water mass transports consistent with the conservation of volume, salt and heat and the equations of the thermal wind. The large-scale gyre circulation (Azores Current, Azores Counter Current, Canary Current and Portugal Current) is well-represented in climatological data. The Azores Current annual mean transport was estimated to be 6.5 ± 0.8 Sv (1 Sv = 106 m3/s) eastward, exhibiting a seasonal signal with minimum transport in the spring (5.3 ± 0.8 Sv) and maximum transport in autumn (7.3 ± 0.8 Sv). The Azores Current transport is twice that of the Azores Counter Current in spring and autumn and is four-times higher in summer and winter. The southward Portugal and Canary Currents show similar seasonal cycles with maximum transports in spring (3.5 ± 0.6 and 6.6 ± 0.4 Sv, respectively).The overturning circulation within the area has an annual mean magnitude of 2.2 ± 0.1 Sv and two seasonal extremes; the highest in summer (2.6 ± 0.1 Sv) and the lowest in winter (1.7 ± 0.1 Sv). Of the annual mean, about two thirds (1.4 Sv) of the overturning circulation results from water mass transformation west of the Strait of Gibraltar: the downwelling and recirculation of upper Central Water (0.6 Sv) in the intermediate layer, the entrainment of Central Water (0.6 Sv) into the Mediterranean Outflow and the contribution of Antarctic Intermediate Water (0.2 Sv) to the Mediterranean Outflow. The remaining 0.8 Sv relates to the overturning in the Mediterranean Sea through the two-layer exchange at the Gibraltar Strait. Accordingly, the density level dividing the upper-inflowing and lower-outflowing limbs of the overturning circulation was found to be σ1 = 31.65 kg m−3 (σ1, potential density referred to 1000 db), which is above the isopycnal that typically separates Central and Mediterranean Water (σ1 = 31.8 kg m−3). In terms of water masses, we describe quantitatively the water mass composition of the main currents. Focusing on the spread of Mediterranean Water, we found that when the northward Mediterranean Water branch weakens in spring and autumn, the westward Mediterranean Water vein strengthens, and vice versa. The maximum net transports of Mediterranean Water across the northern and western sections of the box were estimated at −1.9 ± 0.6 Sv (summer) and −0.8 ± 0.2 Sv (spring), respectively. Within the error bar (0.2 Sv), we found no significant net volume transport of Mediterranean Water across the southern section

[1] Four-year time series of observations in Espartel sill at the western part of the Strait of Gibraltar have been analyzed in order to investigate the variability of the Mediterranean outflow. It is assumed that the observed variability comes from the changing properties of the dense waters that are located at the maximum depth from where they can be uplifted in the upstream basin (Alborán Sea, inside the Mediterranean Sea) and evacuated through the strait. From this perspective, the following three mechanisms are investigated: (1) the replenishment of the deep basin by newly formed Western Mediterranean Deep Water that, depending on its density, can either uplift old resident waters or lay above them leaving in any case a cold signature in the temperature series; (2) the presence/absence of the energetic anticyclonic gyres in the Alborán Sea, particularly the western one, which can transfer momentum to the underlying Mediterranean vein and provide it with additional energy to ascend over the sills of the strait; and (3) the meteorologically enhanced flows that follow the rapid changes of atmospheric pressure over the western Mediterranean basin, which would be able to aspire deeper waters residing in the upstream basin. The three mechanisms act on different timescales, from annual in case (1) to monthly in case (2) to weekly in case (3) although these two latter are modulated annually by the seasonal prevalence of the western Alborán gyre in summer and of the strong meteorologically driven fluctuations in winter. The mechanisms overlap at annual timescales making it difficult to separate out the different contributions.

Abstract. Repeated hydrographic sections provide critically needed data on and understanding of changes in basin-wide ocean CO2 chemistry over multi-decadal timescales. Here, high-quality measurements collected at twelve cruises carried out along the same track between 1991 and 2015 have been used to determine long-term changes in ocean CO2 chemistry and ocean acidification in the Irminger and Iceland basins of the North Atlantic Ocean. Trends were determined for each of the main water masses present and are discussed in the context of the basin-wide circulation. The pH has decreased in all water masses of the Irminger and Iceland basins over the past 25 years with the greatest changes in surface and intermediate waters (between −0.0010 ± 0.0001 and −0.0018 ± 0.0001 pH units yr−1). In order to disentangle the drivers of the pH changes, we decomposed the trends into their principal drivers: changes in temperature, salinity, total alkalinity (AT) and total dissolved inorganic carbon (both its natural and anthropogenic components). The increase in anthropogenic CO2 (Cant) was identified as the main agent of the pH decline, partially offset by AT increases. The acidification of intermediate waters caused by Cant uptake has been reinforced by the aging of the water masses over the period of our analysis. The pH decrease of the deep overflow waters in the Irminger basin was similar to that observed in the upper ocean and was mainly linked to the Cant increase, thus reflecting the recent contact of these deep waters with the atmosphere.

The Mediterranean Outflow Water (MOW) spills from the Mediterranean Sea (east North Atlantic basin) west off the Strait of Gibraltar. As MOW outflows, it entrains eastern North Atlantic Central Waters (ENACW) and Intermediate Waters to form the neutrally buoyant Mediterranean Water (MW) that can be traced over the entire North Atlantic basin. Its high salinity content influences the thermohaline properties of the intermediatedeep water column in the North Atlantic and its dynamics. Here, the composition of MW in its source region (the Gulf of Cádiz, west off Strait of Gibraltar) is investigated on the basis of an optimum multiparameter analysis. The results obtained indicate that mixing of MOW (34.1% 6 0.3%) occurs mainly with overlying ENACW (57.1% 6 0.8%) in a process broadly known as central water entrainment. A diluted form (80% of dilution) of the Antarctic Intermediate Water (AAIW) reaches the region and also takes part in MW formation (8.3% 6 0.5%). Finally, the underlying Labrador Sea Water (LSW) also contributes (0.4% 6 0.1%) to the characteristics of MW. From these results and considering 0.74 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21) as the mean outflow of MOW, the MW exportation rate was inferred (2.2 Sv), which, decomposing MW, means that the MOW outflow is accompanied by 1.24 Sv of entrained ENACW, 0.18 Sv of AAIW, and ,0.01 Sv of LSW.

<p><strong>Abstract.</strong> The GEOVIDE cruise, a collaborative project within the framework of the international GEOTRACES programme, was conducted along the French-led section in the North Atlantic Ocean (Section GA01), between 15 May and 30 June 2014. In this Special Issue, results from GEOVIDE, including physical oceanography and trace element and isotope cyclings, are presented among seventeen articles. Here, the scientific context, project objectives and scientific strategy of GEOVIDE are provided, along with an overview of the main results from the articles published in the special issue.</p>

Three consecutive transects (zonal, meridional, and transverse) formed a closed box to the west of the Strait of Gibraltar. This study aimed to analyze the thermohaline properties, volume transports, and water mass distributions (percentages) along the meridional section (30-41.5º N, 20º W). We identified the main geostrophic current (Azores Current) and its associated volume transport and interannual changes. Data from previous cruises (AZORES I, A16N, CLIVAR, OACES, and CHAOS) with similar tracks were employed to compare with the CAIBOX meridional section. All but one (CHAOS) were summer cruises. We estimated a mean transport for the Azores Current at 20º W of 9.3 ± 2.6 Sv. There appears to be an inverse relation between the position of this current and its associated transport, with relatively high (low) transports when the current is located roughly south (north) of 35º N. Regarding water masses, an increase of 14.4% was found for Mediterranean Water

The overturning circulation lower limb drives a net southward transport of oxygen and nutrients from the North to the South Atlantic • Anomalous circulation in 2010 enhanced nutrient convergence by the overturning upper limb, boosting North Atlantic biological CO2 uptake • We observed a deep silicate divergence in the North Atlantic in 2004 and 2010 compatible with a transient response to reduced overturning Confidential manuscript in review to Global Biogeochemical Cycles