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The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth's climate. However, there are few long series of observations of the AMOC and the study of the mechanisms driving its variability depends mainly on numerical simulations. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyze the seasonal to decadal variability of the subpolar AMOC across the Greenland–Portugal OVIDE line since 1993. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs (linked to the seasonal cycle of density in the East Greenland Current) and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs. This compensation means that the decadal variability of the AMOC is weaker and therefore more difficult to detect than the decadal variability of its velocity-driven and volume-driven components, which is highlighted by the formalism that we propose.

The Atlantic Meridional Overturning Circulation (AMOC) is a key component of the Earth's climate. However, there are few long time series of observations of the AMOC, and the study of the mechanisms driving its variability depends mainly on numerical simulations. Here, we use four ocean circulation estimates produced by different data-driven approaches of increasing complexity to analyse the seasonal to decadal variability of the subpolar AMOC across the Greenland–Portugal OVIDE (Observatoire de la Variabilité Interannuelle à DÉcennale) line since 1993. We decompose the MOC strength variability into a velocity-driven component due to circulation changes and a volume-driven component due to changes in the depth of the overturning maximum isopycnal. We show that the variance of the time series is dominated by seasonal variability, which is due to both seasonal variability in the volume of the AMOC limbs (linked to the seasonal cycle of density in the East Greenland Current) and to seasonal variability in the transport of the Eastern Boundary Current. The decadal variability of the subpolar AMOC is mainly caused by changes in velocity, which after the mid-2000s are partly offset by changes in the volume of the AMOC limbs. This compensation means that the decadal variability of the AMOC is weaker and therefore more difficult to detect than the decadal variability of its velocity-driven and volume-driven components, which is highlighted by the formalism that we propose

The Greenland-Portugal A25 OVIDE line is carried out biennially since 2002. The section is composed of 98 stations where hydrographic, biogeochemical and current measurements are carried out down to the bottom. OVIDE is a contribution to the international programs Go-Ship, IOCCP, and CLIVAR. This data set contains the final (adjusted) CTDO2 data

The Atlantic Meridional Overturning Circulation (AMOC) entails vigorous thermohaline transformations in the subpolar North Atlantic (SPNA). There, warm and saline waters originating in the subtropics are converted into cooler and fresher waters by a combination of surface fluxes and sub-surface thermohaline mixing. Using microstructure measurements and a small-scale variance conservation framework, we quantify the diapycnal and isopycnal contributions to thermohaline mixing within the eastern SPNA. Isopycnal stirring is found to account for 65% of thermal and 84% of haline variance dissipation in the upper 400 m of the eastern SPNA, suggesting an important role of isopycnal stirring in regional water-mass transformations. By applying the tracer variance method to two tracers, we underscore the special significance of isopycnal stirring for tracers weakly coupled to density, such as biologically-active tracers. Our findings thus highlight the central role of isopycnal stirring in both the AMOC and biogeochemical dynamics within the SPNA.

BOCATS 2021 cruise focused on the 10th repetition of the OVIDE section. OVIDE (BOCATS) cruises have been run biennially since 2002, always in June-July, and the section is labelled as a high-resolution reference section in the international program GO-SHIP. With the COVID situation, the cruise couldn’t be run in 2020 but in 2021. BOCATS 2021 started in Vigo on May 26 and ended in Reykjavik on June 29 on the R/V Sarmiento de Gamboa. The 98 stations of the OVIDE section, from Portugal to the tip of Greenland, couldn’t be realized in totality because of the presence of ice at the end of the section. The station 91 isn’t at the same exactly GPS point than Ovide section. The core of the work consisted in acquiring surface-to-bottom hydrographic profiles of pressure, temperature, salinity and dissolved oxygen, and analyzing 17 physical and biogeochemical tracers from 11500 samples drawn from the Niskin bottles of the CTD frame. We report here a focus on the acquisition and calibration steps of the physical variables measured specifically by the hydrographic CTD-O2 probe. After calibration, we find precisions for pressure, temperature, salinity and dissolved oxygen that fit the GO-SHIP international quality requirements. Pressure and temperature were calibrated at the laboratory before and after the cruise, leading to a repeatability better than 1 dbar and 0.001 °C respectively. The calibration of salinity and dissolved oxygen data is obtained by applying polynomial correcting functions that are calculated to statistically minimize the differences between the probe data and the sample data analyzed on board in the chemistry container. These chemical data are also evaluated by the comparison of replicates. After calibration, the differences in salinity and oxygen follow a zero-centered Gaussian-like distribution which standard deviation is used to evaluate the probe precision for each variable. For salinity, we find a standard deviation of 0.001, and for oxygen, 0.025 ml/l

CASCADE is a matlab toolbox designed to qualify and process the data from RDI's Ship Acoustic Doppler Current Profilers (SADCP). This user's guide explains step by step how to run CASCADE (version 7.2) available on https://www.umr-lops.fr/en/Technology/Software/CASCADE-7.2., CASCADE est une boîte à outils matlab conçue pour qualifier et traiter les données des profileurs de courant acoustiques Doppler de RDI (SADCP). Ce guide de l'utilisateur explique étape par étape comment utiliser CASCADE V7.2 disponible ici: https://www.umr-lops.fr/Technologies/Logiciels/ADCP-de-coque.

The GEOVIDE cruise was carried out coast to coast between Portugal and Newfoundland via the south tip of Greenland, following the OVIDE line in the eastern part and crossing the Labrador Sea in the western part. The classical hydrographic rosette was cast 163 times at 78 different geographical positions called stations. While the CTD-O2 probe acquired continuous profiles of the “physical” variables (pressure, temperature, salinity and dissolved oxygen), 22 Niskin bottles were closed at different levels during the upcast to provide samples for biogeochemical analysis. After calibration, we find precisions for pressure, temperature, salinity and dissolved oxygen that fit the GO-SHIP international quality requirements. In parallel, but not simultaneously, a trace-metal rosette (TMR) was cast 53 times, also acquiring profiles of physical variables, and equipped with 24 Go-Flo bottles adapted for the sampling of trace metals. Depending on the number of operations, stations were identified as “Short” (one single CTD cast), “Large” (3 CTD casts), “XLarge” (up to 6) and “Super” (up to 11). All along the track of the ship, current magnitude and direction was measured by Ship Acoustic Doppler Current Profilers, down to 1000m depth

This study proposes new insights for the interpretation of GEOVIDE particle measurements. Potentiel resuspension sites due to internal tides (ITs) have been identified along the western Iberian shelf slope using a tidal regional model. Iberian shelf is mainly a source of lithogenic material whereas Galicia Bank is a source of biogenic matter. The potential pathways for particles from the resuspension sites toward the GEOVIDE stations are identified using IBIRYS reanalysis to infer their transport by the ocean circulation. It appears that each station is influenced by a specific pathway and mixed particles from several resuspension sites. This methodology reveals that the criticality of the bathymetry slope, commonly used to track resuspension sites from internal waves, is not sufficient to investigate ITs. An explicit resolution of ITs using a numerical tidal model is required for an exhaustive identification of the resuspension sites. Key Points Sites of high bottom tidal velocity along the Iberian continental slope were identified using a semi-realistic internal tides model Sediment resuspension was confirmed on five sites based on earlier sediment surveys and Shields criterion threshold Particle trajectories were proposed based on circulation modeling to improve the interpretation of geochemical tracers measurements Plain Language Summary Internal tides are waves with a tidal period generated when tides cross a steep slope of the sea floor. Internal tides then propagate in every stratified layer of the open ocean over hundreds to thousands of kilometers. The encounter of internal tides with underwater relief, either at the generation site or after propagation, results in high bottom friction that enhances sediment resuspension. Evidence of lithogenic particles was found at various depths during the GEOVIDE cruise, from the Iberian continental slope to thousands of kilometers away. The present study identifies the sites where internal tides are responsible for sed

Shipboard hydrography along the A25-Ovide section (2002 – 2018) is combined with a high-resolution mooring array (2014 – 2020) and a regional fleet of Deep-Argo floats (2016 – 2021) to describe temperature changes of overflow-derived water masses in the Irminger Sea. Removing dynamical influences enables to identify a new statistically-significant trend reversal in Iceland Scotland Overflow Water (ISOW) and Denmark Strait Overflow Water (DSOW) core temperatures in the mid-2010s. A basin-wide cooling trend of -16 ± 6 m°C yr-1 during 2016 – 2021 – but reaching as strong as -44 ± 13 m°C yr-1 for DSOW in recent years – is found to interrupt a warming phase that was prevailing since the late 1990’s. The absence of an apparent reversal in the Nordic Seas and the faster changes detected in DSOW compared to ISOW point out the entrainment of subpolar signals within the overflows near the Greenland-Iceland-Scotland sills as a most likely driver. Plain Language Summary The North Atlantic Deep Water is one of the most voluminous water masses of the global ocean. Its deepest constituent – the Lower North Atlantic Deep Water (LNADW) – forms in the Nordic Seas before cascading into the North Atlantic at the Greenland-Iceland-Scotland sills and progressing south towards the Southern Ocean. The temperature and salinity of LNADW are known to obey decadal trends in response to forcing in its source regions as well as subsequent mixing with surrounding and overlying water masses during its Atlantic journey. Here, repeated measurements from oceanographic vessels, continuous monitoring with moored instrumentations, and autonomous Deep-Argo floats in the Irminger Sea (east of Greenland) during 2002 – 2021 are used to reveal a new warming-to-cooling reversal of LNADW in 2014. This signal, which presumably originates in the entrainment of upper and intermediate ocean variability near the Greenland-Iceland-Scotland sills, will progressively travel southward within the lower branch of the Atl

Iceland-Scotland Overflow Water (ISOW) and Labrador Seawater (LSW) are major water masses of the lower Atlantic Meridional Overturning Circulation (AMOC). Therefore, the investigation of their transport pathways is important to understand the structure of the AMOC and how climate properties are exported from the North Atlantic to lower latitudes. There is growing evidence from Lagrangian model simulations and observations that ISOW and LSW detach from boundary currents and spread off-boundary, into the basin interior in the Atlantic Ocean. Nuclear fuel reprocessing facilities of Sellafield and La Hague have been releasing artificial iodine (129I) into the northeastern Atlantic since the 1960ies. As a result, 129I is supplied from north of the Greenland-Scotland passages into the subpolar region labelling waters of the southward flowing lower AMOC. To explore the potential of 129I as tracer of boundary and interior ISOW and LSW transport pathways, we analyzed the tracer concentrations in seawater collected during four oceanographic cruises in the subpolar and subtropical North Atlantic regions between 2017 and 2019. The new tracer observations showed that deep tracer maxima highlighted the spreading of ISOW along the flanks of Reykjanes Ridge, across fracture zones and into the eastern subpolar North Atlantic supporting recent Lagrangian studies. Further, we found that 129I is intruding the Atlantic Ocean at unprecedented rate and labelling much larger extensions and water masses than in the recent past. This has enabled the use of 129I for other purposes aside from tracing ISOW. For example, increasing tracer levels allowed us to differentiate between newly formed 129I-rich LSW and older vintages poorer in 129I content. Further, 129I concentration maxima at intermediate depths could be used to track the spreading of LSW beyond the subpolar region and far into subtropical seas near Bermuda. Considering that 129I releases from Sellafield and La Hague have increased or

The OVIDE section, composed of a hundred top-to-bottom stations from Portugal to Greenland, has been visited biennially since 2002. Collected data show a strong variability of both the Atlantic Meridional Overturning Circulation (AMOC) and of the water mass properties. The OVIDE-AMOC timeseries built upon the Argo array and altimetry has been updated and validated with the in-situ cruise estimates. It shows a strong seasonal variability and, on longer time scales, significant transition in 2014, from moderate (19 Sv) to strong (23 Sv) amplitude, along with the development of a fresh and cold anomaly in the upper 800m over the eastern subpolar Atlantic, discussed in the literature and observed at the OVIDE section. Through a composite analysis of both transport and property data, we compare the 2002-2012 OVIDE average with the 2014-2018 average and analyze the evolutions of the transports of the different water masses with special attention to LSW, which has been largely renewed since 2014 through deep convection in the western subpolar gyre

About 30% of the carbon dioxide derived from human activities (CANTH) has been absorbed by the ocean (DeVries, 2014; Gruber et al., 2019; Friedlingstein et al., 2021), with the North Atlantic (NA) being one of the largest CANTH sinks per unit area (Khatiwala et al., 2013; Sabine et al., 2004). In the NA, oceanic CANTH uptake strongly relies on the meridional overturning circulation and the associated regional winter deep convection. In fact, the formation and deep spreading of Labrador Sea Water stands as a critical CANTH gateway to intermediate and abyssal depths. The NA CANTH uptake has fluctuated over the years according to changes in the North Atlantic Oscillation. Biennial observation of the marine carbonate system along the Go-Ship A25-OVIDE section has allowed us assessing the decadal and interannual variability of the CANTH storage in the subpolarNA from 2002 to 2021. In this study, we investigate 1) the trend of CANTH and 2) the relationship between the CANTH saturation, the apparent oxygen utilization, and the ventilation of the water masses between the A25-OVIDE section and the Greenland-Iceland-Scotland sills during 2002-2021. We divided the A25-OVIDE section into three main basins (Irminger, Iceland, and Eastern NA). Our results show that the Irminger Basin presents a more homogenous CANTH profile and higher CANTH saturation values at depth than the other two basins, which is related to the pronounced convective activity in the Irminger Basin. In contrast, the Eastern NA Basin has higher CANTH values at the surface due to its higher surface temperature, but its deep water masses show the lowest CANTH values since they are the less ventilated in the section. Our analysis also reveals that, overall, the NA CANTH storage has increased during 2002-2021, but varied according to the ventilation changes. While the Eastern NA water masses experienced a relatively constant, although shallower, average ventilation, the Irminger and Iceland Basins underwent a less

Despite covering only 15% of the global oceanic area, the North Atlantic (NA) accumulates one of the highest contents of anthropogenic CO2 (CANTH) in the ocean, storing 23% of the oceanic CANTH (Sabine et al., 2004). In terms of CANTH increase, large regions of the NA recorded average rates of 1.2 ± 0.1 mol m-2 yr-1 during 1994-2007, doubling the global oceanic rate of CANTH increase of 0.65 ± 0.08 mol m-2 yr-1 (Gruber et al., 2019). In the NA, high CANTH concentrations penetrate to mid and abyssal depths as a result of the formation and deep spreading of Labrador Sea Water. These processes are linked to the meridional overturning circulation and winter convection occurring in the NA subpolar gyre. Differences in CANTH storage rates have been observed between different phases of the NA Oscillation (NAO), attributed to both changes in CANTH concentration and decreases in volumetric water mass census (Fröb et al., 2018). There has been a 50% slowdown in the rate of CANTH increase in the NA subpolar gyre between 1994 and 2007, coinciding with a period of low NAO. Biennial observation of the marine carbonate system along the GOSHIP A25-OVIDE section has allowed assessing the long-term and interannual variability of the CANTH storage in the subpolar NA from 2002 to 2018. In this research, we determined the increases of CANTH in the water masses present between the OVIDE section and the Nordic sills. The average accumulation rate CANTH in the water column between the A25-OVIDE section and the sills was 0.90 ± 0.08 mol m-2 yr-1 (0.041 ± 0.003 Pg-C yr-1) for 2002-2021, being 61% higher than the 0.54 ± 0.06 mol m-2 yr-1 observed for 1997-2006 (low NAO). The increase in the CANTH accumulation rate during 2002-2021 is mainly found in the Irminger and Iceland basins and is mainly associated with the period of high NAO that occurred after 2014. In these two basins, the CANTH accumulation rates during the high NAO double those found during the low NAO

Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region.

This study details the sediment resuspension due to internal tides (ITs) and the afterward propagation of suspended particles along and off the Iberian continental slope. Five resuspension sites are precisely identified along the western Iberian shelf slope using a tidal regional model of Bay of Biscay and Iberian continental shelf. Two sites are sources of lithogenic material only, one of biogenic matter only and two are sources of a mixture of both. The potential pathways for particles from the resuspension sites toward the GEOVIDE stations are identified using the IBIRYS reanalysis to infer their transport by the ocean circulation. It appears that each station is influenced by a specific pathway and mixed particles from different resuspension sites. This methodology enables to establish more realistic interpretations of the geochemical in situ observations combining distinct modeling of ITs and oceanic circulation as well as in situ sediment properties. It also reveals that the criticality of the bathymetry slope, commonly used to trigger the potential occurrence of ITs, is not sufficient to investigate their role in sediment resuspension. An explicit resolution of the ITs using a numerical tidal model is required to identify more exhaustively all the actual resuspension sites.

The North Atlantic Basin is a major sink for atmospheric carbon dioxide (CO2) due in part to the extensive plankton blooms which form there supported by nutrients supplied by the three‐dimensional ocean circulation. Hence, changes in ocean circulation and/or stratification may influence primary production and biological carbon export. In this study, we assess this possibility by evaluating inorganic nutrient budgets for 2004 and 2010 in the North Atlantic based on observations from the transatlantic A05‐24.5°N and the Greenland‐Portugal OVIDE hydrographic sections, to which we applied a box inverse model to solve the circulation and estimate the across‐section nutrient transports. Full water column nutrient budgets were split into upper and lower meridional overturning circulation (MOC) limbs. According to our results, anomalous circulation in early 2010, linked to negative‐NAO conditions, led to an enhanced northward advection of more nutrient‐rich waters by the upper overturning limb, which resulted in a significant nitrate and phosphate convergence north of 24.5N. Combined with heaving of the isopycnals, this 'extreme circulation event' in 2010 favoured an enhancement of the nutrient consumption (5.7 ± 4.1 kmol‐P s‐1) and associated biological CO2 uptake (0.25 ± 0.18 Pg‐C yr−1, upper‐bound estimate), which represents a 50% of the mean annual sea–air CO2 flux in the region. Our results also suggest a transient state of deep silicate divergence in both years. Both results are indicative of a MOC‐driven modulation of the biological carbon uptake (by the upper MOC limb) and nutrient inventories (by the lower MOC limb) in the North Atlantic.

Internal waves in the semi-diurnal and near-inertial bands are investigated using an array of seven moorings located over the Reykjanes Ridge in a cross-ridge direction (57.6-59.1°N, 28.5-33.3°W). Continuous measurements of horizontal velocity and temperature for more than two years allow us to estimate the kinetic energy density and the energy fluxes of the waves. We found that there is a remarkable phase locking and linear relationship between the semi-diurnal energy density and the tidal energy conversion at the spring-neap cycle. The energy-to-conversion ratio gives replenishment time scales of 4-5 days on the ridge top vs 7-9 days on the flanks. Altogether, these results demonstrate that the bulk of the tidal energy on the ridge comes from near local sources, with a redistribution of energy from the top to the flanks, which is endorsed by the energy fluxes oriented in the cross-ridge direction. Implications for tidally-driven energy dissipation are discussed. The time-averaged near-inertial kinetic energy is smaller than the semi-diurnal kinetic energy by a factor 2-3, but is much more variable in time. It features a strong seasonal cycle with a winter intensification and sub-seasonal peaks associated with local wind bursts. The ratio of energy to wind work gives replenishment time scales of 13-15 days, which is consistent with the short time scales of observed variability of near-inertial energy. Finally, in the upper ocean (1 km), the highest levels of near-inertial energy are preferentially found in anticyclonic structures, with a twofold increase compared to cyclonic structures, illustrating the funneling effect of anticyclones.

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Li, F., Lozier, M. S., Bacon, S., Bower, A. S., Cunningham, S. A., de Jong, M. F., DeYoung, B., Fraser, N., Fried, N., Han, G., Holliday, N. P., Holte, J., Houpert, L., Inall, M. E., Johns, W. E., Jones, S., Johnson, C., Karstensen, J., Le Bras, I. A., P. Lherminier, X. Lin, H. Mercier, M. Oltmanns, A. Pacini, T. Petit, R. S. Pickart, D. Rayner, F. Straneo, V. Thierry, M. Visbeck, I. Yashayaev & Zhou, C. Subpolar North Atlantic western boundary density anomalies and the Meridional Overturning Circulation. Nature Communications, 12(1), (2021): 3002, https://doi.org/10.1038/s41467-021-23350-2., Changes in the Atlantic Meridional Overturning Circulation, which have the potential to drive societally-important climate impacts, have traditionally been linked to the strength of deep water formation in the subpolar North Atlantic. Yet there is neither clear observational evidence nor agreement among models about how changes in deep water formation influence overturning. Here, we use data from a trans-basin mooring array (OSNAP—Overturning in the Subpolar North Atlantic Program) to show that winter convection during 2014–2018 in the interior basin had minimal impact on density changes in the deep western boundary currents in the subpolar basins. Contrary to previous modeling studies, we find no discernable relationship between western boundary changes and subpolar overturning variability over the observational time scales. Our results require a reconsideration of the notion of deep western boundary changes representing overturning characteristics, with implications for constraining the source of overturning variability within and downstream of the subpolar region., We acknowledge funding from the Physical Oceanography Program of the U.S. National Science Foundation (OCE-1259398, OCE-1756231, OCE-1948335); the U.K. Natural Environment Research Council (NERC) National Capability programs the Extended Ellett Line and CLASS (NE/R015953/1), and NERC grants UK-OSNAP (NE/K010875/1, NE/K010875/2, NE/K010700/1) and U.K. OSNAP Decade (NE/T00858X/1, NE/T008938/1). Additional support was received from the European Union 7th Framework Program (FP7 2007-2013) under grant 308299 (NACLIM), the Horizon 2020 research and innovation program under grants 727852 (Blue-Action), 862626 (EuroSea). We also acknowledge support from the Royal Netherlands Institute for Sea Research, the Surface Water and Ocean Topography-Canada (SWOT-C), Canadian Space Agency, the Aquatic Climate Change Adaptation Services Program (ACCASP), Fisheries and Oceans Canada, an Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant, and from the China’s national key research and development projects (2016YFA0601803), the National Natural Science Foundation of China (41925025) and the Fundamental Research Funds for the Central Universities (201424001). Support for the 53°N array by the RACE program of the German Ministry BMBF is acknowledged, as is the contribution from Fisheries and Oceans Canada’s Atlantic Zone Monitoring Program.

The North Atlantic Basin is a major sink for atmospheric carbon dioxide (CO2) due in part to the extensive plankton blooms which form there supported by nutrients supplied by the three-dimensional ocean circulation. Hence, changes in ocean circulation and/or stratification may influence primary production and biological carbon export. In this study, we assess this possibility by evaluating inorganic nutrient budgets for 2004 and 2010 in the North Atlantic based on observations from the transatlantic A05-24.5°N and the Greenland-Portugal OVIDE hydrographic sections, to which we applied a box inverse model to solve the circulation and estimate the across-section nutrient transports. Full water column nutrient budgets were split into upper and lower meridional overturning circulation (MOC) limbs. According to our results, anomalous circulation in early 2010, linked to extreme negative NAO conditions, led to an enhanced northward advection of more nutrient-rich waters by the upper overturning limb, which resulted in a significant nitrate and phosphate convergence north of 24.5°N. Combined with heaving of the isopycnals, this anomalous circulation event in 2010 favored an enhancement of the nutrient consumption (5.7 ± 4.1 kmol-P s−1) and associated biological CO2 uptake (0.25 ± 0.18 Pg-C yr−1, upper-bound estimate), which represents a 50% of the mean annual sea–air CO2 flux in the region. Our results also suggest a transient state of deep silicate divergence in both years. Both results are indicative of a MOC-driven modulation of the biological carbon uptake (by the upper MOC limb) and nutrient inventories (by the lower MOC limb) in the North Atlantic

Dissolved Fe (DFe) samples from the GEOVIDE voyage (GEOTRACES GA01, May–June 2014) in the North Atlantic Ocean were analysed using a SeaFAST-picoTM coupled to an Element XR HR-ICP-MS and provided interesting insights on the Fe sources in this area. Overall, DFe concentrations ranged from 0.09 ± 0.01 nmol L−1 to 7.8 ± 0.5 nmol L−1. Elevated DFe concentrations were observed above the Iberian, Greenland and Newfoundland Margins likely due to riverine inputs from the Tagus River, meteoric water inputs and sedimentary inputs. Air-sea interactions were suspected to be responsible for the increase in DFe concentrations within subsurface waters of the Irminger Sea due to deep convection occurring the previous winter, that provided iron-to-nitrate ratios sufficient to sustain phytoplankton growth. Increasing DFe concentrations along the flow path of the Labrador Sea Water were attributed to sedimentary inputs from the Newfoundland Margin. Bottom waters from the Irminger Sea displayed high DFe concentrations likely due to the dissolution of Fe-rich particles from the Denmark Strait Overflow Water and the Polar Intermediate Water. Finally, the nepheloid layers were found to act as either a source or a sink of DFe depending on the nature of particles.

In the North Atlantic, there are two main western boundary currents related to the Atlantic Meridional Overturning Circulation (AMOC): the Gulf Stream flowing northward and the Deep Western Boundary Current (DWBC) flowing southward. Here we analyze data from the OVIDE section (GO-SHIP A25 Portugal-Greenland 40–60°N) that crosses the DWBC and the northward extension of the Gulf Stream, the North Atlantic Current. We show that North Atlantic western boundary currents play a key role in the transport of dissolved organic matter, specifically dissolved organic carbon (DOC). Revisited transports and budgets of DOC with new available data identify the eastern Subpolar North Atlantic (eSPNA) as an important source of locally produced organic matter for the North Atlantic and a key region in the supply of bioavailable DOC to the deep ocean. The East Greenland Current, and its upstream source the East Reykjanes Ridge Current on the eastern flank of the mid-Atlantic ridge, are export pathways of bioavailable DOC toward subtropical latitudes. The fast overturning and subsequent remineralization of DOC produced in the autotrophic eSPNA explains up to 38% of the total oxygen consumption in the deep North Atlantic between the OVIDE section and 24°N. Carbon budgets that do not take into account this organic remineralization process overestimates the natural uptake of carbon dioxide (CO2) from the atmosphere by one third. The inclusion of DOC transports in regional carbon budgets reconciles the estimates of CO2 uptake in the North Atlantic between model and observations.

Particulate concentrations of the fourteen Rare Earth Elements (PREE), yttrium and 232-thorium have been measured in two hundred samples collected in the epipelagic (ca. 0–200 m) and the mesopelagic (ca. 200–1000 m) zones of the North Atlantic, during the GEOVIDE cruise (May/June 2014, R/V Pourquoi Pas ?, GEOTRACES GA01). Particulate cerium (PCe) concentrations vary from 0.2 pmol L−1 to 16 pmol L−1, particulate neodymium (PNd) ones from 0.09 pmol L−1 to 6.1 pmol L−1 and particulate ytterbium (PYb) ones from 0.01 pmol L−1 to 0.5 pmol L−1. PREE concentrations are higher close to the Iberian margin and on the Greenland shelf, where PREE concentrations normalized to Post Archean Australian Shale (PAAS) display a positive Ce anomaly between 0.3 and 3, and a light REE (LREE) enrichment compared to heavy REE (HREE) illustrated by high PNdN/PYbN ratios (normalized to PAAS). The lithogenic fraction of the particulate REE concentration is closely related to the margin morphology and the hydrodynamic context: off the Iberian margin, up to 100 % of the PREEs are lithogenic and this lithogenic input spreads westward along isopycnals as intermediate nepheloid layers (INL) up to 1700 km away. Lithogenic inputs are also observed along the Greenland and Newfoundland margins, although the circulation stacks them along the coasts. PREE distributions are also controlled by the biological uptake in the surface layers and remineralization processes deeper. Low surface concentrations and some normalized REE patterns displaying a negative Ce anomaly and HREE enrichment indicate freshly formed biogenic particles. A significant relationship between biogenic silica (BSi) and PHREE is also observed in the diatom blooms occurring in the Labrador and Irminger seas. PHo/PY ratio was calculated in order to identify processes independent of the ionic radius. However, we could not firmly assess the role of the iron hydroxides in the scavenging prates of these elements.

The Atlantic Meridional Overturning Circulation (AMOC) is a key mechanism of heat, freshwater, and carbon redistribution in the climate system. The precept that the AMOC has changed abruptly in the past, notably during and at the end of the last ice age, and that it is ‘very likely’ to weaken in the coming century due to anthropogenic climate change is a key motivation for sustained observations of the AMOC. This paper reviews the methodology and technology used to observe the AMOC and assesses these ideas and systems for accuracy, shortcomings, potential improvements and sustainability. We review hydrographic techniques and look at how these traditional techniques can meet modern requirements. Transport mooring arrays provide the ‘gold standard’ for sustained AMOC observing, utilizing dynamic height, current meter, and other instrumentation and techniques to produce continuous observations of the AMOC. We consider the principle of these systems and how they can be sustained and improved into the future. Techniques utilizing indirect measurements, such as satellite altimetry, coupled with in‐situ measurements, such as the Argo float array, are also discussed. Existing technologies that perhaps have not been fully exploited for estimating AMOC are reviewed and considered for this purpose. Technology is constantly evolving and we look to the future of technology and how it can be deployed for sustained and expanded AMOC measurements. Finally, all of these methodologies and technologies are considered with a view to a sustained and sustainable future for AMOC observation. Plain Language Summary The Atlantic Meridional Overturning Circulation (AMOC) is a system of ocean currents (sometimes known as the Gulf Stream System or the Great Ocean Conveyor Belt) that is important because of how it moves heat and carbon around the planet. Due to human‐induced climate change, the AMOC is predicted to weaken substantially, with adverse impacts for regions dependent on the supply o

In the North Atlantic, there are two main western boundary currents related to the Atlantic Meridional Overturning Circulation (AMOC): the Gulf Stream flowing northward and the Deep Western Boundary Current (DWBC) flowing southward. Here we analyze data from the OVIDE section (GO-SHIP A25 Portugal-Greenland 40–60°N) that crosses the DWBC and the northward extension of the Gulf Stream, the North Atlantic Current. We show that North Atlantic western boundary currents play a key role in the transport of dissolved organic matter, specifically dissolved organic carbon (DOC). Revisited transports and budgets of DOC with new available data identify the eastern Subpolar North Atlantic (eSPNA) as an important source of locally produced organic matter for the North Atlantic and a key region in the supply of bioavailable DOC to the deep ocean. The East Greenland Current, and its upstream source the East Reykjanes Ridge Current on the eastern flank of the mid-Atlantic ridge, are export pathways of bioavailable DOC toward subtropical latitudes. The fast overturning and subsequent remineralization of DOC produced in the autotrophic eSPNA explains up to 38% of the total oxygen consumption in the deep North Atlantic between the OVIDE section and 24°N. Carbon budgets that do not take into account this organic remineralization process overestimates the natural uptake of carbon dioxide (CO2) from the atmosphere by one third. The inclusion of DOC transports in regional carbon budgets reconciles the estimates of CO2 uptake in the North Atlantic between model and observations