Total alkalinity (TA) measurements in seawater are crucial for characterizing and monitoring the oceanic carbonate system. While international best practices and guidelines exist, the field still lacks widely available traceable reference materials and a well-established uncertainty budget of the measurement method. In this study, we applied key metrological principles - development of reference materials, inter-laboratory comparison and uncertainty quantification - to TA measurements. We developed two reference materials, including an artificial material with a rigorously characterized reference value and an associated uncertainty budget, being traceable to the International System of units (SI). These materials were tested in an inter-laboratory comparison involving five laboratories and demonstrated the applicability of the reference materials developed for quality control. Additionally, we established an uncertainty budget for the TA measurement method using two metrological approaches. The resulting expanded uncertainty was 5 mu mol kg-1 (with a coverage factor k=2) in TA, approaching the 4 mu mol kg-1 target set by the Global Ocean Acidification Observing Network for climate monitoring. These findings mark a significant step toward improving the quality and comparability of TA measurements, thereby strengthening long-term ocean carbonate system monitoring.

Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with global change (ocean carbon sinks and sources, ocean acidification) and ultimately to find carbon-based solutions or mitigation procedures (marine carbon removal). We present an extended database (SNAPO-CO2; Metzl et al., 2024c) with 24 700 new additional data for the period 2002 to 2023. The full database now includes more than 67 000 AT and CT observations along with basic ancillary data (time and space location, depth, temperature, and salinity) in various oceanic regions obtained since 1993 mainly in the framework of French research projects. This includes both surface and water columns data acquired in open oceans, coastal zones, rivers, the Mediterranean Sea, and either from time series stations or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with certified reference material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The same technique was used on board for underway measurements during cruises conducted in the southern Indian and Southern oceans. The AT and CT data from these cruises are also added to this synthesis. The data are provided in one dataset for the global ocean (https://doi.org/10.17882/102337, Metzl et al., 2024c) that offers a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT and AT reconstructed fields, ocean carbon and coupled climate–carbon models simulations, and data derived from Biogeochemical Argo (BGC-Argo) floats. These data can also be used to calculate pH, fugacity of CO2 (fCO2), and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

The Amazon shelf of South America is known to be highly contrasted in its surface carbon dioxide concentrations, from very high concentrations near the estuary, and very low concentrations downstream in the saltier Amazon plume, which results in a great contrast in carbon dioxide exchange with the atmosphere. During three cruises in 2020-2023 (Eurec4A-OA, Tara-Microbiomes legs 5, 6 and 7, Amaryllis), dissolved inorganic carbon (DIC) concentration, its isotopic composition (δ13C-DIC), the water isotopic composition (d18O-H2O and d2H-H2O), as well as inorganic nutrients and surface CO2 partial pressure (pCO2) were measured on the Amazon shelf of South America during three cruises in different seasons. These data are used to better understand mixing in the continuum between river water and open-ocean waters, and the biogeochemical processes taking place on the shelf close to the Amazon and Para river estuaries. The water isotopes are furthermore used to identify different freshwater origins.The accuracy of the data is discussed as well as its representativeness. The data are then combined to first identify large variations of the river freshwater sources, compatible with 2021 being a year of very large discharge, and 2023 a year of exceptional low discharge. In addition, the data mostly from August and September 2021 identify a smaller influence of sources and sinks of dissolved inorganic carbon in the mixing shelf region than what had been earlier observed during the Amasseds cruise data in November-December 1991, a much lower river discharge period. This indicates that there might be a larger seasonal and/or interannual variability of these processes than what was earlier assessed. Measured pCO2 data on the Amazon shelf in 2021 are then discussed in this context.

Measurements of dissolved inorganic carbon (DIC) concentration and its isotopic composition (δ13CDIC) are essential to study chemical and biological processes involved in the ocean carbon cycle, including photosynthesis, respiration, and air-sea CO2 fluxes. Anthropogenic CO2 emissions from fossil fuel combustion have caused an increase in DIC accompanied by a decline in δ13CDIC (called the Suess effect). δ13CDIC is thus a useful tracer to assess the oceanic uptake of anthropogenic CO2. Annual assessments of the Global Carbon Budget (e.g. Friedlingstein et al., 2023) have revealed a growing deviation over the last 10 to 15 years between the estimates of the ocean carbon sink based on observations and models, with the growth of the observation-based ocean CO2 sink being larger compared to the models. Discrepancies in the multi-decadal trend originate from all latitudes but are greatest in the Southern Ocean. Here, we present DIC and δ13CDIC measurements from surface and water column samples collected in the South-West Indian Ocean during repeated summer cruises over the last two decades (1998-2021) conducted on board the RV Marion Dufresne within the French monitoring program OISO (Océan Indien Service d’Observation). We compare these measurements with the DIC and δ13CDIC simulated over the same period by the δ13C-enabled version of the NEMO-PISCES ocean-biogeochemical model. We use different methods to separate the natural and anthropogenic signals over the last 20 years. Our analysis reveals some inconsistencies between simulated and observed DIC and δ13CDIC, as well as between other simulated and observed biogeochemical parameters, whereas physical parameters are generally well reproduced by the model. Identifying the cause for this mismatch bears the potential to explain all or part of the divergence between the observation-based and model-based estimates of oceanic carbon uptake.

Total alkalinity (AT) and dissolved inorganic carbon (CT) in the oceans are important properties with respect to understanding the ocean carbon cycle and its link to global change (ocean carbon sinks and sources, ocean acidification) and ultimately finding carbon-based solutions or mitigation procedures (marine carbon removal). We present a database of more than 44 400 AT and CT observations along with basic ancillary data (spatiotemporal location, depth, temperature and salinity) from various ocean regions obtained, mainly in the framework of French projects, since 1993. This includes both surface and water column data acquired in the open ocean, coastal zones and in the Mediterranean Sea and either from time series or dedicated one-off cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ±4 µmol kg−1 for both AT and CT. The data are provided in two separate datasets – for the Global Ocean and the Mediterranean Sea (https://doi.org/10.17882/95414, Metzl et al., 2023), respectively – that offer a direct use for regional or global purposes, e.g., AT–salinity relationships, long-term CT estimates, and constraint and validation of diagnostic CT and AT reconstructed fields or ocean carbon and coupled climate–carbon models simulations as well as data derived from Biogeochemical-Argo (BGC-Argo) floats. When associated with other properties, these data can also be used to calculate pH, the fugacity of CO2 (fCO2) and other carbon system properties to derive ocean acidification rates or air–sea CO2 fluxes.

Dédiée aux sciences de l'atmosphère, au climat et à d'autres domaines connexes, tels que l'océanographie ou la glaciologie, La Météorologie, révisée par des pairs et publiée en français, s'adresse aux professionnels de la météo et du climat, aux enseignants, aux étudiants, aux amateurs et aux utilisateurs. La Météorologie a succédé en 1925 à l'Annuaire de la Société météorologique de France (1852-1924) qui avait lui-même succédé à l'Annuaire météorologique de la France (1849-1851).

The Amazon shelf of South America is known to be highly contrasted in its surface carbon dioxide concentrations, from very high concentrations near the estuary, and very low concentrations downstream in the saltier Amazon plume, which results in a great contrast in carbon dioxide exchange with the atmosphere. During three cruises in 2020-2023 (Eurec4A-OA, Tara-Microbiomes legs 5, 6 and 7, Amaryllis), dissolved inorganic carbon (DIC) concentration, its isotopic composition (δ13C-DIC), the water isotopic composition (d18O-H2O and d2H-H2O), as well as inorganic nutrients and surface CO2 partial pressure (pCO2) were measured on the Amazon shelf of South America during three cruises in different seasons. These data are used to better understand mixing in the continuum between river water and open-ocean waters, and the biogeochemical processes taking place on the shelf close to the Amazon and Para river estuaries. The water isotopes are furthermore used to identify different freshwater origins. The accuracy of the data is discussed as well as its representativeness. The data are then combined to first identify large variations of the river freshwater sources, compatible with 2021 being a year of very large discharge, and 2023 a year of exceptional low discharge. In addition, the data mostly from August and September 2021 identify a smaller influence of sources and sinks of dissolved inorganic carbon in the mixing shelf region than what had been earlier observed during the Amasseds cruise data in November-December 1991, a much lower river discharge period. This indicates that there might be a larger seasonal and/or interannual variability of these processes than what was earlier assessed. Measured pCO2 data on the Amazon shelf in 2021 are then discussed in this context.

With climate projections questioning the future survival of stony corals and their dominance as tropical reef builders, it is critical to understand the adaptive capacity of corals to ongoing climate change. Biological mediation of the carbonate chemistry of the coral calcifying fluid is a fundamental component for assessing the response of corals to global threats. The Tara Pacific expedition (2016–2018) provided an opportunity to investigate calcification patterns in extant corals throughout the Pacific Ocean. Cores from colonies of the massive Porites and Diploastrea genera were collected from different environments to assess calcification parameters of long-lived reef-building corals. At the basin scale of the Pacific Ocean, we show that both genera systematically up-regulate their calcifying fluid pH and dissolved inorganic carbon to achieve efficient skeletal precipitation. However, while Porites corals increase the aragonite saturation state of the calcifying fluid (Ω cf ) at higher temperatures to enhance their calcification capacity, Diploastrea show a steady homeostatic Ω cf across the Pacific temperature gradient. Thus, the extent to which Diploastrea responds to ocean warming and/or acidification is unclear, and it deserves further attention whether this is beneficial or detrimental to future survival of this coral genus.

Abstract. Total alkalinity (AT) and total dissolved inorganic carbon (CT) in the oceans are important properties to understand the ocean carbon cycle and its link with climate change (ocean carbon sinks and sources) or global change (ocean acidification). We present a data-base of more than 44 400 AT and CT observations in various ocean regions obtained since 1993 mainly in the frame of French projects. This includes both surface and water columns data acquired in open oceans, coastal zones and in the Mediterranean Sea and either from time-series or punctual cruises. Most AT and CT data in this synthesis were measured from discrete samples using the same closed-cell potentiometric titration calibrated with Certified Reference Material, with an overall accuracy of ± 4 µmol kg-1 for both AT and CT. Given the lack of observations in the Indian and Southern Oceans, we added sea surface underway AT and CT data obtained in 1998–2018 in the frame of OISO cruises and in 2019 during the CLIM-EPARSES cruise measured onboard using the same technique. Separate datasets for the global ocean, and for the Mediterranean Sea are provided in a single format (https://doi.org/10.17882/95414, Metzl et al., 2023) that offers a direct use for regional or global purposes, e.g. AT/Salinity relationships, long-term CT estimates, constraint and validation of diagnostics CT-AT reconstructed fields or ocean carbon and coupled climate/carbon models simulations, as well as data derived from BG-ARGO floats. When associated with other properties, these data can also be used to calculate pH, fugacity of CO2 (fCO2) and other carbon systems properties to derive ocean acidification rates or air-sea CO2 fluxes.

The Tara Pacific expedition (2016–2018) sampled coral ecosystems around 32 islands in the Pacific Ocean and the ocean surface waters at 249 locations, resulting in the collection of nearly 58 000 samples. The expedition was designed to systematically study warm-water coral reefs and included the collection of corals, fish, plankton, and seawater samples for advanced biogeochemical, molecular, and imaging analysis. Here we provide a complete description of the sampling methodology, and we explain how to explore and access the different datasets generated by the expedition. Environmental context data were obtained from taxonomic registries, gazetteers, almanacs, climatologies, operational biogeochemical models, and satellite observations. The quality of the different environmental measures has been validated not only by various quality control steps, but also through a global analysis allowing the comparison with known environmental large-scale structures. Such publicly released datasets open the perspective to address a wide range of scientific questions.

The decadal changes in the fugacity of CO2 (fCO2) and pH in surface waters are investigated in the southern Indian Ocean (45–57∘ S) using repeated summer observations, including measurements of fCO2, total alkalinity (AT) and total carbon (CT) collected over the period 1998–2019 in the frame of the French monitoring programme OISO (Océan Indien Service d'Observation). We used three datasets (underway fCO2, underway AT–CT and station AT–CT) to evaluate the trends of fCO2 and pH and their drivers, including the accumulation of anthropogenic CO2 (Cant). The study region is separated into six domains based on the frontal system and biogeochemical characteristics: (i) high-nutrient low-chlorophyll (HNLC) waters in the polar front zone (PFZ) and (ii) north part and (iii) south part of HNLC waters south of the polar front (PF), as well as the highly productive zones in fertilised waters near (iv) Crozet Island and (v) north and (vi) south of Kerguelen Island. Almost everywhere, we obtained similar trends in surface fCO2 and pH using the fCO2 or AT–CT datasets. Over the period 1998–2019, we observed an increase in surface fCO2 and a decrease in pH ranging from +1.0 to +4.0 µatm yr−1 and from −0.0015 to −0.0043 yr−1, respectively. South of the PF, the fCO2 trend is close to the atmospheric CO2 rise (+2.0 µatm yr−1), and the decrease in pH is in the range of the mean trend for the global ocean (around −0.0020 yr−1); these trends are driven by the warming of surface waters (up to +0.04 ∘C yr−1) and the increase in CT mainly due to the accumulation of Cant (around +0.6 µmol kg−1 yr−1). In the PFZ, our data show slower fCO2 and pH trends (around +1.3 µatm yr−1 and −0.0013 yr−1, respectively) associated with an increase in AT (around +0.4 µmol kg−1 yr−1) that limited the impact of a more rapid accumulation of Cant north of the PF (up to +1.1 µmol kg−1 yr−1). In the fertilised waters near Crozet and Kerguelen islands, fCO2 increased and pH decreased faster than in the other domains, between +2.2 and +4.0 µatm yr−1 and between −0.0023 and −0.0043 yr−1. The fastest trends of fCO2 and pH are found around Kerguelen Island north and south of the PF. These trends result from both a significant warming (up to +0.07 ∘C yr−1) and a rapid increase in CT (up to +1.4 µmol kg−1 yr−1) mainly explained by the uptake of Cant. Our data also show rapid changes in short periods and a relative stability of both fCO2 and pH in recent years at several locations both north and south of the PF, which leaves many open questions, notably the tipping point for the saturation state of carbonate minerals that remains highly uncertain. This highlights the need to maintain observations in the long-term in order to explore how the carbonate system will evolve in this region in the next decades.

Abstract. We described new sea surface CO2 observations in the southwestern Indian Ocean obtained in January 2020 when a strong bloom event occurred south-east of Madagascar and extended eastward in the oligotrophic Indian Ocean subtropical domain. Compared to previous years (1991–2019) we observed very low fCO2 and dissolved inorganic carbon concentrations (CT) in austral summer 2020, indicative of a biologically driven process. In the bloom the anomaly of fCO2 and CT reached respectively −33 µatm and −42 µmol.kg-1 whereas no change is observed for alkalinity (AT). In January 2020 we estimated a local maximum of air-sea CO2 flux at 27° S of −6.9 mmol.m-2.d-1 (ocean sink) and −4.3 mmol.m-2.d-1 when averaging the flux in the band 26–30° S. In the domain 25–30° S/50–60° E we estimated that the bloom led to a regional carbon uptake of about −1 TgC.month-1 in January 2020 whereas this region was previously recognized as an ocean CO2 source or near equilibrium during this season. Using a neural network approach that reconstructs the monthly fCO2 fields we estimated that when the bloom was at peak in December 2019 the CO2 sink reached −3.1 (±1.0) mmol.m-2.d-1 in the band 25–30° S, i.e. the model captured the impact of the bloom. Integrated in the domain restricted to 25–30° S/50–60° E the region was a CO2 sink in December 2019 of −0.8 TgC.month-1 compared to a CO2 source of +0.12 (± 0.10) TgC.month-1 in December when averaged over the period 1996–2018. Consequently in 2019 this region was a stronger CO2 annual sink of −8.8 TgC.yr-1 compared to −7.0 (±0.5) TgC.yr-1 averaged over 1996–2018. In austral summer 2019/2020, the bloom was likely controlled by relatively deep mixed-layer depth during preceding winter (July–September 2019) that would supply macro and/or micro-nutrients as iron to the surface layer to promote the bloom that started in November 2019 in two large rings in the Madagascar Basin. Based on measurements in January 2020, we observed relatively high N2 fixation rates (up to 18 nmol N.L-1.d-1) suggesting that diazotrophs could play a role on the bloom in the nutrient depleted waters. The bloom event in austral summer 2020, along with the new carbonate system observations, represents a benchmark case for complex biogeochemical model sensitivity studies (including N2-fixation process and iron supplies) for a better understanding on the origin and termination of this still mysterious sporadic bloom and its impact on ocean carbon uptake in the future.

The Southern Ocean is recognized as a major player in the sequestration of anthropogenic carbon. As pH is naturally low at high latitudes, the increase in oceanic CO2 raises particular concerns in this region were surface waters could become rapidly under-saturated with respect to carbonate. We used repeated observations collected by the French monitoring program OISO (Ocean Indien Service d’Observation) in the surface ocean and the mixed layer over the last two decades (1998-2018), conducted on board the Marion Dufresne (IPEV/IFREMER). We used complementary data, available in SOCAT, to expand the study area, in order to investigate the evolution of CO2 and ocean acidification in the Southern Indian Ocean (45°S-57°S). South of the polar front in the High Nutrients Low Chlorophyll (HNLC) region, our results show an increase in the fugacity of CO2 (fCO2) in surface waters during summer, close to the increase in the atmosphere (on the order of +2 µatm yr-1) associated with a decrease in pH in the range of the mean global ocean trend (on the order of -0.0020 yr-1). However much larger changes are found in the phytoplankton blooms in the vicinity of Crozet and Kerguelen Islands for both fCO2 (between +3.0 µatm yr-1 and +5.0 µatm yr-1) and pH (ranging from -0.0033 yr-1 to -0.0059 yr-1). In all regions, the trends observed during summer are mainly driven by an increase in total carbon that is consistent with the accumulation of anthropogenic carbon evaluated below the summer mixed layer.

We report new oceanic carbonate system observations obtained during two cruises conducted in January 2004 (OISO-11) and April 2019 (CLIM-EPARSES) in the Mozambique Channel and estimate the long-term trend of sea surface fugacity of CO2 (fCO(2)) and pH using historical data. While in January 2004 the region was a large CO2 source, the ocean was near equilibrium in April 2019. Although this region experienced a dramatic cyclone event "Idai" in March 2019 leading to low salinity and low dissolved inorganic carbon (C-T) and total alkalinity (A(T)) concentrations in the central channel, salinity normalized A(T) were unchanged and C-T concentrations were higher in 2019 compared to 2004 by about 12 mu mol.kg(-1), likely due to anthropogenic CO2 uptake over 15 years. Compared to fCO(2) observations of 1963 in the channel, the oceanic fCO(2) was higher in 2004/2019 by about 100 mu atm, an increase close to that observed in the atmosphere (90 ppm). A part of the fCO(2) increase from 1963 to 2019 (about +10 mu atm) is due to the long-term ocean warming in this region (+0.011.C.decade(-1)). We estimated a mean decrease of 0.087 (+/- 0.007) pH unit between 1963 and 2019, typical of the preindustrial versus modern change in the global ocean. Using other observations in the southern part of the Mozambique Channel (around 25.S) we estimated a pH trend of 0.0129.decade(-1) (+/- 0.0042) for 1963-1995 and 0.0227.decade-1 (+/- 0.0048) for 1995-2019 suggesting a strengthening of acidification trend in the Mozambique Channel in agreement with the anthropogenic CO2 forcing. For the recent period, these rates were confirmed by reconstructed fCO(2) and pH monthly fields using a neural network model. We noted however that the pH trend in the Mozambique Channel appeared lower than previous estimates at the scale of the Indian Ocean. Based on historical atmospheric CO2 data we estimated that pH in the Mozambique Channel was about 8.18 (+/- 0.014) in the year 1800, i.e. 0.13 higher than in 2019. The concentration of C-T in the year 1800 was likely around 1915 (+/- 10) mu mol.kg(-1). These results will contribute to a better understanding of the impacts of ocean acidification on coral reefs since the industrial revolution by (1) providing a reference level for the reconstruction of pH from coral core samples that were collected at different locations in this region in 2019 and (2) by informing environmental authorities aiming at preserving and protecting those threatened ecosystems.

Antarctic Bottom Water (AABW) is known as a long-term sink for anthropogenic CO2 (Cant), but the sink is hardly quantified because of the scarcity of observations, specifically at an interannual scale. We present in this paper an original dataset combining 40 years of carbonate system observations in the Indian sector of the Southern Ocean (Enderby Basin) to evaluate and interpret the interannual variability of Cant in the AABW. This investigation is based on regular observations collected at the same location (63∘ E–56.5∘ S) in the framework of the French observatory OISO from 1998 to 2018 extended by GEOSECS and INDIGO observations (1978, 1985 and 1987). At this location the main sources of AABW sampled is the low-salinity Cape Darnley Bottom Water (CDBW) and the Weddell Sea Deep Water (WSDW). Our calculations reveal that Cant concentrations increased significantly in the AABW, from an average concentration of 7 µmol kg−1 calculated for the period 1978–1987 to an average concentration of 13 µmol kg−1 for the period 2010–2018. This is comparable to previous estimates in other Southern Ocean (SO) basins, with the exception of bottom water close to formation sites where Cant concentrations are about twice as large. Our analysis shows that total carbon (CT) and Cant increasing rates in the AABW are about the same over the period 1978–2018, and we conclude that the long-term change in CT is mainly due to the uptake of Cant in the different formation regions. This is, however, modulated by significant interannual to multi-annual variability associated with variations in hydrographic (potential temperature, Θ; salinity, S) and biogeochemical (CT; total alkalinity, AT; dissolved oxygen, O2) properties. A surprising result is the apparent stability of Cant concentrations in recent years despite the increase in CT and the gradual acceleration of atmospheric CO2. The interannual variability at play in AABW needs to be carefully considered in the extrapolated estimation of Cant sequestration based on sparse observations over several years.

The North Atlantic is one of the major ocean sinks for natural and anthropogenic atmospheric CO2. Given the variability of the circulation, convective processes or warming–cooling recognized in the high latitudes in this region, a better understanding of the CO2 sink temporal variability and associated acidification needs a close inspection of seasonal, interannual to multidecadal observations. In this study, we investigate the evolution of CO2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50–64∘ N) using repeated observations collected over the last 3 decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Over the full period (1993–2017) pH decreases (−0.0017 yr−1) and fugacity of CO2 (fCO2) increases (+1.70 µatm yr−1). The trend of fCO2 in surface water is slightly less than the atmospheric rate (+1.96 µatm yr−1). This is mainly due to dissolved inorganic carbon (DIC) increase associated with the anthropogenic signal. However, over shorter periods (4–10 years) and depending on the season, we detect significant variability investigated in more detail in this study. Data obtained between 1993 and 1997 suggest a rapid increase in fCO2 in summer (up to +14 µatm yr−1) that was driven by a significant warming and an increase in DIC for a short period. Similar fCO2 trends are observed between 2001 and 2007 during both summer and winter, but, without significant warming detected, these trends are mainly explained by an increase in DIC and a decrease in alkalinity. This also leads to a pH decrease but with contrasting trends depending on the region and season (between −0.006 and −0.013 yr−1). Conversely, data obtained during the last decade (2008–2017) in summer show a cooling of surface waters and an increase in alkalinity, leading to a strong decrease in surface fCO2 (between −4.4 and −2.3 µatm yr−1; i.e., the ocean CO2 sink increases). Surprisingly, during summer, pH increases up to +0.0052 yr−1 in the southern subpolar gyre. Overall, our results show that, in addition to the accumulation of anthropogenic CO2, the temporal changes in the uptake of CO2 and ocean acidification in the North Atlantic Subpolar Gyre present significant multiannual variability, not clearly directly associated with the North Atlantic Oscillation (NAO). With such variability it is uncertain to predict the near-future evolution of air–sea CO2 fluxes and pH in this region. Thus, it is highly recommended to maintain long-term observations to monitor these properties in the next decade.

We investigated the evolution of anthropogenic CO2 (Cant) in the Antarctic Bottom Water in the Southern Indian Ocean since 1978 based on observations from 16 reocupations. We found that the Cant and the dissolved inorganic carbon increased at about the same rate over the 40-years period. However, the data also show large interannual variations and a surprising stability of Cant in the last decade likely reflecting the variability of bottom waters formations and circulation in the Southern Ocean.

Abstract. The North Atlantic is one of the major sinks for anthropogenic CO2. In this study, we investigate the evolution of CO2 uptake and ocean acidification in the North Atlantic Subpolar Gyre (50° N–64° N) using repeated observations collected over the last three decades in the framework of the long-term monitoring program SURATLANT (SURveillance de l'ATLANTique). Data obtained between 1993 and 1997 suggest an important reduction in the capacity of the ocean to absorb CO2 from the atmosphere during summer, due to a rapid increase in the fugacity of CO2 (fCO2) in surface waters (5 times faster than the increase in the atmosphere). This was associated with a rapid decrease in surface pH (of the order of −0.014/yr) and was mainly driven by a significant warming and increase in DIC. Similar trends are observed between 2001 and 2007 during both summer and winter with a mean decrease of pH between −0.006/yr and −0.013/yr. These rapid trends are mainly explained by a significant warming of surface waters, a decrease in alkalinity during summer and an increase in DIC during winter. On the contrary, data obtained during the last decade (2008–2017) show a stagnation of surface fCO2 (increasing the ocean sink for CO2) and pH. These recent trends are explained by the cooling of surface waters, a small decrease of total alkalinity and the near-stagnation of dissolved inorganic carbon. Overall our results show that the uptake of CO2 and ocean acidification in the North Atlantic Subpolar Gyre is substantially impacted by multi-decadal variability, in addition to the accumulation of anthropogenic CO2. As a consequence, the future evolution of air-sea CO2 fluxes, pH and the saturation state of surface waters with regards to aragonite and calcite remain highly uncertain in this region.