Marine carbon observations (MCOs) provide essential data to trace historical and current changes in marine carbon storage and fluxes that ultimately feed into the Global Carbon Budget and the Intergovernmental Panel on Climate Change report. Therefore, MCOs play a key role in informing global climate policy as well as ocean governance. However, they only achieve this potential if multiple sources of observations are combined and analyzed jointly. This implies an immense coordination effort by the international MCO community which developed, e.g., joint standards for the collection of (meta‐)data, quality control processes, data platforms, etc. This article traces the value chain of MCOs, concretely for CO2, from data collection to the Intergovernmental Panel on Climate Change report. Based on an interdisciplinary research project, the study illuminates which structures and practices the marine carbon community has developed to integrate different observations and measurement technologies, starting from German research institutes and agencies and expanding to the European and international networks to which they contribute. Combining a social network analysis with qualitative insights from in‐depth interviews, the article identifies key information providers and brokers and pinpoints systemic vulnerabilities, e.g., where connections between observation networks or data platforms are maintained based on personal relationships or ad‐hoc interactions rather than automated data submissions, or where temporally limited third party funding threatens the continued existence of the observation network.

The Argo Programme is a major component of both the Global Ocean Observing System (GOOS) and the Global Climate Observing System (GCOS), providing near-real time data for ocean and atmospheric services and high-quality data for climate research. Although originally designed to provide temperature and salinity profiles in the upper 2 km of the ice-free ocean, the array has been expanded into seasonal ice zones. In addition, regional pilot programmes have demonstrated that some Argo floats can now measure biogeochemical parameters to address oceanic uptake of carbon, acidification, and deoxygenation (BioGeoChemical, BGC-Argo) and some floats are also able to make measurements throughout the water column down to 6000 m depth (Deep-Argo). These new BGC-Argo and Deep-Argo Missions, together with the initial Core-Argo Mission form the new global, full-depth and multidisciplinary OneArgo programme (Roemmich et al. 2019). Euro-Argo aims at maintaining ¼ of the global OneArgo array, with a regional perspective focusing on European marginal seas (Mediterranean, Black and Baltic seas) and the European part of the Arctic seas. The Euro-Argo strategy focuses on providing sustained high quality oceanic data to the scientific community for better understanding of the role of the Ocean in the Earth’s climate. Addressing issues of climate change along with expanding floats’ capabilities and coverage are at the centre of the Euro-Argo strategy. Another domain of grand challenges is related to the health of the oceanic ecosystem and its impacts on society. The recent technological advances in biogeochemical instrumentation on Argo floats have greatly improved the ability to address ecosystem monitoring, and gather data in the European marine areas to support climate and biodiversity policies set up by the European Union. Moreover, Argo is a major source of information for operational centres such as the Copernicus Marine and Climate Services and the European Centre for Medium-Range Weather Forecasts (ECMWF) in Europe, for the provision of ocean and weather forecasts and seasonal predictions. Euro-Argo supports the enhancement of monitoring and observing systems at regional scales for model-assimilation and model-validation purposes. In particular, the extensions of Argo into the deep ocean and ecosystem parameters offer new possibilities and will help to constrain and improve the models and resulting products. Within this context, Euro-Argo is currently revising its deployment and coverage strategy for the next decade, taking into consideration specific European needs in terms of in situ ocean observations, while contributing to the global OneArgo new ambitious design. We will present this strategy and provide some highlights on the challenges for the years to come.

The ocean plays an essential role in regulating the global climate, absorbing around 25 % of global CO2 emissions. Scientific knowledge of the ocean's capacity as a carbon sink is therefore essential for policy-making at the national and international level. However, the capacity of the existing marine science system to deliver this information at sufficiently high quality, without geographical and temporal gaps, and with equitable contributions by and access for less affluent national science systems, is far from assured. This contribution applies the six guiding principles of Open Science as a yardstick for science in the service of society to assess the current state of marine (carbon) science, pointing out strengths and shortcomings, and deriving specific recommendations for science policy. This contribution results from a three-year interdisciplinary research project with researchers from Brazil and Germany and was discussed within the UN Ocean Decade Program Ocean Acidification Research for Sustainability (OARS) to validate the applicability of insights and recommendations beyond these particular contexts.

Since a pH sensor has become available that is principally suitable for use on demanding autonomous measurement platforms, the marine CO2 system can be observed independently and continuously by Biogeochemical Argo floats. This opens the potential to detect variability and long-term changes in interior ocean inorganic carbon storage and quantify the ocean sink for atmospheric CO2. In combination with a second parameter of the marine CO2 system, pH can be a useful tool to derive the surface ocean CO2 partial pressure (pCO2). The large spatiotemporal variability in the marine CO2 system requires sustained observations to decipher trends and study the impacts of short-term events (e.g., eddies, storms, phytoplankton blooms) but also puts a high emphasis on the quality control of float-based pH measurements. In consequence, a consistent and rigorous quality control procedure is being established to correct sensor offsets or drifts as the interpretation of changes depends on accurate data. By applying current standardized routines of the Argo data management to pH measurements from a pH / O2 float pilot array in the subpolar North Atlantic Ocean, we assess the uncertainties and lack of objective criteria associated with the standardized routines, notably the choice of the reference method for the pH correction (CANYON-B, LIR-pH, ESPER-NN, and ESPER-LIR) and the reference depth for this adjustment. For the studied float array, significant differences ranging between ca. 0.003 pH units and ca. 0.04 pH units are observed between the four reference methods which have been proposed to correct float pH data. Through comparison against discrete and underway pH data from other platforms, an assessment of the adjusted float pH data quality is presented. The results point out noticeable discrepancies near the surface of > 0.004 pH units. In the context of converting surface ocean pH measurements into pCO2 data for the purpose of deriving air–sea CO2 fluxes, we conclude that an accuracy requirement of 0.01 pH units (equivalent to a pCO2 accuracy of 10 µatm as a minimum requirement for potential future inclusion in the Surface Ocean CO2 Atlas, SOCAT, database) is not systematically achieved in the upper ocean. While the limited dataset and regional focus of our study do not allow for firm conclusions, the evidence presented still calls for the inclusion of an additional independent pH reference in the surface ocean in the quality control routines. We therefore propose a way forward to enhance the float pH quality control procedure. In our analysis, the current philosophy of pH data correction against climatological reference data at one single depth in the deep ocean appears insufficient to assure adequate data quality in the surface ocean. Ideally, an additional reference point should be taken at or near the surface where the resulting pCO2 data are of the highest importance to monitor the air–sea exchange of CO2 and would have the potential to very significantly augment the impact of the current observation network.

Abstract. Since a pH sensor has become available that is suitable for this demanding autonomous measurement platform, the marine CO2 system can be observed independently and continuously by BGC-Argo floats. This opens the possibility to detect variability and long-term changes in interior ocean inorganic carbon storage and quantify the ocean sink for atmospheric CO2. In combination with a second parameter of the marine CO2 system, pH can be a useful tool to derive the surface ocean CO2 partial pressure (pCO2). The large spatiotemporal variability of the marine CO2 system requires sustained observations to decipher trends and punctual events (e.g., river discharge, phytoplankton bloom) but also puts a high emphasis on the quality control of float-based pH measurements. In consequence, as the interpretation of changes depends on accurate data, and because sensor offsets or drifts might appear, a consistent and rigorous correction procedure to process and quality-control the data has been established. By applying standardized routines of the Ago data management to pH measurements from a pH/O2 float pilot array in the subpolar North Atlantic Ocean, we investigate the uncertainties and lack of objective criteria associated with the standardized routines, notably the choice of the reference method for the pH correction (CANYON-B or LIRPH) as well the reference depth for this correction. For the studied float array, significant differences of ca. 0.02 pH units are observed between the two reference methods which can be used to correct float-pH data from water samples. Through comparison against discrete pH data from water samples, an assessment of the adjusted float-pH data quality is presented. The results point out noticeable discrepancies near the surface of > 0.01 pH units. In the context of converting surface ocean pH measurements into pCO2 data for the purpose to derive air-sea CO2 fluxes, we conclude that the minimum accuracy requirement of 0.01 pH units (equivalent to the minimum pCO2 accuracy of 10 µatm for potential future inclusion into the SOCAT database) is not systematically achieved in the upper ocean. While the limited dataset and regional focus of our study provides only one showcase, it still calls for an additional independent pH reference in the surface ocean. We therefore propose a way forward to enhance the float-pH quality control procedure. In our analysis, the current philosophy of pH data correction against climatological reference data at one single depth in the deep ocean appears insufficient to assure adequate data quality in the surface ocean. Ideally, an additional reference point should be taken at or near the surface where the resulting pCO2 data are of the highest importance to monitor the air-sea exchange of CO2 and would have the potential to very significantly augment the impact of the current observation network.

The Eastern Boundary Current is an essential part of the water mass exchange between the subtropical and subpolar North Atlantic. Here, we study the offshore branch of the European Shelf Current (ESC) over the Goban Spur slope area off Ireland. Our analysis is based on current measurements obtained from a multiyear mooring effort, complemented by ship-board observations along a hydrographic section, satellite-derived estimates of absolute dynamic topography with geostrophic currents, and float trajectories. These data serve to quantify the offshore branch of the ESC on intraannual to interannual timescales. From the moored observations, we derive a mean poleward along-slope volume flux of 3.7 +/- 0.7 Sv for the period 2017-2019. Using a multilinear regression model and geostrophic surface velocities, we extend the time series to the period 1993-2019 and obtain a long-term mean transport of 3.2 +/- 0.4 Sv. Both time series show strong variability ranging from -7.5 to 15.7 Sv. The variability is linked to a dynamic eddy field, especially a stationary cyclonic circulation pattern near the mooring array, and meandering of current branches originating from the North Atlantic Current. We find no evidence of a consistent deep boundary current extending from the shelf break to the position of the offshore mooring (4,500 m depth), but confirm a persistent along-slope flow at the shallower slope (1,500 m depth). Geostrophic surface velocities and float trajectories reveal that the offshore branch of the ESC does not follow a clear northward path from the eastern subtropical regions but rather indicate the intermittent character of the flow.

In the past two decades, the Argo Program has collected, processed and distributed over two million vertical profiles of temperature and salinity from the upper two kilometers of the global ocean. A similar number of subsurface velocity observations near 1000 dbar have also been collected. This paper recounts the history of the global Argo Program, from its aspiration arising out of the World Ocean Circulation Experiment, to the development and implementation of its instrumentation and telecommunication systems, and the various technical problems encountered. We describe the Argo data system and its quality control procedures, and the gradual changes in the vertical resolution and spatial coverage of Argo data from 1999 to 2019. The accuracies of the float data have been assessed by comparison with high-quality shipboard measurements, and are concluded to be 0.002°C for temperature, 2.4 dbar for pressure, and 0.01 PSS-78 for salinity, after delayed-mode adjustments. Finally, the challenges faced by the vision of an expanding Argo Program beyond 2020 are discussed.

Since 2016 a moored observatory is operated at the eastern extension of the “North Atlantic Changes (NOAC)” array at 47°/48°N. This observatory is installed across the shelf break at Goban Spur and consists of two deep-sea moorings that are separated by about 60 km. The aim of this ongoing monitoring program is to quantify the variability and trends in the properties and transport rates of water masses that are advected northwards along the North Atlantic Eastern Boundary and modify the adjacent regions, i.e. the Northwest European Shelf, North Sea, Nordic Seas and Arctic Ocean. Furthermore, the continuous long term time series are essential for a thorough understanding of the circulation system in the eastern North Atlantic and the underlying physical mechanisms that govern its variability. Here, we present results of the analysis of temperature, salinity and current velocity time series from 2016 to 2019. These provide a descriptive view of the complex current structure and variability of water masses on daily to intra- and inter-annual time scales. The most pronounced signal in the variability of temperature and salinity is caused by the presence of Mediterranean Outflow Water located at about 1000 m depth. During the observation period we find significant positive trends in temperature and salinity in the depth range of 500 to 1500 m. The velocity measurements of the onshore mooring show a northeastward directed mean flow following the topography with along-slope variations, while the flow at the offshore mooring position is more unstable with predominantly cross-slope variations. The combination of our observations with float and altimeter data indicates that the presence of eddies and the interaction with the topography seems to play a crucial role for setting the variability of the flow in this region. Finally, we present an approach to evaluate the volume fluxes at the eastern boundary that will add toward an integrated estimate of the strength of the Atlantic Meridional Overturning Circulation at 47°/48°N.

The Baltic and North Seas Climatology (BNSC) presented here is a new climatology calculated solely from marine in situ observations. Created in cooperation between University of Hamburg (UHH), Federal Maritime and Hydrographic Agency [Bundesamt fur Seeschifffahrt und Hydrographie (BSH)] and German Meteorological Service [Deutscher Wetterdienst (DWD)], the BNSC is an update of the KLIWAS ("Klimawandel und WasserstraBen") climatology for the North Sea and is extended to the Baltic Sea. A thorough quality control, the reduction of the temporal sampling error and spatial and temporal averaging were applied to the observations, yielding time series of gridded fields of atmospheric and hydrographic parameters in the region of the Baltic, the North Sea and adjacent regions of the North Atlantic. The atmospheric subset of the BNSC consists of time series of monthly mean gridded fields of 2 m air and dew point temperature and air pressure at sea level for the period 1950-2015 on a horizontal 1 degrees x 1 degrees grid. Climatological fields are provided as well. The hydrographic part of the BNSC comprises the variables water temperature and salinity on 105 depth levels for the time interval 1873-2015. The grid boxes' edge length is 0.25 degrees in both zonal and meridional direction. Monthly and annual mean fields are provided as well as decadal monthly mean fields. To create homogenous fields, the method of objective analysis was applied to the fields of decadal means. Furthermore, an extensive sensitivity study was carried out to assess the sensitivity of the data product to the amount of observational data. The BNSC introduced here is compared to several different data products: three reanalyses (ERA-Interim, ERA40 and COSMO-REA6), the corresponding KUWAS product and meteorological station data for the atmospheric part. The hydrographic subset is compared to the KLIWAS climatology, the BALTIC ATLAS and the Baltic Sea Physical Reanalysis Product. The BNSC data product allows studying of climate variability but also holds the chance to validate regional numerical climate simulations, which makes it a valuable reference data set. The BSNC is freely available via the website of University of Hamburg's Integrated Climate Data Center.

As decadal predictions become operational, the need to use, understand, and extract information from them becomes essential. A climate index is a simple diagnostic quantity that can be used to characterize integral aspects of a geophysical system such as circulation patterns, and thus can be used to evaluate decadal forecasts. One of the most studied and well documented regions of the World Ocean is the North Atlantic. The North Atlantic subpolar gyre is an important region for the modulation of European climate and where skillful predictions of up to a decade can be obtained. Ocean re-analysis (ORA-S4) data from 1959 to 2017 are used to introduce a new methodology to compute a climate index of the North Atlantic subpolar gyre that captures both the variability in its strength and shape, the latter was to our knowledge never investigated previously as part of the variability of the gyre on interannual to decadal time scales. The methodology reveals two states of the gyre (before and after 2000), the former is mainly driven by temperature and the latter by a combination of mechanisms that interact to sustain a relatively stable subpolar gyre in terms of strength. Plain Language Summary Decadal predictions is a relatively new topic in climate science and is driven by the needs of industry and society to adapt to future climate states. To that end, elaborate earth system models provide forecasts that nevertheless need to be validated. Some regions of the world ocean are easier to predict such as the North Atlantic subpolar gyre. One way to validate the forecasts is through climate indeces. To that extent, we follow a new approach to define a climate index for the subpolar gyre that captures many features of the circulation that relate to the strength and the size of the gyre.

An ensemble of regional climate change scenarios for the North Sea is validated and analyzed. Five Coupled Model Intercomparison Project Phase 5 (CMIP5) General Circulation Models (GCMs) using three different Representative Concentration Pathways (RCPs) have been downscaled with the coupled atmosphere–ice–ocean model RCA4-NEMO. Validation of sea surface temperature (SST) against different datasets suggests that the model results are well within the spread of observational datasets. The ensemble mean SST with a bias of less than 1 ∘ C is the solution that fits the observations best and underlines the importance of ensemble modeling. The exchange of momentum, heat, and freshwater between atmosphere and ocean in the regional, coupled model compares well with available datasets. The climatological seasonal cycles of these fluxes are within the 95% confidence limits of the datasets. Towards the end of the 21st century the projected North Sea SST increases by 1.5 ∘ C (RCP 2.6), 2 ∘ C (RCP 4.5), and 4 ∘ C (RCP 8.5), respectively. Under this change the North Sea develops a specific pattern of the climate change signal for the air–sea temperature difference and latent heat flux in the RCP 4.5 and 8.5 scenarios. In the RCP 8.5 scenario the amplitude of the spatial heat flux anomaly increases to 5 W/m 2 at the end of the century. Different hypotheses are discussed that could contribute to the spatially non-uniform change in air–sea interaction. The most likely cause for an increased latent heat loss in the central western North Sea is a drier atmosphere towards the end of the century. Drier air in the lee of the British Isles affects the balance of the surface heat budget of the North Sea. This effect is an example of how regional characteristics modulate global climate change. For climate change projections on regional scales it is important to resolve processes and feedbacks at regional scales.

Tritium and helium isotope data provide key information on ocean circulation, ventilation, and mixing, as well as the rates of biogeochemical processes and deep-ocean hydrothermal processes. We present here global oceanic datasets of tritium and helium isotope measurements made by numerous researchers and laboratories over a period exceeding 60 years. The dataset's DOI is https://doi.org/10.25921/c1sn-9631, and the data are available at https://www.nodc.noaa.gov/ocads/data/0176626.xml (last access: 15 March 2019) or alternately http://odv.awi.de/data/ocean/jenkins-tritium-helium-data-compilation/ (last access: 13 March 2019) and includes approximately 60 000 valid tritium measurements, 63 000 valid helium isotope determinations, 57 000 dissolved helium concentrations, and 34 000 dissolved neon concentrations. Some quality control has been applied in that questionable data have been flagged and clearly compromised data excluded entirely. Appropriate metadata have been included, including geographic location, date, and sample depth. When available, we include water temperature, salinity, and dissolved oxygen. Data quality flags and data originator information (including methodology) are also included. This paper provides an introduction to the dataset along with some discussion of its broader qualities and graphics.

In diesem Kapitel werden die aktuellen Erkenntnisse bzgl. der vergangenen, derzeitigen und künftigen klimatischen Bedingungen in der Deutschen Bucht zusammengefasst und die Erkenntnisse des 1. Hamburger Klimaberichtes (kurz 1. HKB, von Storch und Claussen 2011) aktualisiert. Das Klima der Metropolregion Hamburg (MRH) wird maßgeblich von den ozeanographischen und meteorologischen Verhältnissen in und über der Nordsee beeinflusst, insbesondere aber von den Verhältnissen in der Deutschen Bucht. Diese grenzt direkt an die Metropolregion und ist auch die seewärtige Begrenzung der Tideelbe (◉ Abb. 4.1).

The strong warming and salinification of the Eastern North Atlantic starting in the mid 1990s has been attributed to a westward contraction of the sub-polar gyre and stronger inflow of waters from the sub-tropical gyre. Temporal changes in the shape and strength of the two gyres have been related to the major mode of atmospheric variability in the Atlantic sector, the NAO. Hydrographic conditions along the Northwest European shelf are thus the result of different processes such as variations in transports, varying relative contributions of water masses from the two gyres and property trends in the source water masses.