The CNES-CLS22 Mean Dynamic Topography (MDT; https://doi.org/10.24400/527896/A01-2023.003, Jousset, 2023) represents an incremental update to previous CNES-CLS solutions, combining altimetry, satellite gravity, and in situ observations (drifters, hydrography profiles, and HF radar data). The main improvement lies in the Arctic, where enhanced Mean Sea Surface (MSS) coverage eliminate artifacts present in CNES-CLS18 and enable a more physically consistent representation of circulation, including the Norwegian Atlantic Front Current along the Mohn Ridge. Globally, CNES-CLS22 remains close to CNES-CLS18, with modest improvements in validation against independent datasets: RMS differences in geostrophic velocities decrease by only ∼ 0.2 %–0.5 % at the global scale and the average variance reduction at the global scale compared to heights derived from profiles is ∼ 0.5 %. Though regional gains are significant in the Arctic and Nordic Seas. HF radar integration in the Mid-Atlantic Bight demonstrates progress but highlights persistent challenges in shelf regions dominated by ageostrophic processes. At very small scales (< 40 km), noise from in situ data may introduce unrealistic kinetic energy, underscoring the need for improved filtering. Overall, CNES-CLS22 consolidates previous advances and provides better representation of key circulation features, but further progress will require enhanced coastal observations and refined processing methods, particularly for high-latitude and shelf areas.

Climate change impinges on the Arctic Ocean, leading to sea-ice loss and potentially drastic cascading ecosystem changes. A recent process is atlantification, the growing influence of warm and salty waters from the Atlantic on the Arctic with increasing ocean volume transport from the Nordic Seas to the Barents Sea playing a key role. Despite its importance and a multitude of hypotheses that have been tested, this trend remains mainly unexplained. Here we explore nonlinear effects and successfully link the flow trend through the Barents Sea Opening to a frequency shift of atmospheric synoptic. We show that a part of the flow through Barents Sea Opening is driven by topographic Rossby waves, and that they have a very sensitive response to atmospheric frequency over the Nordic Seas. These findings highlight how anthropogenic changes to the atmosphere are altering ocean processes, with implications for sea-ice extent and ecosystems in the Arctic.

Marine fish trade globalizes nutrients and contaminants. Using trade data, human demographic information, and nutrient and contaminant exposure data, the estimated direct consumption of traded fish from Northeast Atlantic Ocean (NEAO) catches varied among 155 importer countries/regions. The associated trade pathways globalised high amounts of important nutrients including iodine, selenium, and eicosapentaenoic acid and docosahexaenoic acid (EPA+DHA) and contributed greatly to annual domestic EPA+DHA requirements for small-population importers (e.g., Lithuania: 62.8%) but not for high-population importers (e.g., Chinese mainland). Traded amounts of mercury, dioxin, and dioxin-like polychlorinated biphenyls (dl-PCBs) from the NEAO fish were low, and associated pathway contributions to total domestic mercury exposures were <4%. Changes in fish body size affected nutrient and contaminant fillet concentrations and subsequently trade dynamics of nutrients and contaminants. Our study provides valuable insights regarding seafood globalization and marine fish trade that can be used to support adaptive management strategies for contaminants and nutrition-sensitive policies.

Micronutrient deficiency or 'hidden hunger' is of growing importance regionally and globally. Marine fish have the potential to mitigate hidden hunger although certain contaminants they often contain may also pose a health risk. Understanding biological and environmental drivers behind essential and hazardous element concentrations is therefore important to develop evidence-based advice for adaptive management strategies. We use Bayesian models to predict concentrations of ten essential and two hazardous elements in fillets of 14 marine fish species in the Northeast Atlantic Ocean. Data from 15,709 individuals of six lean, five semi-fatty, and three fatty species were included. Fish length, fat content, ocean basin, sea temperature and salinity were used as predictor variables. We found good model predictability and identified some important trends in driver effects. Fish length was the most important driver of element concentrations for most species with a negative effect for calcium, copper, manganese, and arsenic, and a positive effect for mercury, suggesting that smaller individuals may be a safer and better source of essential elements. Ocean basin was also an important driver in most cases. For concentrations of selenium, zinc, and mercury, effect sizes of ocean basins increased from north to south for several species. Fat content exhibited a small negative effect on concentrations of calcium, iron, and mercury, and a small positive effect on phosphorus and arsenic concentrations in many species. Temperature showed a small negative effect on zinc concentration for most species, while the effect of salinity varied among species without an apparent trend. This is the first multi-species and multi-element study to investigate drivers of element concentrations in marine fish at a large spatial scale using a Bayesian approach. The robust model predictability indicates the models' potential to further understand nutrient yield dynamics from fisheries, thereby empowering the implementation of informed strategies against hidden hunger.

Abstract. In this paper, the Copernicus Ocean State Report offers detailed scientific analysis of the ocean under climate change, ocean variability, and ocean extremes in the northeastern Atlantic and adjacent seas. Major results show that the northeastern Atlantic Ocean and adjacent seas have experienced consistent warming, with sea surface temperatures increasing at a rate of 0.25 ± 0.03 °C per decade since 1982, doubling the global average trend. This warming is most pronounced in the Black Sea, Mediterranean Sea, and Baltic Sea. Sea levels have risen significantly over the past 30 years, particularly in the Baltic and Mediterranean seas. Ocean acidification has also increased, with pH decreasing at a rate of −0.017 ± 0.001 units per decade. Marine heatwaves have intensified and expanded, affecting over 60 % of the region in 2022 and 2023. Over the past 16 years, most extreme wind speeds exceeding 22 m s−1 prevailed in the central and subpolar North Atlantic and northern Mediterranean Sea. The region has also seen significant variability in ocean climate indicators and circulation patterns, including increased Atlantic Water transport to the Arctic Ocean through the Fram Strait and notable variations in the Mediterranean Sea's meridional overturning circulation. No major Baltic inflow occurred in winter 2022/23.

Abstract. Anomalously warm oceanic events, often termed marine heatwaves (MHWs), can potentially impact the ecosystem in the affected region and have therefore become a hot topic for research in recent years. Determining the intensity and spatial extent of marine heatwaves, however, depends on the definition and climatological average used. Moreover, the stress applied by the heatwave to the marine ecosystem will depend on which component of the ecosystem is considered. Here, we utilize a model reanalysis (1991–2022) to explore the frequency, intensity, and duration of marine heatwaves in the Barents Sea, as well as regional heterogeneities. We find that major marine heatwaves are rather coherent throughout the region, but surface marine heatwaves occur more frequently while heatwaves on the ocean floor have a longer duration. Moreover, we investigate the sensitivity to the choice of climatological average length when calculating marine heatwave statistics. Our results indicate that severe marine heatwaves may become more frequent in a future Barents Sea due to ongoing climate change.

This dataset contains biogeochemical samples analyzed by the Plankton Chemistry Laboratory at the Institute of Marine Research (IMR), from the Norwegian, Greenland and Iceland Seas. Number of surveys and stations have varied greatly over the last 3 decades. IMR is conducting one annual Ecosystem Survey in April-May each year, with multiple trawl surveys and net tows, but only CTD water collections are reported here. This month-long exercise also has companion vessels from Iceland and the Faroe Islands surveying their own territorial waters. Three transects are the core of the time-series, visited multiple times each year (Svinøy-NorthWest, Gimsøy-NorthWest, Bjørnøya-West). On each station, the CTD cast is sampled for dissolved inorganic nutrients (nitrate, nitrite, phosphate, silicate) and phytoplankton chlorophyll- a and phaeopigments (ChlA, Phaeo) at predetermined depths. At times, short-term projects have collected samples for Winkler dissolved oxygen titrations (DOW) and particulate organic carbon and nitrogen (POC, PN) determinations. This unique data set has seen limited use over the years but is a great contribution towards global ocean research and climate change investigations.

Abstract. Anomalously warm oceanic events, often termed marine heatwaves, can potentially impact the ecosystem in the affected region and has therefore become a hot topic for research in recent years. Determining the amplitudes and extent of marine heatwaves, however, depends on the definition and climatological baseline used. Moreover, the stress applied by the heatwave to the marine ecosystem will depend on which component of the ecosystem is considered. Here, we utilize a model reanalysis (1991–2021) to explore the frequency, intensity and duration of marine heatwaves in the Barents Sea, as well as their regional expression. We find that major marine heatwaves are rather coherent throughout the region and have comparable surface and bottom expressions. Moreover, we utilize a 60-year regional model hindcast to show the impact of changing baselines on marine heatwave statistics. Our results indicate that severe marine heatwaves are likely becoming more frequent in a future Barents Sea due to ongoing climate change.

The Barents Sea is one of the Polar regions where current climate and ecosystem change is most pronounced. Here we review the current state of knowledge of the physical, chemical and biological systems in the Barents Sea. Physical conditions in this area are characterized by large seasonal contrasts between partial sea-ice cover in winter and spring versus predominantly open water in summer and autumn. Observations over recent decades show that surface air and ocean temperatures have increased, sea-ice extent has decreased, ocean stratification has weakened, and water chemistry and ecosystem components have changed, the latter in a direction often described as “Atlantification” or “borealisation,” with a less “Arctic” appearance. Temporal and spatial changes in the Barents Sea have a wider relevance, both in the context of large-scale climatic (air, water mass and sea-ice) transport processes and in comparison to other Arctic regions. These observed changes also have socioeconomic consequences, including for fisheries and other human activities. While several of the ongoing changes are monitored and quantified, observation and knowledge gaps remain, especially for winter months when field observations and sample collections are still sparse. Knowledge of the interplay of physical and biogeochemical drivers and ecosystem responses, including complex feedback processes, needs further development.

Young-of-the-year (0-group) fish in the Barents Sea have been investigated in an annual joint Norwegian-Russian pelagic trawl survey in autumn, using a standardized procedure since 1980. We use a conceptual framework of `upstream' spawning areas and 'downstream' nursery areas, recorded as 0-group distribution in the Barents Sea, to address spatial (geographical) and temporal (1980-2017) variation in 0-group length. Four boreal species (cod Gadus morhua, haddock Melanogrammus aeglefinus, herring Clupea harengus, and deepwater redfish Sebastes mentella) tended to have smaller 0-group individuals in the northern and eastern parts of the Barents Sea, with the largest individuals found in the central part where they were also most abundant. We interpret this to reflect slower growth as the lore-runners' of the seasonal cohort of juveniles are transported into colder waters (through lateral mixing). The Arctic species (capelin Mallotus villosus and polar cod Boreogadus saida) showed a different pattern with increasing 0-group length with increasing distance away from the spawning areas, seen most clearly for capelin. The longer juveniles in northern areas are probably older and stemming from early spawning. There was temporal covariation in 0-group length between the six species over the 38-year time series, with highest correlation between cod and haddock. The covariation reflected similar fluctuations in four decadal 'waves', with maxima in 0-group length in the early/mid 1980s, 1990s, 2000s, and 2010s. There was a high degree of spatial consistency in the temporal patterns of variation in 0-group length, with synchronous variations in different geographical areas. There were also increasing linear trends over the time series for cod, haddock, and polar cod, which represented increase of about 20%, 40%, and 15% of the initial length for the three species, respectively. The fluctuations and trends in 0-group length were positively correlated with seawater temperature, which suggests a strong effect of climate variability and warming (by 1.5-2.0 degrees C since 1980) on 0-group length. The clear differences among the species, and the limited fraction of variance explained by temperature, suggest that other factors such as food play additional roles. Zooplankton biomass integrated over the water column had low explanatory power, but this may reflect intrinsic limitations in the data (e.g., depth-integrated, end of season) in providing an adequate representation of feeding conditions, rather than suggesting that food was not important.

This dataset contains biogeochemical samples from the Barents Sea and Arctic region analyzed by the Plankton Chemistry Laboratory at the Institute of Marine Research (IMR). Number of surveys and stations visited in the Barents Sea and Arctic has varied over the last 30 years. One major effort is the annual Ecosystem Survey in the fall, with multiple trawl surveys, net tows and CTD water sampling. Additionally, two transects are visited multiple times each year (Fugløya-Bjørnøya and Vardø-North). Only samples collected from water bottles are reported here. Bottle samples from each CTD cast were collected for dissolved inorganic nutrients (nitrate, nitrite, phosphate, silicate) and phytoplankton chlorophyll- a and phaeopigments (ChlA, PHAEO) at predetermined depths and for later analysis at IMR. On occasion, short-term projects have performed Winkler dissolved oxygen titrations (DOW) and particulate organic carbon and nitrogen (POC, PN) determinations. This unique data set has seen limited use over the years but is a great contribution towards global ocean research and climate change investigations.

Chapter 1: CMEMS OSR5 1 1.1 IntroductionKarina von Schuckmann and Pierre-Yves Le Traon 1 1.2 Knowledge and data for international Ocean governancePaula Kellett, Brittany E. Alexander and Jo...

We present the in-situ biogeochemical data products distributed by the Copernicus Marine Service since 2018. The products offer available data of chlorophyll, oxygen, and nutrients (nitrate, silicate and phosphate), both in near-real time and as re-processed data, collected across the globe. The re-processing involves careful quality control utilizing tailored automated quality control procedures combined with visual inspection of questionable values by experts. Moreover, oxygen data are provided with uniform units for modelers (µmol/l) and other oceanic applications and monitoring purposes (µmol/kg) The products integrate observations aggregated from the Regional EuroGOOS consortium, as well as from SeaDataNet2, National Data Centers (NODCs) and JCOMM global systems, among others.We highlight some use cases, including a study showing an overall decline in the nutrient concentration (nitrate and silicate) of the Atlantic Water flowing though the Nordic Seas en-route to the Arctic Ocean, during the period 1990-2019. Moreover, the study shows indications of a delayed-response reduction further downstream in the Arctic Water exiting the Arctic Ocean through Fram Strait. Other use cases include the study of variability in the concentration of dissolved oxygen in the Mediterranean Sea, showing an association with dynamical processes.The in-situ near-real time biogeochemical product is updated every month whereas the re-processed product is updated two times per year. Products are delivered on NetCDF4 format compliant with the CF1.7 standard and well-documented quality control procedures.

Data on copepodid abundance and stage composition of Calanus finmarchicus was collected with seasonal resolution (5-6 times a year) from four stations along an oceanographic transect across the western Barents Sea Opening over 25 years (1995-2019). The stations were located in the Norwegian Coastal Current (NCC), inflowing Atlantic water (two stations), and near the Arctic polar front south of Bear Island. Mean copepodite abundance in the water column for the four stations increased from a level around 1000-10,000 individuals m(-2) in winter to 30,000-100,000 individuals m(-2) in summer (maximum 670,000 m(-2)). The overwintering (G0) population was dominated by copepodite stage 5 (CV) (40-70%) but with a relatively high fraction also of stage CIV (25-35%) in January. During winter, there was a progressive development of CVs into adult males and females, with mean abundances from 1500 to 4000 adult females m(-2) in April and May for the stations in the NCC and Atlantic water. Young copepodids of the new spring generation (G1) appeared with relatively high abundances in April at the NCC station and in May at the Atlantic water stations. The data showed a gradient of later development from south to north, reflected both in the maturation of the overwintering generation and in the development of the spring generation. Back-calculations based on temperature-dependent development time suggested peak spawning around late April to mid-May for the Atlantic water stations, and a month or so earlier (in March) for the NCC station, during an early phase of the spring phytoplankton growth. The spring generation developed as a distinct cohort but with large interannual variability in numbers. In Atlantic water, the composition of the copepodid developmental stages in August resembled the situation in June, although it was slightly more advanced. This is interpreted to reflect development of a second generation (G2) spawned upstream sometime in June or early July. The June data suggests that only a small fraction of the G1 cohort develops into adults that spawn and form the G2. There was a tendency of increased summer abundance of C. finmarchicus in the inflow region of the southwestern Barents Sea after about 2005. This may be related to the wind regime with more eastward wind-stress at the Barents Sea opening in recent 'high Calanus' years compared to 'low Calanus' years in the early 2000s.