Sea ice and snow volume are essential variables for polar predictions, but operational systems still struggle to accurately capture their evolution. Satellite measurements now provide estimates of sea ice freeboard and snow depth. The combined assimilation of sea ice concentration (SIC), along-track altimetry radar freeboard data from Cryosat-2 and observations of snow depth from Cryosat-2 and SARAL is implemented in a multivariate approach in a global 1/4 degrees ocean/sea ice coupled NEMO4.2/SI3 model. A multivariate experiment, performed on two full seasonal cycles 2017-2018, is compared to a free (no assimilation) and a SIC-only assimilation simulations. The multivariate technique increases the sea ice volume, even in the absence of freeboard and snow measurements during summer, and rapidly changes the spatial patterns of ice and snow thicknesses in both hemispheres, in accordance with the assimilated observations. The sea ice volume from the multivariate approach compares better with independent (not assimilated) estimates from ICESat-2 and CS2SMOS or SMOS in both hemispheres. The multivariate system performs better in the Arctic than in Antarctica where the ice and ocean separate analyses are not designed to handle properly the strong interactions between upper oceanic layers and sea ice cover in the Southern Ocean. These results also confirm the importance of using variable snow and ice densities in a freeboard assimilation context. This study shows promising results for enhancing the capacity of assimilation systems to monitor the volume of sea ice and snow and paves the way for future satellite missions.

Surface Water and Ocean Topography (SWOT) high-resolution sea surface height (SSH) data extend the capabilities of nadir altimetry, enabling the detection of small ocean features, up to submesoscales. Assimilating these new measurements has great potential to enhance the accuracy of high-resolution global models but requires a detailed understanding of the data physical content to adapt the assimilation system accordingly. The Cal/Val 1-day repeat phase of the SWOT mission offers a unique opportunity to evaluate model performance in the high-frequency and high-wavenumber domain. SWOT SSH data are compared with outputs from Mercator Ocean International 1/12° global analysis and forecasting system. This comparison aims to characterize differences between the dynamics captured by SWOT and those represented in the model. Maps of SSH variability and spectral analyses are presented. Frequency spectra reveal good agreement between SWOT and the model at large scales but significant differences at higher frequencies. These differences are attributed to submesoscale signals in SWOT observations that were not captured by nadir altimeters or that are too small for the model grid resolution. The analyses also reveal coherent and non-coherent internal tide residuals in SWOT data. These residuals are quantified to improve the characterization of the representation error for future assimilation experiments. Insights from this study will inform and pave the way for effectively integrating SWOT data into operational systems.

The 10th Surface Water and Ocean Topography (SWOT) Applications Meeting, held one year after the satellite's launch, highlighted significant milestones in mission progress and showcased the innovative work of SWOT Early Adopters (EA) using mission data products. Over 100 participants from diverse sectors convened to discuss operational applications leveraging SWOT's unprecedented water surface measurements. The meeting emphasized applied science efforts to enhance hydrology and oceanographic models. This summary highlights the breadth of operational and private‐sector uses of SWOT data, emphasizing its potential to drive new innovations and deliver societal benefits, such as improved water resource management, flood prediction, and climate resilience.

Ocean Reanalyses Workshop of the European Copernicus Marine Service What: Gather together ocean reanalyses users and producers to identify users' needs of ocean reanalyses and design the strategy to improve ocean reanalyses to fulfill users' needs When: 10-12 October 2023 Where: Toulouse, France, and online

Forecasting across different Earth system components has initially been achieved independently, but increasing computer power, increasing model accuracy, increasing connectivity between experts, and increasing need for multi-hazard weather warning is changing the scene. Coupling methods, which involve exchanging information between discrete modelling systems, enable us to gain accuracy and consistency across Earth system components. This paper explains the principles of two-way coupling, where models run simultaneously and exchange information both ways. As individual models reach better accuracy, coupling becomes a key factor to improve forecasting capability because it reproduces the natural complexity of the environment: a wealth of literature shows the benefits of coupling. However, coupling is still limited in operational oceanography by its large demands on computational resources, by data assimilation techniques (currently not very well harmonised between the different models), and by administrative separation of forecasts across different Earth system components. Overcoming these barriers will support ocean predictions towards a multi-hazard approach and a more accurate representation of the Earth system component interactions and improve collaborations between multi-disciplinary forecasting communities.

The ocean plays an essential role in regulating Earth’s climate, influencing weather conditions, providing sustenance for large populations, moderating anthropogenic climate change, encompassing massive biodiversity, and sustaining the global economy. Human activities are changing the oceans, stressing ocean health, threatening the critical services the ocean provides to society, with significant consequences for human well-being and safety, and economic prosperity. Effective and sustainable monitoring of the physical, biogeochemical state and ecosystem structure of the ocean, to enable climate adaptation, carbon management and sustainable marine resource management is urgently needed. The Argo program, a cornerstone of the Global Ocean Observing System (GOOS), has revolutionized ocean observation by providing real-time, freely accessible global temperature and salinity data of the upper 2,000m of the ocean (Core Argo) using cost-effective simple robotics. For the past 25 years, Argo data have underpinned many ocean, climate and weather forecasting services, playing a fundamental role in safeguarding goods and lives. Argo data have enabled clearer assessments of ocean warming, sea level change and underlying driving processes, as well as scientific breakthroughs while supporting public awareness and education. Building on Argo’s success, OneArgo aims to greatly expand Argo’s capabilities by 2030, expanding to full-ocean depth, collecting biogeochemical parameters, and observing the rapidly changing polar regions. Providing a synergistic subsurface and global extension to several key space-based Earth Observation missions and GOOS components, OneArgo will enable biogeochemical and ecosystem forecasting and new long-term climate predictions for which the deep ocean is a key component. Driving forward a revolution in our understanding of marine ecosystems and the poorly-measured polar and deep oceans, OneArgo will be instrumental to assess sea level change, ocean carbon fluxes, acidification and deoxygenation. Emerging OneArgo applications include new views of ocean mixing, ocean bathymetry and sediment transport, and ecosystem resilience assessment. Implementing OneArgo requires about $100 million annually, a significant increase compared to present Argo funding. OneArgo is a strategic and cost-effective investment which will provide decision-makers, in both government and industry, with the critical knowledge needed to navigate the present and future environmental challenges, and safeguard both the ocean and human wellbeing for generations to come.

The EU funded project EuroSea brought together key actors of the European ocean observing and forecasting communities with key users of the ocean observing products and services in order to better integrate existing ocean observation systems and tools, and to improve the delivery of ocean information to users. EuroSea was constructed around the ocean observing value chain that connects observations to users of ocean information, and, just as intended, the value chain concept was a useful prism to improve the system. In this article, we summarize some of the main take-home messages from EuroSea on the needs for developing the European Ocean Observing System and its links with modeling and forecasting systems. During the project, the challenges and gaps in the design and coordination of the European ocean observing and forecasting system were identified and mapped. Many gaps and challenges related to the observations of physical, chemical and biological Essential Ocean Variables were identified. Some of these gaps are related to technological developments, while others are caused by insufficient and short-term funding leading to a not sustainable system, management, and cooperation between different entities, as well as limitations in foresight activities, policies and decisions. This article represents a compilation of the broader needs for advancing the observing and forecasting system, and is meant as a guide for the community, and to funders and investors to advance ocean observing and the delivery of ocean information in Europe. To enhance the sustainability of ocean observations, which is paramount for a reliable provision of quality oceanographic data and services, several recommendations were compiled for ocean observing networks, frameworks, initiatives, as well as the ocean observing funders within the European nations, and the European Commission.

Abstract. Modelling our planet is challenging, and forecasting the ocean component is crucial for better understating physical processes in a changing climate. To achieve this objective, numerical ocean models require more advanced approaches that aim at connecting different Earth system’s components in a more sophisticated way: this is offered by coupling methods, that involve exchanging information between discrete modelling systems. The paper explains the principles of two-way coupling, where models run simultaneously and exchange information both ways. As individual models reach better accuracy, coupling becomes a key factor to improve forecasting capability because it reproduces the natural complexity of the environment: a wealth of literature shows the benefits of coupling. However, coupling is still limited in operational oceanography by its large demands on computational resources, by data assimilation techniques (currently not very well harmonized between the different models) and by administrative separation of forecasts across different earth-system components. Overcoming these barriers will support ocean predictions towards a multi-hazard approach and a more accurate representation of the Earth systems’ components interaction, and improve collaborations between multi-disciplinary forecasting communities.

Predicting the ocean state in a reliable and interoperable way, while ensuring high-quality products, requires forecasting systems that synergistically combine science-based methodologies with advanced technologies for timely, user-oriented solutions. Achieving this objective necessitates the adoption of best practices when implementing ocean forecasting services, resulting in the proper design of system components and the capacity to evolve through different levels of complexity. The vision of OceanPrediction Decade Collaborative Center, endorsed by the UN Decade of Ocean Science for Sustainable Development 2021-2030, is to support this challenge by developing a “predicted ocean based on a shared and coordinated global effort” and by working within a collaborative framework that encompasses worldwide expertise in ocean science and technology. To measure the capacity of ocean forecasting systems, the OceanPrediction Decade Collaborative Center proposes a novel approach based on the definition of an Operational Readiness Level (ORL). This approach is designed to guide and promote the adoption of best practices by qualifying and quantifying the overall operational status. Considering three identified operational categories - production, validation, and data dissemination - the proposed ORL is computed through a cumulative scoring system. This method is determined by fulfilling specific criteria, starting from a given base level and progressively advancing to higher levels. The goal of ORL and the computed scores per operational category is to support ocean forecasters in using and producing ocean data, information, and knowledge. This is achieved through systems that attain progressively higher levels of readiness, accessibility, and interoperability by adopting best practices that will be linked to the future design of standards and tools. This paper discusses examples of the application of this methodology, concluding on the advantages of its adoption as a reference tool to encourage and endorse services in joining common frameworks.

Observing System Simulation Experiments (OSSEs) with the Mercator Ocean/Copernicus Marine global 1/12° data assimilation system have been carried out to compare and quantify the expected performance of two high resolution altimetry mission concepts envisioned for the long-term evolution (post-2032) of the Copernicus Sentinel-3 topography mission. The two mission concepts are a constellation of two wide-swath altimeters and a constellation of 12 nadir altimeters. These two configurations greatly improve ocean forecasting and monitoring capabilities. Compared to a constellation of three nadir altimeters (the present configuration), analysis and forecast errors are reduced by a factor of 2. Our results also show that a constellation of two wide-swath altimeters has better performance than a constellation of 12 nadirs. Compared to a constellation of 12 nadirs, the error of the Sea Surface Height (SSH) forecast of a two wide swath constellation is reduced by 14% overall. Improvements are also observed when analyzing surface currents and Lagrangian diagnostics. A constellation of two wide-swath altimeters thus seems to be a very promising concept for the long-term evolution of the Sentinel-3 topography mission.

Abstract. Observations are a fundamental element in ocean predictions: they are crucial not only for monitoring the ocean state, but also for improving the forecasting systems and validating the model outputs. In this framework, it is essential to adequately access, manage, and integrate such information in the ocean value chain. Data providers are in charge of collecting, processing and analyzing these observations, delivering comprehensive datasets that can be used for informed decision making and by forecasters to improve ocean models. In this paper, several examples of data services are discussed – ranging from the Copernicus Marine In-Situ Thematic Assembly Center to European Marine Observation and Data network (EMODnet) to SeaDataNet – recognized as key players in the framework of monitoring and management of the marine resource. The paper offers an outlook on future directions in ocean data integration, particularly on the opportunities offered by the standardization of protocols for data dissemination and the role of cost-effective and citizen-based data collection.

Abstract. Ocean prediction relies on the integration between models, satellite and in-situ observations through data assimilation techniques. Satellites offer nowadays high-resolution observations of essential ocean variables at the surface, widely adopted in combination with precise but sparse in-situ measurements that, from the surface to the deep ocean, can constrain large scale variability in models. Moreover, observations are a valuable source of information for validating and assessing model products, for improving them and for developing the next generation of machine learning algorithms aimed at enhancing the accuracy and scope of ocean forecasts. The authors discuss the role of observations in operational ocean forecasting systems, describing the state-of-the-art of satellite and in-situ observing networks and defining the paths for addressing multi-scale monitoring and forecasting.

Over the period 2021-2028, the Copernicus Marine Service will provide a continuity of service with incremental evolutions of products and services. Yet, major evolutions are proposed to better answer user needs, to keep the service at the state-of-the-art and to meet the new ocean monitoring and forecasting challenges required by European policies and users. In this context, a priority evolution for the Copernicus Marine Service to be implemented by 2028 is a major extension of the service towards the coastal ocean.In this presentation, an overview of the proposed evolutions of the Copernicus Marine Service towards the coastal ocean will be provided.Incremental evolutions are planned within the Copernicus Marine Service core service to better address coastal zones, including an improved representation of processes and forcings relevant for coastal zones, enhanced assimilation of observations over continental shelves, improved algorithms to provide satellite derived information on the ocean state and winds in the coastal zone, etc.In addition, new short, mid- and long-term activities are scheduled to develop new services. In the short-term, a Copernicus Coastal Thematic Hub will be implemented to gather in a single platform and access point the ensemble of information generated by several Copernicus Services on coastal zones. In addition, the Copernicus Marine Service will develop an improved coastal zone monitoring with new pan-European satellite-based products.In the mid-term, a co-design and co-production of marine information will be developed between Copernicus Marine and EU Member States. In that regard, a selection of coastal systems operated by Member States will be coupled to Copernicus Marine monitoring and forecasting operational systems.In the long-term, climate projections of the marine environment (physics, biogeochemistry, marine ecosystems) will be developed for the 21st century at basin scale. This long-term evolution is building on precursor R&D European projects and aims at developing regional to local ocean climate services to support policy implementation, including for coastal zones.

Changes in the state of the ocean affect the well-being of the planet, its ecosystems, and human societies (IPCC, 2019). The ocean is home to a vast array of plant and animal species, many of which are still undiscovered (Census of Marine Life, 2010), and it is hence critical in supporting global biodiversity and sustaining complex ecosystems (Pörtner et al., 2021; IPCC, 2022). The ocean supports various industries, such as fishing, shipping, tourism, and renewable and non-renewable resources, and provides job opportunities, income, and economic growth (OECD, 2016; European Commission, 2022). The ocean plays a vital role in regulating Earth’s climate. It acts as a massive heat sink, absorbing and storing vast amounts of surplus heat accumulated in the Earth system from human activities (von Schuckmann et al., 2023). It absorbs significant quantities of anthropogenic carbon through physical and biological processes, helping to mitigate climate change by reducing the greenhouse gas concentration in the atmosphere (Friedlingstein et al., 2022; Crisp et al., 2022). The ocean is intricately affected by climate change. Rising sea levels, warming, deoxygenation and acidification of the ocean, changing currents, and loss of sea ice and biodiversity all pose significant risks to coastal communities, infrastructure, economies, and vulnerable ecosystems (IPCC, 2021, 2022). In addition, overexploitation and ocean pollution, including plastic waste and chemical contaminants, can harm both marine life and human health, posing risks through the food chain and direct exposure (e.g., Landrigan et al., 2020; Lamb et al., 2018). The degradation of the ocean and increased pressure on its functioning and services can lead to economic losses, unemployment, and reduced opportunities for sustainable development (OECD, 2016). Protecting and preserving the ocean is essential for ensuring a sustainable future, maintaining biodiversity, regulating climate, supporting economies, and safeguarding human health and well-being (IPCC, 2019; Pörtner et al., 2021). Monitoring and reporting on any change in the state of the ocean can help to achieve this goal, while at the same time it allows developing timely strategies for adaptation measures as well as to raise awareness, inform decision-making, encourage action to protect and conserve the ocean, and hence transition towards sustainable ocean stewardship (e.g., von Schuckmann et al., 2020; Blunden and Boyer, 2022; IOCUNESCO, 2022). Most recently, on 19 June 2023, the Treaty of the High Seas was adopted by consensus and standing ovation during the United Nations meeting in New York. This treaty, also known as BBNJ (Biodiversity Beyond National Jurisdiction), is key to protecting the ocean, promoting equity and fairness, tackling environmental degradation, fighting climate change, and preventing biodiversity loss in the high seas (An historic achievement: Treaty of the High Seas is adopted, 2023). Regular reporting on the state, variability, and change of the ocean provides state-of-the-art science-driven findings to various stakeholders, including policymakers, scientists, industries, and the general public. Ocean reporting involves several components as part of a so-called added value chain (von Schuckmann et al., 2020). Firstly, data collection from different methods including satellite observations (e.g., sea level, sea surface temperature (SST), sea surface salinity (SSS), ocean color, waves) and in situ measurements – autonomous, moored, and taken during research expeditions – are paramount for building data products from the global to regional scale (IOC, 2019). Reprocessed data are also used to feed ocean reanalyses that combine them with numerical models through data assimilation schemes (Decade Collaborative Centre for Ocean Prediction, DCC-OP, 2023). All these different products build the baseline of the Copernicus Marine Service and include information about ocean physics (blue ocean; e.g., temperature, salinity, currents, waves), biogeochemistry and biodiversity (green ocean; e.g., chlorophyll

This paper reviews the recent progress in our estimation of ocean dynamic topography and the derived surface geostrophic currents, mainly based on multiple nadir radar altimeter missions. These altimetric observations provide the cornerstone of our ocean circulation observing system from space. The largest signal in sea surface topography is from the mean surface dominated by the marine geoid, and we will discuss recent progress in observing the mean ocean circulation from altimetry, once the geoid and other corrections have been estimated and removed. We then address the recent advances in our observations of the large-scale and mesoscale ocean circulation from space, and the particular challenges and opportunities for new observations in the polar regions. The active research in the ocean barotropic tides and internal tidal circulation is also presented. The paper also addresses how our networks of global multi-satellite and in situ observations are being combined and assimilated to characterize the four-dimensional ocean circulation, for climate research and ocean forecasting systems. For the future of ocean circulation from space, the need for continuity of our current observing system is crucial, and we discuss the exciting enhancement to come with global wide-swath altimetry, the extension into the coastal and high-latitude regions, and proposals for direct total surface current satellites in the 2030 period.

Ocean monitoring and forecasting systems combine information from ocean observations and numerical models through advanced data assimilation techniques. They are essential to monitor and report on past, present and future oceanic conditions. However, given the continuous development of oceanic models and data assimilation techniques in addition to the increased diversity of assimilated platforms, it becomes more and more difficult to establish how information from observations is used, and to determine the utility and relevance of a change of the global ocean observing system on ocean analyses. Here, a series of observing system simulation experiments (OSSE), which consist in simulating synthetic observations from a realistic simulation to be subsequently assimilated in an experimental analysis system, was performed. An original multiscale approach is then used to investigate (i) the impact of various observing system components by distinguishing between satellites and in situ (Argo floats and tropical moorings), and (ii) the impact of recommended changes in observing systems, in particular the impact of Argo floats doubling and enhancements of tropical moorings, on the fidelity of ocean analyses. This multiscale approach is key to better understand how observing system components, with their distinct sampling characteristics, help to constrain physical processes. The study demonstrates the ability of the analysis system to represent 40-80% of the temperature variance at mesoscale (20-30% for salinity), and more than 80% for larger scales. Satellite information, mostly through altimetric data, strongly constrains mesoscale variability, while the impact of in situ temperature and salinity profiles are essential to constrain large scale variability. It is also shown that future enhancements of Argo and tropical mooring arrays observations will likely be beneficial to ocean analyses at both intermediate and large scales, with a higher impact for salinity-related quantities. This work provides a better understanding on the respective role of major satellite and in situ observing system components in the integrated ocean observing system.

Human activities are causing a sustained increase in the concentration of carbon dioxide (CO2) and other greenhouse gases in the atmosphere. The resulting harmful effects on Earth’s climate require decarbonizing the economy and, given the slow pace and inherent limitations of decarbonization of some industries such as aviation, also the active removal and safe sequestration of CO2 away from the atmosphere (i.e., carbon dioxide removal or CDR; NASEM, 2022). Limiting global warming to 1.5°C—a target that may already have been exceeded—would require CDR on the order of 100–1000 Gt CO2 over the twenty-first century (IPCC, 2018).

A global deep-ocean observing system, the Argo Program, was enabled by technology advances and implemented by a multi-national partnership of academic and governmental institutions and scientists, national agencies, and commercial providers. Development of the autonomous profiling float made Argo possible. Formation of key partnerships made it a reality. The Argo Steering Team and Argo Data Management Team coordinate among 25 national Argo Programs for deployment of the globally distributed array, and for uniform data communications, quality control, and data distribution. Deployment of the Argo array began in 1999, achieved 3000 floats in 2007, and has been sustained at about 4000 through the present. Ongoing innovations are expanding the partnerships, with Deep Argo floats profiling to the sea floor and Biogeochemical Argo floats carrying dissolved oxygen, pH, nitrate, and bio-optical sensors. The Argo partnership has created and sustained an unprecedented observational network for climate assessment, research, education and operational oceanography.

The Copernicus Marine Environment Monitoring Service (CMEMS) provides regular and systematic reference information on the physical state, variability and dynamics of the ocean, ice and marine ecosystems for the global ocean and the European regional seas. The Copernicus Marine Service has run a successful initial phase over the past five years. Operational capabilities have been demonstrated, user uptake and user base have been steadily increasing and service evolution activities have allowed regular improvements of the products and services provided to users. CMEMS now serves a wide range of users (more than 21,000 subscribers are registered to the service) and applications (maritime safety, marine resources, coastal and marine environment, weather, seasonal forecast and climate). An overview of CMEMS achievements will be given and the presentation will highlight the essential role of R&D activities. CMEMS priorities and scientific challenges for Copernicus 2 (2021-2027) will then be discussed.