The sea surface temperature (SST) variability in the Tropical North Atlantic (TNA) plays a crucial role in the regional climate by modulating the Intertropical Convergence Zone (ITCZ) and influencing precipitation, convective systems, and tropical cyclones. While atmospheric synoptic-scale intraseasonal variability in this region is dominated by African Easterly Waves (AEWs), their impact on SST remains poorly understood. This study investigates the modulation of SST by AEWs using a regional configuration of a coupled ocean-atmosphere model and moored surface buoy air-sea observations. The results reveal a significant AEWs signature in SST anomalies, with typical temperature fluctuations of approximately +/- 0.3 degrees C (reaching up to +/- 0.5 degrees C for the strongest events). A heat budget analysis shows that AEWs mainly influence SST through modulation of the latent heat flux, shortwave radiation, and vertical mixing. The contribution of ocean mixing and that of the air-sea fluxes appear to be of similar order. The dominant 3-5 d AEWs exhibit a stronger impact than their 6-9 d counterparts. These findings highlight the role of AEWs in driving SST variability and mixed-layer dynamics, underscore the importance of accurately representing them in coupled climate models, and call for further investigation into their influence on the mean and seasonal upper-ocean state.

This study investigates the ability of a global ocean reanalysis at 1/12 degrees horizontal resolution, GLORYS12, to represent oceanic processes at intraseasonal and higher-frequency scales. GLORYS12, which includes data assimilation of satellite and multi-instrument in situ observations, is compared to a twin-free simulation (with no assimilation) in the tropical Pacific Ocean. Spectral analyses show that data assimilation improves the realism of sea surface height intraseasonal variability in the entire tropical Pacific Ocean, in both amplitude and phase, with an increase in the amplitude of more than 50% for the 20-90-day band and up to 15% for the 2-20-day band. The improvement is largest along the 5 degrees N/S latitudes, where the magnitude of tropical instability waves is maximum, but is limited along the equator where steric height variability is dominated by intraseasonal oceanic Kelvin waves, already well represented in the free simulation. Wavenumber-frequency spectra show that data assimilation constraint improves both the spatial and temporal scales of intraseasonal waves and their timing. Data assimilation impacts the realism of oceanic simulations in two ways. By modifying the background oceanic stratification, it corrects the phase speed of westward-propagating waves. It is also shown that the intraseasonal component of analysis increments (data assimilation corrections applied) is dynamically consistent and exhibits clear intraseasonal propagation. By demonstrating the benefits of data assimilation for intraseasonal processes in the tropical Pacific Ocean, this study highlights the high value of both in situ and satellite observations to constrain ocean models in a wide range of time scales.

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 diurnal cycles of near‐surface velocity and temperature, also known as diurnal jet and diurnal warm layer (DWL), are ubiquitous in the tropical oceans, affecting the heat and momentum budget of the ocean surface layer, air‐sea interactions, and vertical mixing. Here, we analyze the presence and descent of near‐surface diurnal shear and stratification in the upper 20 m of the equatorial Atlantic as a function of wind speed using ocean current velocity and hydrographic data taken during two trans‐Atlantic cruises along the equator in October 2019 and May 2022, data from three types of surface drifters, and data from Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) moorings along the equator. The observations during two seasons with similar mean wind speeds but varying surface heat fluxes reveal similar diurnal jets with an amplitude of about 0.11 m s−1 and similar DWLs when averaging along the equator. We find that higher wind speeds lead to earlier diurnal peaks, deeper penetration depths, and faster descent rates of DWL and diurnal jet. While the diurnal amplitude of stratification is maximum for minimal wind speeds, the diurnal amplitude of shear is maximum at 6 m depth for moderate wind speeds of about 5 m s−1. The inferred wind dependence of the descent rates of DWL and diurnal jet is consistent with the earlier onset of deep‐cycle turbulence for higher wind speeds. The DWL and the diurnal jet not only trigger deep‐cycle turbulence but are also observed to modify the wind power input and thus the amount of energy available for mixing.

Analysis of global numerical experiments with physical and BGC forecasting model to estimate the impact of new observing system design.

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.

The ocean is the main heat reservoir in Earth's climate system, absorbing most of the top-of-the-atmosphere excess radiation. As the climate warms, anomalously warm and fresh ocean waters in the densest layers formed near Antarctica spread northward through the abyssal ocean, while successions of warming and cooling events are seen in the deep-ocean layers formed near Greenland. The abyssal warming and freshening expands the ocean volume and raises sea level. While temperature and salinity characteristics and large-scale circulation of upper 2000 m ocean waters are well monitored, the present ocean observing network is limited by sparse sampling of the deep ocean below 2000 m. Recently developed autonomous robotic platforms, Deep Argo floats, collect profiles from the surface to the seafloor. These instruments supplement satellite, Core Argo float, and ship-based observations to measure heat and freshwater content in the full ocean volume and close the sea level budget. Here, the value of Deep Argo and planned strategy to implement the global array are described. Additional objectives of Deep Argo may include dissolved oxygen measurements, and testing of ocean mixing and optical scattering sensors. The development of an emerging ocean bathymetry dataset using Deep Argo measurements is also described.

Regional processes play a key role in the global carbon budget. Major ocean carbon uptake at mid-latitudes counteracts carbon release in the tropics, which is modulated by episodes of marine heatwaves (MHWs). Yet, we lack essential knowledge on persistent MHWs (PMHWs), and their effect on the carbon sensitive areas. Here, based on a 1985-2017 joint analysis of reconstructions, ocean reanalysis, in situ and satellite data, we show that PMHWs occur in major carbon uptake and release areas. Air-sea CO2 flux changes from PMHWs are strongest in the Pacific Ocean with a 35 +/- 2 % reduction in CO2 release in the tropics linked to ENSO, and a reduction in CO2 uptake of 28 +/- 9 % in the North Pacific. These results provide new insights into the interplay of extreme variability and a critical regulating ocean ecosystem service, and pave the way for future investigations on its evolution under climate change.

Design of numerical experiments assimilating in situ physical and BGC observations to assess and enhance their impact in CMEMS ocean monitoring and forecasting systems.

Design of numerical experiments assimilating in situ physical and BGC observations to assess and enhance their impact in CMEMS ocean monitoring and forecasting systems.

GLORYS12 is a global eddy-resolving physical ocean and sea ice reanalysis at 1/12° horizontal resolution covering the 1993-present altimetry period, designed and implemented in the framework of the Copernicus Marine Environment Monitoring Service (CMEMS). The model component is the NEMO platform driven at the surface by atmospheric conditions from the ECMWF ERA-Interim reanalysis. Ocean observations are assimilated by means of a reduced-order Kalman filter. Along track altimeter sea level anomaly, satellite sea surface temperature and sea ice concentration, as well as in situ temperature and salinity vertical profiles are jointly assimilated. A 3D-VAR scheme provides an additional correction for the slowly-evolving large-scale biases in temperature and salinity. The performance of the reanalysis shows a clear dependency on the time-dependent in situ observation system. The general assessment of GLORYS12 highlights a level of performance at the state-of-the-art and the capacity of the system to capture the main expected climatic interannual variability signals for ocean and sea ice, the general circulation and the inter-basins exchanges. In terms of trends, GLORYS12 shows a higher than observed warming trend together with a slightly lower than observed global mean sea level rise. Comparisons made with an experiment carried out on the same platform without assimilation show the benefit of data assimilation in controlling water mass properties and sea ice cover and their low frequency variability. Moreover, GLORYS12 represents particularly well the small-scale variability of surface dynamics and compares well with independent (non-assimilated) data. Comparisons made with a twin experiment carried out at 1/4° resolution allows characterizing and quantifying the strengthened contribution of the 1/12° resolution onto the downscaled dynamics. GLORYS12 provides a reliable physical ocean state for climate variability and supports applications such as seasonal forecasts. In addition, this reanalysis has strong assets to serve regional applications and provide relevant physical conditions for applications such as marine biogeochemistry. In the near future, GLORYS12 will be maintained to be as close as possible to real time and could therefore provide relevant and continuous reference past ocean states for many operational applications.

Monitoring and predicting global ocean biogeochemistry and marine ecosystems is one of the biggest challenges for the coming decade. In operational systems, biogeochemical (BGC) models are forced – or coupled – with physical ocean models fields that are generally constrained by data assimilation of temperature, salinity and sea level anomalies. Yet, while physical data assimilation substantially improves simulated physical fields, BGC models forced by such analyses are commonly degraded, and more especially in equatorial regions. Here impacts of physical data assimilation on surface chlorophyll and nitrate concentrations are investigated in the tropical Pacific, based on three ocean reanalysis runs using the same physical–BGC model configuration but differing in their level of physical data assimilation. It is shown that, in the Mercator Ocean operational system, the assimilation of satellite altimetry and sea surface temperature in addition to temperature and salinity in situ profiles leads to spurious vertical velocities in the western equatorial Pacific. Our analysis suggests that these unrealistic vertical velocities are explained by the use of an inaccurate mean dynamic topography for the assimilation of altimetry that modifies the pressure-driven horizontal circulation in the upper ocean layer. Moreover, the biases found in this key region modify the subtle dynamical and BGC balances in the whole tropical Pacific and result in unrealistic trends of ocean heat content and nitrate concentration. This study demonstrates that looking into details of the physics is indispensable to improve physical data assimilation systems and to ensure that they make the best use of observations. This is also a key point to refine the strategy of the BGC models forcing and further improve ocean predictions.

Trabajo presentado en la AGU Fall Meeting (2020), celebrada online del 1 al 17 de diciembre de 2020.

This report presents the work plan of the Task 2.3: Observing System Simulation Experiments: impact of multi-platform observations for the validation of satellite observations

Global ocean sampling with autonomous floats going to 4000–6000 m, known as the deep Argo array, constitutes one of the next challenges for tracking climate change. The question here is how such a global deep array will impact ocean reanalyses. Based on the different behavior of four ocean reanalyses, we first identified that large uncertainty exists in current reanalyses in representing local heat and freshwater fluxes in the deep ocean (1 W m−2 and 10 cm yr−1 regionally). Additionally, temperature and salinity comparison with deep Argo observations demonstrates that reanalysis errors in the deep ocean are of the same size as, or even stronger than, the deep ocean signal. An experimental approach, using the 1/4° GLORYS2V4 (Global Ocean Reanalysis and Simulation) system, is then presented to anticipate how the evolution of the global ocean observing system (GOOS), with the advent of deep Argo, would contribute to ocean reanalyses. Based on observing system simulation experiments (OSSE), which consist in extracting observing system datasets from a realistic simulation to be subsequently assimilated in an experimental system, this study suggests that a global deep Argo array of 1200 floats will significantly constrain the deep ocean by reducing temperature and salinity errors by around 50%. Our results also show that such a deep global array will help ocean reanalyses to reduce error in temperature changes below 2000 m, equivalent to global ocean heat fluxes from 0.15 to 0.07 W m−2, and from 0.26 to 0.19 W m−2 for the entire water column. This work exploits the capabilities of operational systems to provide comprehensive information for the evolution of the GOOS.

The Ocean State Report is an annual publication of the Copernicus Marine Service that provides a comprehensive and state-of-the-art report on the current state, natural variations, and ongoing changes in the global ocean and European regional seas.It is meant to act as a reference European Union report for the scientific community, international and national bodies, and the general public.Using satellite data, models and reanalyses, and in situ measurements, the Copernicus Marine Ocean State Report provides a 4-dimensional view (latitude, longitude, depth, and time) of the blue (e.g.hydrography and currents), white (e.g.sea ice) and green (e.g.biogeochemical) ocean.It draws on expert analysis and is written by over 100 scientific experts from more than 30 European institutions.Scientific integrity is assured through a process of independent peer review in collaboration with the Journal of Operational Oceanography.This document is a summary of the fourth issue of the Copernicus Marine Ocean State Report and highlights the current state, natural variations, and ongoing changes in the global ocean.It draws on the Copernicus Marine Ocean Monitoring Indicator (OMI) framework.It approaches the topic from several angles, presenting the state of key ocean variables, examining ongoing changes to the ocean in line with climate change, analysing natural variability and extreme events, and discussing the services that the ocean provides to humanity.Finally, new tools and success stories from the Copernicus Marine Service illustrate how accurate, timely information is key to understanding and adapting to the evolving ocean and seas.Society, a sustainable economy, and the environment -the three pillars of sustainable development-rely on the ocean.This section explores the importance of the ocean in the framework of the UN Sustainable Development Goals, supported by ocean data and information.Page 3. Key indicators are used to track the vital health signs of the global ocean. This section presents the indicators fromCopernicus Marine used to monitor and understand changes already in motion and presents notable changes to the ocean over the last quarter of a century.Page 4.The ocean is undergoing sweeping, severe, and unavoidable changes, with major impacts on marine ecosystems and humanity.The IPCC Special Report on Ocean and Cryosphere, and the Copernicus Marine Service Ocean State Report both show that the global ocean is becoming warmer and more acidic, sea level is rising, and that sea ice is retreating.This section presents the most notable impacts of climate change on the ocean.Page 6.Humans depend heavily upon the ocean through the goods, cultural importance and services provided by marine ecosystems.This section provides an overview and specific examples of ocean ecosystem services and details the key ocean variables that underlie these services.Page 12.The Copernicus Marine Service provides stateof-the-art analyses and ocean forecasts, offering a valuable capability to observe, understand, and anticipate changes and extreme events in the marine environment.This section presents advancements in the Copernicus Marine service, and successful examples of applying these tools in practice.Page 16.

This paper summarizes recent efforts on Observing System Evaluation (OS-Eval) by the Ocean Data Assimilation and Prediction (ODAP) communities such as GODAE OceanView and CLIVAR-GSOP. It provides some examples of existing OS-Eval methodologies, and attempts to discuss the potential and limitation of the existing approaches. Observing System Experiment (OSE) studies illustrate the impacts of the severe decrease in the number of TAO buoys during 2012-2014 and TRITON buoys since 2013 on ODAP system performance. Multi-system evaluation of the impacts of assimilating satellite sea surface salinity data based on OSEs has been performed to demonstrate the need to continue and enhance satellite salinity missions. Impacts of underwater gliders have been assessed using Observing System Simulation Experiments (OSSEs) to provide guidance on the effective coordination of the western North Atlantic observing system elements. OSSEs are also being performed under H2020 AtlantOS project with the goal to enhance and optimize the Atlantic in-situ networks. Potential of future satellite missions of wide-swath altimetry and surface ocean currents monitoring is explored through OSSEs and evaluation of Degrees of Freedomfor Signal (DFS). Forecast Sensitivity Observation Impacts (FSOI) are routinely evaluated for monitoring the ocean observation impacts in the US Navy's ODAP system. Perspectives on the extension of OS-Eval to coastal regions, the deep ocean, polar regions, coupled data assimilation, and biogeochemical applications are also presented. Based on the examples above, we identify the limitations of OS-Eval, indicating that the most significant limitation is reduction of robustness and reliability of the results due to their system-dependency. The difficulty of performing evaluation in near real time is also critical. A strategy to mitigate the limitation and to strengthen the impact of evaluations is discussed. In particular, we emphasize the importance of collaboration within the ODAP community for multi-system evaluation and of communication with ocean observational communities on the design of OS-Eval, required resources, and effective distribution of the results. Finally, we recommend further developing OS-Eval activities at international level with the support of the international ODAP (e.g., OceanPredict and CLIVAR-GSOP) and observational communities.

The Copernicus Marine Environment Monitoring Service (CMEMS) provides regular and systematic reference information on the physical and biogeochemical ocean and sea-ice state for the global ocean and the European regional seas. CMEMS serves a wide range of users (more than 15,000 users are now registered to the service) and applications. Observations are a fundamental pillar of the CMEMS value-added chain that goes from observation to information and users. Observations are used by CMEMS Thematic Assembly Centres (TACs) to derive high-level data products and by CMEMS Monitoring and Forecasting Centres (MFCs) to validate and constrain their global and regional ocean analysis and forecasting systems. This paper presents an overview of CMEMS, its evolution, and how the value of in situ and satellite observations is increased through the generation of high-level products ready to be used by downstream applications and services. The complementary nature of satellite and in situ observations is highlighted. Long-term perspectives for the development of CMEMS are described and implications for the evolution of the in situ and satellite observing systems are outlined. Results from Observing System Evaluations (OSEs) and Observing System Simulation Experiments (OSSEs) illustrate the high dependencies of CMEMS systems on observations. Finally future CMEMS requirements for both satellite and in situ observations are detailed.

The Copernicus Marine Services State of Pacific Ocean analysis of available data demonstrates that the ocean surrounding the Pacific Islands is warmer, has higher heat content, with sea level rising at rates higher