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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 timedependent 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 ad

The future Surface Water Ocean Topography (SWOT) mission due to be launched in 2022 will extend the capability of existing nadir altimeters to enable two-dimensional mapping at a much higher effective resolution. A significant challenge will be to assimilate this kind of data in high-resolution models. In this context, Observing System Simulation Experiments (OSSEs) have been performed to assess the impact of SWOT on the Mercator Ocean and Copernicus Marine Environment Monitoring Service (CMEMS) global, high-resolution analysis and forecasting system. This paper focusses on the design of these OSSEs, in terms of simulated observations and assimilation systems (ocean model and data assimilation schemes). The main results are discussed in a companion paper. Two main updates of the current Mercator Ocean data assimilation scheme have been made to improve the assimilation of information from SWOT data. The first one is related to a different parametrisation of the model error covariance, and the second to the use of a four-dimensional (4D) version of the data assimilation scheme. These improvements are described in detail and their contribution is quantified. The Nature Run (NR) used to represent the “truth ocean” is validated by comparing it with altimeter observations, and is then used to simulate pseudo-observations required for the OSSEs. Finally, the design of the OSSEs is evaluated by ensuring that the differences between the assimilation system and the NR are statistically consistent with the misfits between real ocean observations and real-time operational systems.

A first attempt was made to quantify the impact of the assimilation of Surface Water Ocean Topography (SWOT) swath altimeter data in a global 1/12° high resolution analysis and forecasting system through a series of Observing System Simulation Experiments (OSSEs). The OSSE framework (Nature Run and Free Run) and data assimilation scheme have been described in detail in a companion article (Benkiran et al., 2021). The impact of assimilating data from SWOT and three nadir altimeters was quantified by estimating analysis and forecast error variances for sea surface height (SSH), temperature, salinity, zonal, and meridional velocities. Wave-number spectra and coherence analyses of SSH errors were also computed. SWOT data will significantly improve the quality of ocean analyses and forecasts. Adding SWOT observations to those of three nadir altimeters globally reduces the variance of SSH and surface velocities in analyses and forecasts by about 30 and 20%, respectively. Improvements are greater in high-latitude regions where space/time coverage of SWOT is much denser. The combination of SWOT data with data from three nadir altimeters provides a better resolution of wavelengths between 50 and 200 km with a more than 40% improvement outside tropical regions with respect to data from three nadir altimeters alone. The study has also highlighted that the impact of using SWOT data is likely to be very different depending on geographical areas. Constraining smaller spatial scales (wavelengths below 100 km) remains challenging as they are also associated with small time scales. Although this is only a first step, the study has demonstrated that SWOT data could be readily assimilated in a global high-resolution analysis and forecasting system with a positive impact at all latitudes and outstanding performances.

The Atlantic Water (AW) inflow through Fram Strait, largest oceanic heat source to the Arctic Ocean, undergoes substantial modifications in the Western Nansen Basin (WNB). Evaluation of the Mercator system in the WNB, using 1,500 independent temperature‐salinity profiles and five years of mooring data, highlighted its performance in representing realistic AW inflow and hydrographic properties. In particular, favorable comparisons with mooring time‐series documenting deep winter mixed layers and changes in AW properties led us to examine winter conditions in the WNB over the 2007–2020 period. The model helped describe the interannual variations of winter mixed layers and documented several processes at stake in modifying AW beyond winter convection: trough outflows and lateral exchange through vigorous eddies. Recently modified AW, either via local convection or trough outflows, were identified as homogeneous layers of low buoyancy frequency. Over the 2007–2020 period, two winters stood out with extreme deep mixed layers in areas that used to be ice‐covered: 2017/18 over the northern Yermak Plateau‐Sofia Deep; 2012/13 on the continental slope northeast of Svalbard with the coldest and freshest modified AW of the 12‐year time series. The northern Yermak Plateau‐Sofia Deep and continental slope areas became “Marginal Convection Zones” in 2011 with, from then on, occasionally ice‐free conditions, 50‐m‐ocean temperatures always above 0 °C and highly variable mixed layer depths and ocean‐to‐atmosphere heat fluxes. In the WNB where observations require considerable efforts and resources, the Mercator system proved to be a good tool to assess Atlantic Water modifications in winter.

Atlantic Water (AW) enters the Arctic through Fram Strait as the West Spitsbergen Current (WSC). When reaching the south of Yermak Plateau, the WSC splits into the Svalbard, Yermak Pass and Yermak Branches. Downstream of Yermak Plateau, AW pathways remain unclear and uncertainties persist on how AW branches eventually merge and contribute to the boundary current along the continental slope. We took advantage of the good performance of the 1/12° Mercator Ocean model in the Western Nansen Basin (WNB) to examine the AW circulation and volume transports in the area. The model showed that the circulation changed in 2008-2020. The Yermak Branch strengthened over the northern Yermak Plateau, feeding the Return Yermak Branch along the eastern flank of the Plateau. West of Yermak Plateau, the Transpolar Drift likely shifted westward while AW recirculations progressed further north. Downstream of the Yermak Plateau, an offshore current developed above the 3800 m isobath, fed by waters from the Yermak Plateau tip. East of 18°E, enhanced mesoscale activity from the boundary current injected additional AW basin-ward, further contributing to the offshore circulation. A recurrent anticyclonic circulation in Sofia Deep developed, which also occasionally fed the western part of the offshore flow. The intensification of the circulation coincided with an overall warming in the upper WNB (0-1000 m), consistent with the progression of AW. This regional description of the changing circulation provides a background for the interpretation of upcoming observations.

Ocean reanalyses combine ocean models, atmospheric forcing fluxes, and observations using data assimilation to give a four-dimensional description of the ocean. Metrics assessing their reliability have improved over time, allowing reanalyses to become an important tool in climate services that provide a more complete picture of the changing ocean to end users. Besides climate monitoring and research, ocean reanalyses are used to initialize sub-seasonal to multi-annual predictions, to support observational network monitoring, and to evaluate climate model simulations. These applications demand robust uncertainty estimates and fit-for-purpose assessments, achievable through sustained advances in data assimilation and coordinated inter-comparison activities. Ocean reanalyses face specific challenges: (i) dealing with intermittent or discontinued observing networks, (ii) reproducing inter-annual variability and trends of integrated diagnostics for climate monitoring, (iii) accounting for drift and bias due, e.g., to air-sea flux or ocean mixing errors, and (iv) optimizing initialization and improving performances during periods and in regions with sparse data. Other challenges such as multi-scale data assimilation to reconcile mesoscale and large-scale variability and flow-dependent error characterization for rapidly evolving processes, are amplified in long-term reanalyses. The demand to extend reanalyses backward in time requires tackling all these challenges, especially in the emerging context of earth system reanalyses and coupled data assimilation. This mini-review aims at documenting recent advances from the ocean reanalysis community, discussing unsolved challenges that require sustained activities for maximizing the utility of ocean observations, supporting data rescue and advancing specific research and development requirements for reanalyses.