Evaluating the benefits of SWOT data assimilation and implications for future operational oceanography

Abstract. We present the first evaluation of SWOT data assimilation in a high-resolution ocean forecasting system at the global scale under operational conditions. Observing System Experiments performed with the Mercator Ocean operational data assimilation system are used to assess the gain in model performance associated with the assimilation of SWOT observations into Copernicus Marine Service products. We find that SWOT assimilation improves the global representation of sea level anomaly (SLA), particularly at spatial scales below 100 km. The global gain is of the order of 4 %, which is lower than those reported in previous studies, likely reflecting the strong constraint provided by the numerous nadir altimeters already assimilated in the system. Independent validation using drifters deployed during the SWOT Cal/Val phase shows improvements in both the zonal and meridional current components, highlighting the ability of wide-swath observations to provide a more balanced constraint on surface circulation than conventional nadir altimetry. Assimilation experiments with data from both the high-frequency Cal/Val and spatially extensive Science phases show that broader spatial coverage provides a larger global benefit than higher revisit frequency. Experiments with two model configurations further show that the higher-resolution model, which resolves more physical processes, derives greater benefits from SWOT data assimilation. These results provide guidance for the future development of operational oceanography based on wide-swath altimetry, highlighting the need for broader and more frequent satellite sampling, higher-resolution models with improved physical representation, and data assimilation methods adapted to dense, high-resolution, two-dimensional observations.