Jen-Ping Peng (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain); Elisabet Verger-Miralles (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain); Laura Gómez-Navarro (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain); Máximo García-Jove (Balearic Islands Coastal Observing and Forecasting System, ICTS SOCIB, Spain); Eugenio Cutolo (IMT-Atlantique, France); Bàrbara Barcelo-Llull (Mediterranean Institute for Advanced Studies, IMEDEA (CSIC-UIB), Spain); Daniel R. Tarry (Applied Physics Laboratory, University of Washington, USA); Nikolaos Zarokanellos (Balearic Islands Coastal Observing and Forecasting System, ICTS SOCIB, Spain)

SWOT sea level observations and derived estimates of surface currents and vorticity fields are evaluated based on in-situ observations collected during two sea trial experiments conducted in the Balearic Sea (Western Mediterranean) during the initial fast-sampling phase of the satellite in April-May 2023. Measurements from ship-based instruments (CTD, Moving Vessel Profiler, thermosalinograph, ADCP) and autonomous platforms (surface drifters and gliders) allowed us to characterize the thermohaline and kinematic properties of a ~20-25km-radius intrathermocline anticyclonic eddy detected under the swath of the satellite, approximately 60 km from the coast of Mallorca and Ibiza Islands. In addition, high-resolution numerical simulations were developed to support the cruise planning, help data analysis and provide a complementary view of the small-scale ocean variability. These include a 2-km resolution data-assimilative model run in both predictive and retrospective modes, as well as 650m-resolution nested simulations.

We evaluate both SWOT observations and high-resolution regional simulations using this unique in-situ dataset. On the one hand, SWOT is found to significantly improve the representation of the signature of this small-scale eddy with respect to conventional altimetry, both in terms of observed sea level and derived geostrophic currents. SWOT, together with glider observations, also reveal a significant temporal variability at a daily timescale. On the other hand, the simulations represent a small-scale anticyclonic eddy in the study area showing similarities with the observed eddy in terms of dimension and vertical structure, yet with a 25km spatial offset. The simulations are used to better understand the observed daily variability and evaluate the importance of ageostrophic processes during the study period.

This assessment of SWOT and model uncertainties paves the way for the integration of SWOT observations into the simulations through data assimilation. We provide here initial results about the contribution of SWOT observations to the improvement of the representation of ocean variables in these high-resolution regional simulations after assimilation using an Ensemble Optimal Interpolation approach.