Barbara Arribas-Rodríguez (Graduate student, Spain); Begoña Tejedor (Associate Professor, Spain); Oscar Alvarez (Associate Professor, Spain); Carlos Jose Gonzalez (Assistant Professor, Spain); Stefano Vignudelli (Research staff, Italy)

In this work, we present the results of the accuracy assessment of time series of SWOT KaRIn Sea Surface Height Anomaly (SSHA) along the Spanish coasts. We used three tide gauges from Puertos del Estado (https://www.puertos.es/servicios/oceanografia) located at Huelva (eastern Gulf of Cadiz), Barcelona (northwestern Mediterranean Sea) and Bilbao (eastern North Atlantic Ocean). We assessed 1.5 years (from July 2023 to February 2025) of Level 2 KaRIn Low Rate (LR) SSHA data product available at: https://hitide.podaac.earthdatacloud.nasa.gov/. SWOT data in LR give regular grids of 2 km x 2 km resolution; they were selected considering a radius of 100 km respect to the location of the tide gauges. We analysed one ascending and one descending orbit at each location. We estimated the standard deviation of the differences (SDD) between time series of KaRIn-dervied SSHAs and tide gauge measurements. The possible use of the ECMWF wet tropospheric correction (EMCWF_wet tropo) instead of the correction derived from the advanced microwave radiometer (AMR_wet_tropo) measurements was analysed. EMCWF_wet tropo showed the same level accuracy as AMR_wet_tropo, but the number of valid grids with ECMWF_wet_tropo was higher than AMR_wet_tropo. We also compared the accuracy of SSHAs by applying the tidal model (FES) with respect to the GOT model. We found that the SDD with FES is slightly better than with GOT. Once the best set of corrections were selected to compute the SSHA from SWOT, we estimated the average SDD at the three study areas. Overall, the SDD ranges between 7.3 cm (Bilbao) and 10.7 cm (Barcelona). We compared the SWOT accuracy with the SSHA from Sentinel-3A (S3A). We selected the closest S3A tracks to the tide gauge locations and estimated the SDD along a track segment of 100-km long. The SDD from SWOT was interpolated to the S3A along-track positions. The mean SDD of S3A / SWOT (two tracks) was: 7.1 (#322) / 8.1 (#154) - 9.1 (#447) cm (Huelva); 9.6 (#356) / 10.2 (#320) - 10.5 (#363) cm (Barcelona); and 6.8 (#051) / 7.8 (#141) - 8.6 (#376) cm (Bilbao). We conclude that S3A shows a better performance than SWOT at all the locations and tracks (considering a radius of 100 km respect to the tide gauge location). More efforts are still needed in the processing of SWOT data to achieve the same accuracy as S3A, but the preliminary results obtained from SWOT are promising. The high SDD observed at Barcelona will be discussed in terms of the hydrodynamic conditions in that area.