SWOT Geostrophic Velocity Validation Against In Situ Measurements in the California Current

Abstract The Surface Water and Ocean Topography (SWOT) mission provides high‐resolution sea surface height (SSH) observations from the Ka‐band Radar Interferometer (KaRIn), with the potential to improve our understanding of ocean circulation dynamics. A key objective of SWOT is to convert SSH measurements into ocean currents, a process that requires rigorous validation beyond direct SSH assessment. In this study, we evaluated SWOT‐derived geostrophic velocities in the California Current System (CCS) at 125.044W, 35.917N by comparing them with in situ velocity measurements from a moored current meter array during the post‐launch Calibration and Validation (CalVal) phase (April 1–4 July 2023). During this period, SWOT operated in a special 1‐day repeat orbit, providing daily sampling that is not representative of the nominal 21‐day science orbit. To derive reliable velocities, SSH fields were low‐pass filtered at 70 km to suppress noise and unbalanced motions. Compared to the in situ velocities, SWOT explained up to 80% (43%) of the meridional (zonal) variance, outperforming DUACS, which explained 63% (15%). SWOT also better captured variability at periods longer than 6–10 days and resolved sharper mesoscale features than the DUACS gridded product, which resolves signals with time scales greater than 25–30 days. When the comparison was repeated using only one SWOT snapshot every 21 days, mimicking the science‐phase orbit, SWOT still retained meaningful skill in representing the velocity field. These results demonstrate SWOT's ability to improve mesoscale and large‐scale velocity estimation, while also highlighting the challenges in regions with strong internal gravity waves.