Sub‐100 km Ocean Processes Revealed by Structure Functions of SWOT Sea Surface Height and In Situ Observing Network

Abstract The Surface Water and Ocean Topography (SWOT) mission, equipped with a Ka‐band Radar Interferometer (KaRIn), provides unprecedented sea surface height anomaly (SSHA) observations at kilometer‐scale resolution over a wide swath. Although regional studies have showcased SWOT's capabilities, its SSHA wavenumber spectra at wavelengths below 70 km exhibit shallower slopes (, where is the along‐track wavenumber) than predicted by geostrophic turbulence theory. We analyzed SWOT SSHA data alongside in situ measurements collected by the mission oceanographic campaign from the California Current System during the April–July 2023 calibration and validation (Cal/Val) period. We analyzed steric height from hourly CTD measurements on 11 moorings and 2 gliders and SWOT SSHA using structure functions, revealing that the shallow SWOT SSHA spectra at sub‐70 km scales primarily result from KaRIn instrument noise, with a notable cross‐track dependence (shallowest at the swath edges). Additionally, high‐frequency internal gravity waves also contribute to the shallow spectral slope. Because of limitations in in situ and SWOT observations, we could not quantitatively partition each individual process's contribution. Nevertheless, our results revealed, for the first time, the impact of instrument noise and high‐frequency internal waves on the SSHA spectrum at sub‐70 km that was previously unknown from conventional nadir altimeters, highlighting the complexity of small‐scale SSHA signals and the need for further research.