Fine-Scale Upper-Ocean Variability in the Kuroshio Extension Region from the Wide-Swath SWOT Measurements

Abstract Fine-scale upper-ocean variability in the 15–150-km wavelength range within the Kuroshio Extension region is investigated using wide-swath sea surface height (SSH) data from the Surface Water and Ocean Topography (SWOT) mission spanning from August 2023 to December 2024. The regional SSH wavenumber spectrum in this fine-scale range follows largely the k −11/3 power law, indicative of control by surface quasigeostrophic dynamics. The fine-scale eddy kinetic energy (EKE) and strain signals exhibit a pronounced asymmetry with respect to the polarity of the large-mesoscale vorticity field, attributable to ageostrophic secondary circulations and the departure from balanced dynamics. Seasonally, the fine-scale EKE is enhanced from January to April, coinciding with winter mixed layer instability, and its spatial influence extends beyond the Kuroshio Extension jet into the broad interior ocean. We identify a transition scale between forward and inverse energy cascades at approximately 50 km—significantly smaller than the ∼150-km scale estimated from gridded nadir altimetry data. Temporally, the amplitude of the inverse cascade is modulated by the total regional EKE level, whereas the amplitude of the forward cascade is linked to the intensity of regions where A − S < 0, with A denoting absolute vorticity and S denoting the strain rate—a proxy for the breakdown of balanced dynamics.