Shuiming Chen (University of Hawaii, United States)

Fine-scale upper ocean variability in the 15 ~ 150 km range of the Kuroshio Extension region is investigated using the wide-swath sea surface height (SSH) data of the Surface Water and Ocean Topography (SWOT) mission from August 2023 to December 2024. The regional SSH wavenumber spectrum in this fine-scale range follows the k^(-11/3) power law, suggesting the control by the surface quasi-geostrophic dynamics. Distinct asymmetric dependence on polarity of the large-mesoscale vorticity field is detected for the fine-scale eddy kinetic energy (EKE) and strain signals due to the ageostrophic secondary circulation and departure from balanced dynamics. As a result of winter mixed layer instability, the fine-scale EKE shows a seasonal enhancement from January to April and its spatial extent is observed to cover the broad interior ocean beyond the Kuroshio Extension jet. We find the length scale separating the forward and inverse energy cascades at 50 km, smaller than the 150-km scale estimated from the gridded nadir altimetry data. Temporally, the inverse cascade amplitude is found to be controlled by the total EKE level in the region, whereas the forward cascade amplitude depends on the areal abundance of the quantity for departure of balanced dynamics: A-S<0, where A is absolute vorticity and S is strain rate.