Vertical transformation of mesoscale eddies crossing the Kuroshio Extension: Evidence from cyclonic and anticyclonic rings

Abstract

The Kuroshio Extension is one of the most energetic frontal systems in the World Ocean, separating subtropical and subarctic water masses. Mesoscale eddies regularly cross this front, enabling cross-jet transport of heat, salt, and hydrographic properties despite the barrier nature of the jet. Previous studies have mainly focused on the horizontal component of this transport, while the vertical evolution of eddies remains poorly understood. Here, we present a quantitative investigation of mesoscale eddy transformation during Kuroshio Extension crossing using two representative events: a cyclonic ring (2002) and an anticyclonic ring (2024). Three-dimensional eddy structure was examined before, during, and after frontal crossing. Results show that crossing a density front causes not only horizontal transport but also substantial vertical adjustment of eddy cores driven by buoyancy changes relative to surrounding waters. The cyclonic ring, transporting dense subarctic waters into the subtropical region, exhibits a 320 m deepening of the σθ = 26.50 kg m−3 isopycnal, a 400 m deepening of the 5 °C isotherm, and a 200–500 m downward displacement of S = 34.0 Practical Salinity Unit isohaline boundaries. In contrast, the anticyclonic ring undergoes a maximum transient uplift of 150 m of the σθ = 26.50 kg m−3 isopycnal during frontal crossing, followed by partial relaxation to a net uplift of 100 m. Similar behavior is observed for the upper saline-core boundary and the 5 °C isotherm. Simultaneously, the warm saline lens transforms into a dome-shaped structure. This restructuring involves changes in eddy geometry and the distributions of temperature, salinity, and velocity. We propose a buoyancy-controlled mechanism of vertical eddy adjustment after frontal crossing, applicable to mesoscale eddies interacting with major density fronts throughout the World Ocean.