Spatiotemporal evolution of submesoscale filaments at the periphery of an anticyclonic mesoscale eddy north of the Kuroshio Extension

Five high-resolution hydrographic sections were conducted at the periphery of an anticyclonic mesoscale eddy north of the Kuroshio Extension (KE) to explore the evolution of submesoscale structure in the subsurface layer associated with mesoscale eddies. The five sections continuously captured patches of low temperature and salinity water originated from the subarctic region with a horizontal scale of 10–20 km and a vertical scale of 100 m in the strain field at the periphery and in the interior of the eddy detected mainly by the surface Okubo-Weiss parameter. Each patch on the same isopycnal constituted a submesoscale filament. A cold and fresh filament in the intermediate layer in and around the eddy was recently ventilated compared with ambient water, suggesting that submesoscale filaments contribute to a rapid water mass transport from the outside to the inside of the eddy, as well as from the subarctic to the KE region. Filaments at the periphery of the eddy were forced by the convergence in the cross-frontal direction. The horizontal tracer gradient of the filaments evolved in the downstream direction, and 10–50% of the evolution was explained by geostrophic forcing. Moreover, the analysis implied that the evolution of the vertical tracer gradient might have been caused by the vertical shear of the horizontal velocity. The resulting patches were expected to contribute to an effective mixing, suggesting the influence of submesoscale filaments on not only the water mass transport but also the transformation in and around mesoscale eddies in the KE region.