Submesoscale Frontal Dynamics in the Kuroshio Extension Jet

The Kuroshio current flowing up off the eastern side of Japan is well known to be controlled by mesoscale dynamics, but effects of submesoscale dynamics, O (10) km or less, on the mean structure, eddies, frontal processes, stratifications, etc. have not been fully understood. To this end, and towards a rigorous submesoscale-resolving Kuroshio model, we conducted detailed numerical downscaling experiments with a double nested ROMS at horizontal resolutions of 3 km and 1 km, forced by the assimilative JCOPE2 at 1/12 degree as the boundary condition. According to an energy analysis along the principal frontal directions, energy conversion systems observed in the Kuroshio extension jet can be approximately categorized into two groups by their locations. The first one is right on the Kuroshio axis where the meandering and scale transitions occur mainly through baroclinic frontal instability, while submesoscale eddies are suppressed by the horizontal shear near the Kuroshio axis. The second one is at 1 - 2 degrees apart from the axis with emergence of coherent submesoscale eddies; by comparing to the case in the eastern boundary current system (Capet et al., 2008), the energy conversion rates on the both sides of the Pacific look quite similar to each other, suggesting that the effect of shear instability weakens as going away from the axis. As a result, if sufficiently distant from the Kuroshio axis, the dynamics becomes much closer to that of the eastern boundary current, leading to the forward energy cascade down to the submesoscale mainly through frontogenesis and associated baroclinic instability. In a meridional transect along 143 deg. E across the Kuroshio front, these submesoscale eddies are associated with strong ageostrophic secondary circulations and vertical transport of potential vorticity in the upper ocean.