Submesoscale Flows Associated with Convergent Strain in an Anticyclonic Eddy of the Kuroshio Extension: A High-resolution Numerical Study

Based on fine-resolution simulations, we present a case study of the frontogenetic generation of submesoscale flows associated with a convergence field inside an anticyclonic mesoscale eddy in the Kuroshio Extension. The results reveal that the generation of submesoscale flows on the edge of the eddy is closely related to the convergent zone in the ageostrophic secondary circulation (ASC) that develops from frontal sharpening. Diagnostic analysis of the frontal tendency based on flow decomposition shows that frontal sharpening is initiated by mesoscale strain and accelerated by submesoscale convergence. With convergence on the cold side and divergence on the warm side of fronts, submesoscale divergent fields contribute to the asymmetry of the frontal tendency because the fronts tend to be strengthened by convergent motions and weakened by divergent motions. During frontal sharpening, the cross-front ASCs generate additional positive vorticity in their downwardbranches characterized by convergence via vortex stretching, which involves neighboring submesoscale vortices. Moreover, the enhanced lateral buoyancy gradients and surface buoyancy loss are conducive to triggering symmetric instability (SI) and ageostrophic anticyclonic instability (AAI) on the edge of the eddy. In response to these submesoscale instabilities and the subsequent slumping of fronts, the removal of kinetic energy (KE) associated with shear flows is comparable to the source of submesoscale KE from the eddy, which indicates that mesoscale energy can be drained by submesoscale instabilities during frontal sharpening. This study focuses on and describes the frontogenetic generation of submesoscale flows in mesoscale eddies in the Kuroshio Extension.