Multiscale energetics and submesoscale instabilities of eddy shedding at the Kuroshio loop current in the South China Sea.

The multiscale energetics and submesoscale instabilities after the eddy shedding of Kuroshio Loop Current (KLC) intrusion into the South China Sea (SCS) remain ambiguous. Here, a typical KLC eddy shedding process is well simulated using a downscaled submesoscale-permitting model. Then, energy and dynamics diagnostics are employed to investigate the cross-scale interactions between mesoscales and submesoscales during and after this process. In energetics, although the forward and inverse energy cascades coexist, the forward cascade of available potential energy (APE) is crucial in energizing submesoscales, while the strength of forward kinetic energy (KE) is relatively weak. The submesoscale KE is primarily charged by strong buoyancy conversion and secondarily by horizontal advection from upstream, which is mainly balanced by turbulence dissipation and vertical pressure work. In dynamics, except for the release of submesoscale APE by baroclinic instability, symmetric instability (SI) can extract KE from geostrophic flows and drive forward KE cascades. Specifically, strain-induced advective frontogenesis can rapidly sharpen submesoscales by enhancing lateral buoyancy gradients, the increased baroclinicity together with atmospheric-forced buoyancy loss causes negative total Ertel potential vorticity and creates favorable conditions for SI. These results highlight the significance of submesoscales in multiscale energetics and dynamical instabilities of the KLC eddy shedding.