Scale-dependent budget and vertical pathway of kinetic energy in the Northwest Pacific

Abstract

The oceanic energy budget across mesoscale and submesoscale plays a crucial role in regulating large-scale ocean circulation and air-sea interactions. Although submesoscale baroclinic instability is known to inject energy near the surface and fuel an inverse energy cascade toward mesoscale eddies, the vertical pathway through which this energy is redistributed remains poorly understood. Using a submesoscale-permitting ocean general circulation model (OGCM) and a coarse-graining approach, we examine the spatial structure and scale dependence of the three-dimensional kinetic energy (KE) budget in the Northwest Pacific, where energetic submeso/mesoscale motions exist. At submesoscales (~10 km), the vertical pressure work (PW) drives a substantial downward transport of KE from the mixed layer into the ocean interior, with a magnitude comparable to the baroclinic instability in enhancing submesoscale motions beneath the mixed layer. In the meanwhile, KE is gradually transferred upscale, except near intense surface fronts where forward energy cascade occurs. As the horizontal scale increases to mesoscale (~100 km), a considerable amount of KE converted via baroclinic instability is transferred upscale via inverse energy cascade; while the vertical PW continues to facilitate a downward energy transport toward greater depth, consistent with the barotropization tendency of mesoscale eddies. The results of this study reveal a tight coupling between the cross-scale energy cascade and vertical energy redistribution. Particularly, as submesoscale energy cascades to larger scales, it also penetrates deeper through PW, ultimately feeding interior mesoscale motions while reducing energy dissipation near the surface. This mechanism highlights the previously underappreciated role of PW in the evolution of the three-dimensional structure of submesoscale eddies.