Seasonal Shift in the Dominant Pathway Energizing Mesoscale Eddies in the California Current

Mesoscale eddies are the dominant reservoir of kinetic energy in the ocean, yet the mechanisms that generate and maintain them in eastern boundary current systems remain incompletely assessed. Here we use a 1-km resolution simulation of the California Current System (CCS) to diagnose and quantify the processes that supply kinetic energy to the mesoscale band. A pronounced seasonal transition is evident in surface vorticity, kinetic energy spectra, and mixed-layer depth. In winter and spring, the mixed layer deepens during periods of strong wind work. Once the forcing relaxes, the mixed layer restratifies, generating submesoscale eddies and filaments. These features merge and transfer energy upscale, supplying the mesoscale through an inverse cascade. These mixed-layer restratification events occur most frequently when the mixed layer is deep (~50m) and highly variable. In contrast, during summer when the mixed layer is shallow, a mixed-layer-averaged geostrophic kinetic energy budget indicates that wind work accelerates the surface layer of the ocean more efficiently. This is associated with enhanced kinetic energy transfer from scales larger than 100km into the mesoscale. Together, these results indicate a seasonally varying balance between inverse and direct kinetic energy transfers in the CCS, implying that the annual cycle of upper-ocean stratification leads to a seasonal shift in the dominant pathway of mesoscale eddy generation.