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Submesoscale processes in the upper ocean vary seasonally, in tight correspondence with mixed layer thickness variability. Based on a global high-resolution MITgcm simulation, seasonal evaluation of strong vorticity and spectral analysis of the kinetic energy in the Kuroshio Extension System show the strongest submesoscales occur in March, implying a lag of about a month behind mixed layer thickness maximum in February. An analysis of spectral energy sources and transfers indicates that the seasonality of the submesoscale energy content is a result of the competition between the conversion of available potential energy into submesoscale kinetic energy via a buoyancy production/vertical buoyancy flux associated with mixed layer instability and nonlinear energy transfers to other scales associated with an energy cascade. The buoyancy production is seasonally in phase with the mixed layer depth, but the transfers of energy across scales makes energizing the reservoir of submesoscale kinetic energy lag behind by a month.

Plain Language Summary Submesoscale processes have spatial and temporal scales of O(1-10) km and O(1) day, linking the meso- and micro-scales. Previous works reveal that mixed layer instability, which has extraction rates scaled with mixed layer depth, releases potential energy for the generation of submesoscale kinetic energy in the upper ocean. However, the seasonality of submesoscale kinetic energy in the Kuroshio Extension System region is determined not only by the potential energy releasing due to the mixed layer instability, but also by the energy transfers between different scales.