Abstract We show that the submesoscale inverse cascade almost entirely drives the mesoscale kinetic energy (KE) seasonal cycle in the interior subtropical gyre. Using a coarse‐graining framework to diagnose cross‐scale energy fluxes (Πh ${ {Pi } }{h}$), we show the forward cascade remains confined to <20 km within the mixed layer, peaking January–March. The inverse cascade exhibits a dramatic upscale shift: its peak scale expands from ∼30 km in January to ∼200 km by June while penetrating vertically below the mixed layer by March. The observed cascade timescale (∼180 days) far exceeds predictions from classical turbulence theory (∼40 days), revealing fundamental departures from idealized quasi‐geostrophic dynamics. This horizontal and vertical expansion establishes the pathway whereby submesoscale eddies energize mesoscale motions. Lead‐lag analysis reveals potential energy conversion precedes frontogenesis by 7–21 days, submesoscale eddies by 9–23 days, and peak Πh ${ {Pi } }{h}$ by 20–90 days, which in turn leads large‐scale KE by 30–70 days.