Recent studies show that the vigorous seasonal cycle of the mixed layer modulates upper ocean submesoscale turbulence. Here we provide model‐based evidence that the seasonally changing upper ocean stratification in the Kuroshio Extension also modulates submesoscale (here 10–100 km) inertia‐gravity waves. Summertime restratification weakens submesoscale turbulence but enhances inertia‐gravity waves near the surface. Thus, submesoscale turbulence and inertia‐gravity waves undergo vigorous out‐of‐phase seasonal cycles. These results imply a strong seasonal modulation of the accuracy of geostrophic velocity diagnosed from submesoscale sea surface height delivered by the Surface Water and Ocean Topography satellite mission. Key Points Upper ocean submesoscale (here 10‐100 km) turbulence and inertia‐gravity waves undergo strong seasonal cycles that are out of phase Submesoscale turbulence dominates the horizontal velocity and sea surface height variability in late winter/early spring Submesoscale inertia‐gravity waves dominate the horizontal velocity and sea surface height variability in late summer/early fall