Observed spectral kinetic energy fluxes at mesoscale and submesoscale in the St. Lawrence maritime estuary

High-resolution numerical simulations have revealed the existence of an inverse kinetic energy cascade starting at submesoscale (30 km) to larger scales at the ocean surface, and a direct kinetic energy cascade at smaller scales. The inverse cascade has been confirmed with satellite altimetric observations only for scales larger than 200 km, since these observations do not resolve well smaller scales. Here, we use surface currents measured by four high-frequency radars with 2-km horizontal resolution every hour since January 2013 to compute spectral kinetic energy fluxes (wich require the horizontal turbulent current fluctuations) in the St. Lawrence maritime estuary at scales ranging from 4 to 40 km. Geostrophic/ageostrophic decomposition is used to determine the separation scale between submesoscale and mesoscale and to quantify the contribution of ageostrophic currents to the spectral kinetic energy fluxes. To determine the processes contributing to the spectral kinetic energy fluxes, observed currents are decomposed into subinertial, near-inertial and super-inertial frequency bands, and spectral kinetic energy fluxes are computed for different combinations of these frequency bands. The spectral fluxes are then seasonally-averaged to highlight the seasonal cycle.