The effect of tides and high-frequency winds on the oceanic cross-scale kinetic energy flux

The flux of kinetic energy between oceanic currents of different horizontal scales is of key importance for the oceans energy balance between wind-forcing on mainly large scales and dissipation on small scales. Oceanic eddies in quasi-geostrophic balance, including submesoscale mixed-layer eddies, are associated with an inverse cascade towards larger scales. In contrast, in regional simulations, internal gravity waves have been shown to reduce the inverse cascade by quasi-geostrophic eddies and to drive a strong forward cascade towards the small dissipative scales. The major forcing mechanisms of internal gravity waves are tides and high-frequency winds. In this study, we investigate the effect of both forcings on the cross-scale kinetic energy flux by comparing the latter in parallel submesoscale-permitting simulations of the full Atlantic i) with both forcings, ii) with only high-frequency wind forcing, and iii) without both forcings. We show that both internal gravity wave forcings contribute to an increase in the forward cascade, which is most pronounced in summer-time when balanced flows are weak. Both forcing effects are present at all investigated scales, but the tidal effect dominates at smaller scale (about 30 km), while at larger scales (about 100 km) the wind-effect dominates. By comparing fluxes from three-day- and hourly-mean velocities, we show that the forward cascade associated with both forcings is a result of high-frequency motions at time-scales less than three days. In spring, the high-frequency forward cascade is overcome by the inverse-cascade of mixed-layer eddies. The comparison of the (mainly inverse) fluxes from the three-day-mean flow between the parallel experiments show that the effect of tides on the low-frequency cascade is very small while the high-frequency winds are responsible for the reduction of the low-frequency inverse cascade. Finally, we show that geostrophic coarse-graining cross-scale kinetic energy fluxes can be computed from SWOT satellite observations despite the gap between the measurement swaths, by applying a SWOT simulator to the ocean model solutions. With this, it is possible to validate the submesoscale geostrophic cross-scale kinetic energy flux in ocean simulations.