Eddy-Internal Wave Interactions: Stimulated Cascades in Cross-Scale Kinetic Energy and Enstrophy Fluxes

The interactions between oceanic mesoscale eddies, submesoscale currents, and internal gravity waves (IWs) are investigated in submesoscale-resolving realistic simulations in the North Atlantic Ocean. Using a novel analysis framework that couples the coarse-graining method in space with temporal fi ltering and a Helmholtz decomposition, we quantify the effects of the interactions on the cross-scale kinetic energy (KE) and enstrophy fl uxes. By systematically comparing solutions with and without IW forcing, we show that externally forced IWs stimulate a reduction in the KE inverse cascade associated with mesoscale rotational motions and an enhancement in the KE forward cascade associated with divergent submesoscale currents, i.e., a " stimulated cascade " process. The corresponding IW effects on the enstrophy fl uxes are seasonally dependent, with a stimulated reduction (enhancement) in the forward enstrophy cascade during summer (winter). Direct KE and enstrophy transfers from currents to IWs are also found, albeit with weaker magnitudes compared with the stimulated cascades. We further fi nd that the forward KE and enstrophy fl uxes associated with IW motions are almost entirely driven by the scattering of the waves by the rotational eddy fi eld, rather than by wave - wave interactions. This process is investigated in detail in a companion manuscript. Finally, we demonstrate that the stimulated cascades are spatially localized in coherent structures. Speci fi cally, the magnitude and direction of the bidirectional KE fl uxes at submesoscales are highly correlated with, and inversely proportional to, divergence-dominated circulations, and the inverse KE fl uxes at mesoscales are highly correlated with strain-dominated circulations. The predominantly forward enstrophy fl uxes in both seasons are also correlated with strain-dominated fl ow structures.