Near-inertial waves and geostrophic turbulence

Wind-forced near-inertial waves form a high-energy wave component in the upper ocean. The weakly dispersive nature of these large horizontal and small vertical scale waves make them suitable candidates for energetic interactions with mesoscale balanced flows. We take advantage of an idealized two-vertical-mode system obtained by projecting the hydrostatic Boussinesq equations onto the barotropic and a single high baroclinic mode to examine wave-balanced flow interactions. Our detailed analysis using results of freely evolving numerical simulations demonstrate how the well established two-mode quasigeostrophic turbulence phenomenology changes in the presence of high-energy near-inertial waves. In the absence of waves, the barotropic flow, which contains most of the balanced energy, undergoes an inverse energy cascade resulting in the formation of large-scale coherent vortices. In contrast, high-energy near-inertial waves transfer energy to the barotropic flow, facilitating a forward energy cascade of the balanced flow. The balanced flow in turn assists in the forward energy cascade of the wave field, which transforms the wave field from low-frequency near-inertial waves to high-frequency inertia-gravity waves. Given that the idealized model we employ is two-dimensional, the forward energy cascade of wave and balanced flow is an unexpected and intriguing feature.