Is the resonant wave interaction approximation consistent with the dynamics of internal wave fields?

Abstract: Nonlinear interaction and breaking of internal ocean waves are responsible for much of the interior ocean mixing, affecting ocean carbon storage and the global overturning circulation. These interactions are also believed to dictate the observed Garrett-Munk wave energy spectrum, which is still unexplained after 50 years of studies. According to the resonant wave interaction approximation, used to derive the kinetic equation for the energy spectrum, the dominant interactions are between wave triads whose wavevectors satisfy $\mathbf{k}=\mathbf{p}+\mathbf{q}$, and whose frequencies satisfy $\omega_{\mathbf{k}}=|\omega_{\mathbf{p}}\pm\omega_{\mathbf{q}}|$. In order to test the validity of the resonant wave interaction approximation, we examine several analytical derivations of the theory. The assumptions underlying each derivation are tested using idealized direct 2d numerical simulations, representing near-observed energy levels of the oceanic internal wave field. We show that the assumptions underlying the derivations are not consistent with the simulated dynamics. In addition, most of the triads satisfying the resonant conditions do not contribute significantly to nonlinear wave energy transfer in our simulations, while some interactions that are dominant in nonlinear energy transfers do not satisfy the resonance conditions. We also point to possible self-consistency issues with some derivations found in the literature.