Spontaneous superharmonic internal wave generation by modal interactions in uniform and nonuniform stratifications.

Triadic resonance is one mechanism via which internal waves dissipate their energy, often at locations away from their generation sites. In this paper, we perform a combined theoretical and numerical study of triadic resonance in internal wave modes in a finite-depth ocean with background rotation and an arbitrary stratification profile. The spatial evolution of the modal amplitudes within a resonant triad are first derived based on the requirement that the nonlinear solution at leading order cannot diverge. The amplitude evolution equations are then numerically solved for two different cases: (i) modes 1 and 2 at a specific frequency ({\omega_0}) in triadic resonance with the mode-1 superharmonic wave (2{\omega_0}) in a uniform stratification, and (ii) a self-interacting mode-3 at a specific frequency {\omega_0} in triadic resonance with the mode-2 (2{\omega_0}) in a nonuniform stratification representative of the ocean. Quantitative estimates of energy transfer rates within the resonant triad show that superharmonic wave generation resulting from modal interactions should be an important consideration alongside other triadic resonances like parametric subharmonic instability (PSI). Remarkably, in case (ii), the amplitude evolution equations suggest that any initial energy in mode-3 at {\omega_0} would get permanently transferred to mode-2 at frequency 2{\omega_0}. Direct numerical simulations (DNS) are then performed to show the spontaneous generation of superharmonic internal waves resulting from modal interactions in the aforementioned two cases, and quantitatively validate the initial spatial evolution of the wave field predicted by the amplitude evolution equations. DNS at off-resonant frequencies are used to identify the range of primary wave frequencies (around the resonant frequency) over which spontaneous superharmonic wave generation occurs.