Nonlinear internal wave penetration via parametric subharmonic instability

Abstract

6 pages, 5 figuresInternational audienceWe present the results of a laboratory experimental study of an internal wave field generated by harmonic, spatially-periodic boundary forcing from above of a density stratification comprising a strongly-stratified, thin upper layer sitting atop a weakly-stratified, deep lower layer. In linear regimes, the energy flux associated with relatively high frequency internal waves excited in the upper layer is prevented from entering the lower layer by virtue of evanescent decay of the wave field. In the experiments, however, we find that the development of parametric subharmonic instability (PSI) in the upper layer transfers energy from the forced primary wave into a pair of subharmonic daughter waves, each capable of penetrating the weakly-stratified lower layer. We find that around $10\%$ of the primary wave energy flux penetrates into the lower layer via this nonlinear wave-wave interaction for the regime we study

Mixing and formation of layers by internal wave forcing

“…7. On the other hand, the smaller frequency secondary waves could still be transporting some of the energy through region B [Ghaemsaidi et al, 2016]. Once the evanescent region is formed, the amount of mixing present in the central region is reduced, but does not vanish completely due to secondary waves.…”

Mixing and Formation of Layers by Internal Wave Forcing

“…Hence, the large‐scale mode 1 forcing is limited to the two narrowing regions A and C, while it is evanescent in the increasing central region B, as shown in Figure . On the other hand, the smaller frequency secondary waves could still be transporting some of the energy through region B (Ghaemsaidi et al, ). Once the evanescent region is formed, the amount of mixing present in the central region is reduced, but does not vanish completely due to secondary waves.…”