Weakly nonlinear non-Boussinesq internal gravity wavepackets

Internal gravity wavepackets induce a horizontal mean flow that interacts nonlinearly with the waves if they are of moderately large amplitude. In this work, a new theoretical derivation for the wave-induced mean flow of internal gravity waves is presented. Using this we examine the weakly nonlinear evolution of internal wavepackets in two dimensions. By restricting the two-dimensional waves to be horizontally periodic and vertically localized, we derive the nonlinear Schrِdinger equation describing the vertical and temporal evolution of the amplitude envelope of non-Boussinesq waves. The results are compared with fully nonlinear numerical simulations restricted to two dimensions. The initially small-amplitude wavepacket grows to become weakly nonlinear as it propagates upward due to non-Boussinesq effects. In comparison with the results of fully nonlinear numerical simulations, the nonlinear Schrِdinger equation is found to capture the dominant initial behaviour of the waves, indicating that the interaction of the waves with the induced horizontal mean flow is the dominant mechanism for weakly nonlinear evolution. In particular, due to modulational stability, hydrostatic waves propagate well above the level at which linear theory predicts they should overturn, whereas strongly non-hydrostatic waves, which are modulationally unstable, break below the overturning level predicted by linear theory.