Laboratory study of the interaction between two internal wave rays

Abstract

Laboratory experiments were conducted to study the interaction between two downward propagating internal wave rays with identical properties but opposite horizontal phase velocities. The intersection of the rays produced a velocity field with stagnation points, and these points propagated vertically upwards within the intersection region. Nonlinear non-resonant interactions between the two rays produced evanescent modes, with frequencies greater than the ambient buoyancy frequency, trapped within the intersection region. These evanescent modes provided a mechanism whereby energy could accumulate locally and, even though the vertical wavelength of the primary resultant wave remained the same, the local isopycnal displacements increased in time. Eventually, the isopycnals were forced to overturn in the region just above the stagnation points by the variation with depth in the local horizontal strain rate.

The gravitationally unstable overturning ultimately broke down releasing its available potential energy and generating turbulence within the intersection region. The results showed that the release of available potential energy was disrupted by the wave motions and even the dissipative scales were directly affected by the ambient stratification and the background wave motion. The distribution of the centred displacement scales was highly skewed towards the Kolmogorov scale and the turbulent Reynolds number Re_t was low. Thus, the net buoyancy flux was very small and almost all turbulent kinetic energy was dissipated over the parameter range investigated. The results also showed that for such dissipative events the square of the strain Froude number (ε/νN²₀) and the turbulent Reynolds number Re_t can be less than one.