Internal wave generation in evanescent regions with variable stratification in experiments, simulations, and linear theory

Abstract

An investigation into the influence of topographical shape and stratification profile on the kinetic energy of propagating internal waves generated by tidal flow in evanescent regions is accomplished using four different methods. Experiments, analytical modeling, and numerical modeling with two different analysis methods are each used to explore resulting propagating internal waves after an evanescent region. Due to varying stratification, just above the evanescent generation region, the waves are propagating and contribute to the internal wave energy available throughout the oceans. Each analysis method captures different dynamics best, and those dynamics are defined here, but general trends are found to be the same. As the relative length of the evanescent region above the topography increases or the average relative buoyancy frequency in this region decreases, the internal wave energy in the propagating region decreases due to enhanced decay distance or rate before reaching the propagating region. It is also found that the average stratification in each of the evanescent and propagating regions may be used instead of the entire profile to estimate propagating wave dynamics—a relevant simplification especially to increase computational speed. Finally, an equation to approximate propagating wave energy from an evanescent region as a function of stratification and topographic parameters is given, based on results from all four methodologies.