Scattering of internal tides by barotropic quasigeostrophic flows

TL;DR

This paper develops a kinetic model to describe how barotropic quasigeostrophic flows scatter internal tides, leading to wavefield isotropization, and validates the model with numerical simulations.

Contribution

It introduces a kinetic equation for internal tide scattering by quasigeostrophic flows and analyzes the resulting wavefield isotropization, a novel approach in ocean wave dynamics.

Findings

Scattering restricts energy transfer to waves with same frequency and vertical structure.

Isotropization occurs over characteristic time and length scales dependent on flow parameters.

Numerical simulations confirm the validity of the kinetic model and its predictions.

Abstract

Oceanic internal tides and other inertia-gravity waves propagate in an energetic turbulent flow whose lengthscales are similar to the wavelengths. Advection and refraction by this flow cause the scattering of the waves, redistributing their energy in wavevector space. As a result, initially plane waves radiated from a source such as a topographic ridge become spatially incoherent away from the source. To examine this process, we derive a kinetic equation which describes the statistics of the scattering under the assumptions that the flow is quasigeostrophic, barotropic, and well represented by a stationary homogeneous random field. Energy transfers are quantified by computing a scattering cross section and shown to be restricted to waves with the same frequency and identical vertical structure, hence the same horizontal wavelength. For isotropic flows, scattering leads to an isotropic wavefield. We estimate the characteristic time and length scales of this isotropisation, and study their dependence on parameters including the energy spectrum of the flow. Simulations of internal tides generated by a planar wavemaker carried out for the linearised shallow-water model confirm the pertinence of these scales. A comparison with the numerical solution of the kinetic equation demonstrates the validity of the latter and illustrates how the interplay between wave scattering and transport shapes the wave statistics.