Near - critical reflection of internal waves

Using a matched asymptotic expansion we analyse the two-dimensional, near-critical reflection of a weakly nonlinear internal gravity wave from a sloping boundary in a uniformly stratified fluid. Taking a distinguished limit in which the amplitude of the incident wave, the dissipation, and the departure from criticality are all small, we obtain a reduced description of the dynamics. This simplification shows how either dissipation or transience heals the singularity which is presented by the solution of Phillips (1966) in the precisely critical case. In the inviscid critical case, an explicit solution of the initial value problem shows that the buoyancy perturbation and the alongslope velocity both grow linearly with time, while the scale of the reflected disturbance is reduced as 1/t. During the course of this scale reduction, the stratification is 'overturned' and the Miles–Howard condition for stratified shear flow stability is violated. However, for all slope angles, the 'overturning' occurs before the Miles–Howard stability condition is violated and so we argue that the first instability is convective. Solutions of the simplified dynamics resemble certain experimental visualizations of the reflection process. In particular, the buoyancy field computed from the analytic solution is in good agreement with visualizations reported by Thorpe & Haines (1987). One curious aspect of the weakly nonlinear theory is that the final reduced description is a linear equation (at the solvability order in the expansion all of the apparently resonant nonlinear contributions cancel amongst themselves). However, the reconstructed fields do contain nonlinearly driven second harmonics which are responsible for an important symmetry breaking in which alternate vortices differ in strength and size from their immediate neighbours. 1. Introduction 1.1. Reflection of internal waves on a sloping bottom Understanding the intensity and spatial distribution of turbulent vertical mixing in the ocean is an important problem in physical oceanography. Ocean models require accurate parametrizations of turbulent mixing to make realistic predictions of the transport of heat, salt and chemical species. Because the ocean is stably stratified, vertical mixing is inhibited and convection to great depth occurs only in restricted high-latitude regions. But the fluid which has reached the abyss by convection must ultimately return to the sea surface so as to maintain a quasi-steady state. How and where this return flow occurs remains obscure to this day. Munk (1966) and Armi