Seminar: Internal gravity wave energy pathways: Many conundrums and some surprising answers

Abstract

Internal gravity waves are a well-known mechanism of energy transport in stratified fluids such as the atmosphere and the ocean. Their abundance and importance for various geophysical processes like ocean mixing and momentum deposition in atmospheric jets are widely accepted. And yet, a number of processes related to the internal gravity wave lifecycle remain only partly understood. While generation mechanisms and individual instability mechanisms as well as wave propagation through varying mean flows are well studied, central questions remain open.

As an example, wave-triadic interactions (in slowly-varying mean flows) have been studied extensively, but there is to-date no complete wave turbulence theory with the corresponding generality. New research reveals, however, that non-rotating wave turbulence including a mean-flow shear, but allowing resonant interactions only, can reproduce solutions predicted by near-resonant interactions with modulated phases. Another example is the pathway of wave energy from propagating waves to turbulence. Often, several mechanisms play a part in a concert of instabilities, ultimately leading to the transition to small-scale turbulence. That directly conflicts the assumption of a single dominant instability mechanism as often made in parameterizations. Furthermore, wave-induced shear of non-breaking internal gravity waves may contribute to the generation of turbulent kinetic energy where it would otherwise not be expected.

And so piecing the puzzle together still yields surprises and usually more new than answered questions.