# Instabilities of Internal Gravity Wave Beams

## TL;DR

This paper explores the instability mechanisms of internal gravity wave beams, which are prevalent in geophysical fluids, highlighting their nonlinear nature and recent experimental insights into their stability.

## Contribution

It analyzes the two main instability mechanisms of internal gravity wave beams, expanding understanding beyond traditional plane wave solutions in stratified fluids.

## Key findings

- Identification of triadic resonant instability as a key destabilization process
- Observation of streaming instability leading to mean flow generation
- Relevance of wave beam instabilities in geophysical fluid dynamics

## Abstract

Internal gravity waves play a primary role in geophysical fluids: they contribute significantly to mixing in the ocean and they redistribute energy and momentum in the middle atmosphere. Until recently, most studies were focused on plane wave solutions. However, these solutions are not a satisfactory description of most geophysical manifestations of internal gravity waves, and it is now recognized that internal wave beams with a confined profile are ubiquitous in the geophysical context. We will discuss the reason for the ubiquity of wave beams in stratified fluids, related to the fact that they are solutions of the nonlinear governing equations. We will focus more specifically on situations with a constant buoyancy frequency. Moreover, in light of recent experimental and analytical studies of internal gravity beams, it is timely to discuss the two main mechanisms of instability for those beams. i) The Triadic Resonant Instability generating two secondary wave beams. ii) The streaming instability corresponding to the spontaneous generation of a mean flow.