African Easterly Waves in an Idealized General Circulation Model: Instability and Wavepacket Diagnostics

TL;DR

This study uses an idealized model to analyze the dynamics of African easterly waves, showing they often remain within the jet and can grow spontaneously, influenced by external energy sources.

Contribution

It demonstrates that AEWs can emerge spontaneously within a localized jet and explores the conditions for their stability and growth using wavepacket diagnostics.

Findings

AEWs often stay within the African easterly jet

Spontaneous growth of AEWs occurs even with damping

External energy sources likely influence AEW intermittency

Abstract

We examine the group dynamic of African easterly waves (AEW) generated in a realistic, spatially non-homogeneous African easterly jet (AEJ) using an idealized general circulation model. Our objective is to investigate whether the limited zonal extent of the AEJ is an impediment to AEW development. We construct a series of basic states using global reanalysis fields and initialize waves via transient heating over West Africa. The dominant response is a localized wavepacket that disperses upstream and downstream. The inclusion of a crude representation of boundary layer damping stabilizes the waves in most cases. In some basic states, however, exponential growth occurs even in the presence of damping. This shows that AEWs can occasionally emerge spontaneously. The key result is that the wavepacket in almost all cases remains within the AEJ instead of being swept away. Drawing from other studies, this also suggests that even the damped waves can grow if coupled with additional sources of energy such as moist convection and dust radiative feedback. The wavepacket in the localized AEJ appears to satisfy a condition for absolute instability, a form of spatial hydrodynamic instability. However, this needs to be verified more rigorously. Our results also suggest that the intermittent nature of AEWs is mediated, not by transitions between convective and absolute instability, but likely by external sources such as propagating equatorial wave modes