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Weather and Climate Dynamics An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/wcd-2020-47
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wcd-2020-47
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  22 Sep 2020

22 Sep 2020

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This preprint is currently under review for the journal WCD.

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

Joshua White and Anantha Aiyyer Joshua White and Anantha Aiyyer
  • Department of Marine, Earth and Atmospheric Sciences, North Carolina State University

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.

Joshua White and Anantha Aiyyer

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Joshua White and Anantha Aiyyer

Joshua White and Anantha Aiyyer

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Latest update: 03 Dec 2020
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Short summary
Using a simple general circulation model, we examine the structure of waves in the mid-tropospheric jet over North Africa. We show that waves occur in near-stationary groups or wavepackets. As they are not swept out of the jet, this may provide the opportunity for the packets to amplify via feedback from other energy sources like rain-producing cloud complexes and mineral dust that are known to operate here. Our results address the criticism that the easterly jet is too short to sustain waves.
Using a simple general circulation model, we examine the structure of waves in the...
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