What is it about?
We found, for the first time, a strong link between turbulence and the quasi-biennial oscillation (QBO), a phenomenon where stratospheric winds reverse direction approximately every two years. The QBO propagates downward through the stratosphere such that, during transitions, the wind direction changes with height. Turbulence is strongly dependent on wind direction and peaks during the QBO transition where westward winds overlie eastward winds. The relationship between wind direction and turbulence is driven by atmospheric waves. Tropical thunderstorms generate many types of waves, some of which are confined near the equator. One such type, equatorial Kelvin waves, propagate in eastward and weak westward winds. During the QBO transition, these waves are able to propagate upward from the top of thunderstorms into the stratosphere. However, as they continue to propagate upward in the stratosphere, they encounter strong westward winds and dissipate, generating turbulence. The frequency of tropical stratospheric turbulence varies by a factor of ten depending on wind direction. This finding has significant implications for aviation, atmospheric modelling, and understanding the transfer of momentum across scales in the tropical stratosphere.
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Why is it important?
Air enters the stratosphere primarily in the tropics, carrying gases and aerosols from natural and human sources in the lower atmosphere. As it rises, it is churned by turbulence, which slows its ascent and may alter its chemical composition. Because most aircraft cannot reach the tropical stratosphere, turbulence in this region is poorly understood, limiting efforts to model global atmospheric chemistry. Lightweight aircraft have recently been proposed for tropical stratospheric operations, for purposes such as environmental monitoring, communication, and injecting aerosols to reflect sunlight and mitigate greenhouse gas warming. Understanding tropical stratospheric turbulence is crucial for the successful operation of these aircraft.
Perspectives
I switched topics when I started my postdoc and I found it very rewarding to make a discovery on a topic where so little is currently known. This topic is particularly important now because the tropical stratosphere has recently become a region of great interest for the operation of aircraft and for geoengineering Earth's climate. I also appreciated the elegance of using radiosonde data, which is simple and inexpensive to collect compared to many other data types, but can still be enormously useful especially when you have a very long data record, as we do here.
Rachel Atlas
Centre National de la Recherche Scientifique
Read the Original
This page is a summary of: Turbulence in the tropical stratosphere, equatorial Kelvin waves, and the quasi-biennial oscillation, Proceedings of the National Academy of Sciences, January 2025, Proceedings of the National Academy of Sciences,
DOI: 10.1073/pnas.2409791122.
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