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A major step forward in understanding extreme precipitation in arid zones
In our atmosphere, massive air masses oscillate at high altitudes, influencing our daily weather: these are known as Rossby waves. A team from UNIL has revealed that extreme precipitation in arid regions is linked to strong perturbations in these waves— an important breakthrough in understanding these little-studied areas.
More than a third of the world’s population lives in drylands and is disproportionately at risk from hydrometeorological hazards such as drought and flooding. A recent emblematic example is the collapse of the Derna dams in Libya in September 2023 following torrential rains, causing the death of more than 12,000 people.
Despite recurrent flood disasters in such regions, very few studies have been carried out on the weather systems governing the formation of precipitation in arid zones, in contrast to wetlands, which have attracted a great deal of attention. Researchers at the University of Lausanne have revealed that a peculiar meteorological phenomenon, known as Rossby waves—the north-south meandering of the jet stream—plays a crucial role in triggering extreme precipitation in these regions. When these waves in air mass amplify in the north-south direction, they can overturn and break, very much like ocean waves breaking upon approaching the shore. This phenomenon contributes to over 90% of extreme precipitation in arid regions around the globe. The results of this study have been published in Communications Earth & Environment, a journal from the Nature portfolio.
“Our results provide a new understanding of the atmospheric processes leading to torrential rains in societally vulnerable regions that are threatened by both freshwater shortages and flood hazards”, says Andries-Jan de Vries, first author of the study and researcher at the University of Lausanne's Faculty of Geosciences. “With global warming, both precipitation deficits and extremes are predicted to intensify although these changes remain uncertain. Our findings can help improving weather and climate prediction of flood and drought hazards in these regions.”
Rossby waves: a definition
Rossby waves are gigantic north-south oscillations in air masses that propagate around the globe in mid-latitudes at around 10 km above the Earth’s surface. They carry warm air from the tropics poleward and displace cold air from the Arctic equatorward. All in balance. As they move, they create zones of high and low pressure in certain regions. However, Rossby waves can also amplify and become unstable, leading to rapid growth and the overturning and breaking of these waves, generating rotating circulation patterns in the upper troposphere. This phenomenon is responsible for intense precipitation over the regions concerned. Likewise, this phenomenon is responsible for up to 80% of annual precipitation, essential for refilling scarce freshwater resources. Breaking of Rossby waves is most important for precipitation falling during the cool season in regions where climate models predict a strong precipitation decline, such as the Mediterranean, North Africa, the Middle East, southern Africa, and southern Australia.
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For further information: A. J. de Vries, M. Armon, K. Klingmüller, R. Portmann, M.Röthlisberger & D. I. V. Domeisen, Breaking Rossby waves drive extreme precipitation in the world's arid regions, Communications Earth & Environment, 2024.
Are you interested in climate extremes? Visit the website of the Expertise Center for Climate Extremes (ECCE) at the University of Lausanne.
