In the stratosphere, at altitudes above 20 kilometers, polar temperatures north of Europe repeatedly rise quite explosively. The air there can warm from below minus 60 degrees Celsius to above zero within days, according to data from international weather services. This happens on average once per year in the winter months and lasts about one to two weeks. Climate researchers from the Wegener Center of the University of Graz, together with partners from the Chinese Academy of Sciences, have investigated how these sudden strong warmings have developed over the past decades. “Employing a new method, we found that the duration of the sudden warmings in the stratosphere has increased by about 50 percent since the 1980s,” reports Gottfried Kirchengast, lead author of the study, which was published in the journal Atmospheric Chemistry and Physics. Since the severe temperature increases are accompanied by a massive weakening of the rotating polar vortex, this also affects the winter weather at middle latitudes.
“If the circulation of the winter polar vortex is disturbed by strong air exchange between the weather layer and the stratosphere, then large-scale warming of up to 50 degrees Celsius or more can occur at altitudes above 20 kilometers. This is the strongest form of atmospheric temperature increase worldwide and can completely halt the vortex winds,” explains Gottfried Kirchengast. The scientists analyzed data from the last four decades, from November to April in each winter. “We found ninety-five percent of the warming events to occur from December to February, and more than three quarters of them located with their area of strongest temperature anomaly north of Europe and Asia,” says the climate researcher. In addition to the significant increase in the duration of these warmings, from around 10 to 15 days, there was also a tendency towards an increased intensity, although this is not yet statistically significant in a robust way.
“Our results reinforce the findings of other recent studies that such polar vortex disturbances are becoming increasingly pronounced due to climate change. Ironically, this effect of global warming leads to more severe short-duration cold extremes here in Europe,” says Kirchengast. The study is part of further work by the researchers at the Wegener Center of the University of Graz that analyzes the complex global-warming-induced interplay of circulation and weather extremes between polar and middle latitudes in greater detail.
The research is embedded in the Field of Excellence Climate Change Graz of the University of Graz. The contribution of the Wegener Center to the study was supported by the Austrian Research Promotion Agency FFG based on funds from the Austrian Space Applications Programme ASAP endowed by the Federal Ministry for Climate Action, Environment, Energy, Mobility, Innovation and Technology (BMK).
Li, Y., G. Kirchengast, M. Schwärz, and Y. Yuan (2023): Monitoring sudden stratospheric warmings under climate change since 1980 based on reanalysis data verified by radio occultation, Atmospheric Chemistry and Physics, https://doi.org/10.5194/acp-23-1259-2023