Frequent extreme weather is likely due to atmosphere stalling of giant waves

COMPUTER simulations predicted a strong increase of events in which the undulations of the jet stream in the atmosphere stop moving along and grow very large, favoring more frequent extreme weather events on the ground: the westerly winds stop pushing forward weather systems which hence become more persistent—a few sunny days grow into heatwaves, extended rains lead to floods.

An international team of scientists at the Potsdam Institute for Climate Impact Research (PIK) linked this prediction to human-caused warming, specifically in the Arctic.

“We expect a roughly 50-percent increase in the incidence of atmospheric conditions that favor a slow, broadly meandering jet stream and stalled weather extremes,” said Michael Mann of the Pennsylvania State University and lead author of the PIK study shared to Database.

“We call this quasi-resonant amplification of planetary waves, but what it means is rather simple: people will likely experience extreme and potentially dangerous weather events more often,” Mann pointed out. He added: “Such planetary wave events were behind the 2016 Canadian wildfire, the 2013 European floods and the 2010 Russian heat wave.”

For the first time, the future occurrence of conditions leading to growth and stalling of the planetary waves have been computed in a large set of state-of-the-art climate simulations called CMIP5. While the connection between stalling atmospheric waves and extreme weather events has already been demonstrated by previous studies and for past events, looking into the future by projecting what might happen if greenhouse-gas emissions rise unabated brought a surprise.

“Looking at a large number of different computer models, we found interesting differences,” said Stefan Rahmstorf of the PIK, a coauthor of the study.

“Distinct climate models provide quite diverging forecasts for future climate resonance events. However, on average they show a clear increase in such events. Our analysis clearly shows that there seem to be two crucial factors determining the frequency of these events,” Rahmstorf added.

Arctic warming and air pollution

Computer simulations that take into account the fact that the Arctic is warming much faster than the rest of the planet showed a more pronounced increase of the stalling events. The same is true for simulations that include the effects of air pollution with tiny particles, so-called aerosols.

Strong Arctic warming reduces the temperature difference between the North Pole and the subtropics, and it is this difference that is a major driver of the jet stream—hence the relevance of the Arctic. Air pollution blocks a part of the sunshine, for instance, in regions with lots of dirty coal-fired power plants and thus results in slight temporary local cooling. This also reduces the temperature difference to the North Pole, the PIK study revealed.

“So one implication of our findings is that a reduction of air pollution in the industrialized countries could actually restore some of the natural temperature difference between the mid-latitudes and the Arctic—which, in turn, would help to prevent the future increase in stalling of planetary waves and the related weather extremes,” said Rahmstorf.

“It is interesting that closing down coal-fired power stations helps to avoid climate destabilization in two ways: by reducing greenhouse-gas emissions that drive global and Arctic warming and also by reducing air pollution. Thus, if we want to limit the increase of dangerous weather extremes, phasing out coal fast seems like a pretty good idea,” he said.

“The climate models are not yet advanced enough to predict these types of extreme weather episodes for specific times or locations, because the models are still too coarse,” noted study coauthor Dim Coumou, who worked at PIK and Vrije Universiteit Amsterdam.

However, the models are able to produce large-scale patterns of temperature change realistically, added PIK coauthor Kai Kornhuber.

“In combination with previous research on the topic, we see a rather worrying trend in our simulations—weather extremes linked to greenhouse-gas emissions are on the rise, and more frequent stalling of giant airstreams high up in the atmosphere seems to be one major factor,” he explained.

Also in the PIK group, aside from Mann, Rahmstorf, Coumou and Kornhuber, were Byron A. Steinman, Sonya K. Miller and Stefan Petri.

The PIK, a member of the prestigious Leibniz Association, is one of the leading research institutions addressing relevant questions in the fields of global change, climate impacts and sustainable development. Natural and social scientists work closely together to generate interdisciplinary insights that provide a sound basis for decision-making for society, businesses and politics.


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