Nice sunny days grow into heat waves, rain into floods: summer weather stalling

An international team of scientists, among who is VU climate researcher Dim Coumou, made an overview of recent studies into stagnant and therefore extreme summer weather.

08/20/2018 | 12:54 PM

Be it heavy rainfall or downright hot, summer weather becomes more persistent in North America, Europe and parts of Asia. When weather conditions stall for several days or weeks, it can turn into extremes: heat-waves resulting in droughts, wildfires and health risks; or continuous rainfall resulting in floods. Scientists now present the first comprehensive review of research on summer weather stalling focusing on the influence of the extra strong warming in the Arctic caused by greenhouse gas emissions from burning fossil fuels. Evidence is mounting, they show, that we likely meddle with circulation patterns high up in the sky. These are affecting weather patterns - with sometimes disastrous effects on the ground.

“While it might not sound so bad to have more prolonged sunny episodes in summer, this is in fact a major climate risk,” says Dim Coumou from the Vrije Universiteit Amsterdam and the Potsdam Institute for Climate Impact Research (PIK), lead-author of the study now published in Nature Communications. “We have rising temperatures due to human-caused global warming which intensifies heat waves and heavy rainfall, and on top of that we could get dynamical changes that make weather extremes even stronger – this is quite worrying.” This summer is an impressive example of how stalling weather can impact society: Persistent hot and dry conditions in western Europe, Russia and parts of the US threaten cereal yields in these breadbaskets.

Investigating the Arctic factor – and the influence of human-caused warming
Tons of studies have appeared on this topic in recent years, sometimes with seemingly conflicting results. The scientists now set out to review the existing research and tried to connect the dots, with a focus on the Arctic factor. Under global warming, the Arctic warms more than the Northern hemisphere’s land masses. This reduces the temperature difference between the North Pole and the equator, yet this temperature difference is a main driver for airstreams. “There’re many studies now, and they point to a number of factors that could contribute to increased airstream stalling in the mid-latitudes – besides Arctic warming, there’s also the possibility of climate-change-induced shifting of the storm tracks, as well as changes in the tropical monsoons,” says Simon Wang from Utah State University in the US. Under global warming, the Indian summer monsoon rainfall will likely intensify and this will also influence the global airstreams and might ultimately contribute to more stalling weather patterns. All of these mechanisms do not work in isolation but interact,” says Wang. “There is strong evidence that winds associated with summer weather systems are weakening and this can interact with so-called amplified quasi-stationary waves. These combined effects point towards more persistent weather patterns, and hence more extreme weather.”

Changes in airstreams can contribute to “extreme extremes”
“Computer simulations generally support the observations and our theoretical understanding of the processes, so this seems pretty robust,” concludes Coumou. “However, the observed changes are typically more pronounced than those seen in climate models.” So either the simulations are too conservative, or the observed changes are strongly influenced by natural variability. “Our review aims at identifying knowledge gaps and ways forward for future research,” says Coumou. “So there’s still a lot to do, including machine learning and the use of big data. While we do not have perfect certainty, the state of research indicates that changes in airstreams can, together with other factors, lead to a phenomenon that sounds funny but isn’t: extreme extremes.”