The influence of Arctic amplification on mid-latitude summer circulation
While it might not sound so bad to have more prolonged sunny episodes in summer, this is in fact a major climate risk
Stalling summer weather
Be it heavy downpours or super-hot spells, summer weather becomes more persistent in North America, Europe and parts of Asia. When those conditions stall for several days or weeks, they can turn into extremes: heatwaves resulting in droughts, health risks and wildfires; or relentless rainfall resulting in floods. A team of scientists now presents the first comprehensive review of research on summer weather stalling focusing on the influence of the disproportionally strong warming of the Arctic as 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, in turn, regional and local weather patterns – with sometimes disastrous effects on the ground. This has been the case with the 2016 wildfire in Canada, another team of scientists show in a second study.
Warm tail of summer temperature distribution warms faster than the cold tail. This figure plots the trend in the temperature of very hot summer days minus the trend in the temperature of cold summer days.
The Arctic Factor
“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 Potsdam Institute for Climate Impact Research (PIK) and Vrije Universiteit Amsterdam, lead-author of the review paper and co-author of the wildfire case study. “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 societies: persistent hot and dry conditions in Western Europe, Russia and parts of the US threaten cereal yields in these breadbaskets.
Tons of studies have appeared on this topic in recent years, sometimes with seemingly conflicting results. For the paper now published in Nature Communications, an international team of scientists 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 rest of the Northern hemisphere. This reduces the temperature difference between the North Pole and the equator, yet this very difference is a main driving force for the airstreams. “There are 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, a co-author of the review paper.
“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.”
D. Coumou, G. Di Capua, S. Vavrus, L. Wang, S. Wang (2018): The influence of Arctic amplification on mid-latitude summer circulation. Nature Communications [DOI:10.1038/s41467-018-05256-8]