When weather patterns hit a traffic jam, extreme events often result. This is particularly the case during the summer months, a new study argues.
The research, published Friday in the journal Science Advances, focuses on a devastating flood event that struck Serbia and the nation of Bosnia and Herzegovina in May of 2014.
The flooding, which resulted from a relentless four-day deluge, killed at least 48, displaced about 150,000 and caused about 3.5 billion euros in damage.
The study pinpoints a phenomenon known as "planetary wave resonance" for trapping the storm above the Balkans. More specifically, the study says a "quasi-stationary circumglobal Rossby wave train" helped deny the storm an escape route.
Rossby waves are named after the Swiss-American meteorologist Karl Gustaf-Rossby, who first described these phenomena in the early to mid-20th century.
The study, from a group of scientists at the Potsdam Institute for Climate Impact Research as well as the University of Potsdam, is the latest research to link an extreme weather event both to changes in the moisture and energy available for storms as a result of global warming, as well as to these unusual configurations of wind patterns in the upper atmosphere.
Planetary waves exist at the jet stream level, forming north-south undulations between the poles and the equator, about 30,000 to 45,000 feet above the ground.
To understand what they are, picture a circle of people holding hands. The planetary waves would be represented by two people moving their arm up and down and passing the wave like a snake from one person to another, around the circle.
Wave resonance occurs when these upper level winds form particular patterns, much like ocean waves do, that helps form standing, building waves that can remain in place longer than others do.
To extend the hand snake comparison, this would be like freezing the game in place for hours.
Here's how researcher Dim Coumou, who is one of the co-authors of this study, describes the planetary wave phenomenon:
Image: Earth simulator
"Ridges and troughs in the jet stream are expressions of Rossby waves in the atmosphere. For this study we studied a particular subclass of these waves: quasi-stationary waves, i.e. the position of the ridges and troughs of these waves is almost fixed in time," he said in an email to Mashable.
Other studies published in the past few years have tied planetary wave resonance to extreme summertime weather events, including the 2003 European heat wave, 2010 Russian heat wave, deadly flooding in Pakistan's Indus River Valley as well as heat waves in the U.S. in 2011.
"It turns out that many of the record-breaking extremes of the past two decades are associated with extreme planetary wave amplitudes," said study co-author Stefan Rahmstorf, in an email to Mashable.
The link to the planetary waves is not conclusive, though, the study notes. Instead, it is based more on an analysis of world weather patterns at the time of the Balkans flood.
“This does not prove causality, but the co-occurrence is at least suspicious – particularly since we had a similar situation for instance in 1997 in Germany with cyclone ‘Zoe’, resulting in the devastating Elbe [River] flooding,” Coumou said in a press release.
While some research has shown that stuck weather patterns may be becoming more common and extreme as global warming heats up the Arctic, the authors of this study say the jury is still out on that.
"There is still large uncertainty in how climate change might exactly alter the characteristics of these waves, but it is clear that only little changes can have large consequences," Coumou told Mashable.
In addition to the planetary waves connection, the study also shows that extreme rainfall events are increasing faster in the Balkans than the global average rate.
While the mean daily rainfall in the Balkans has increased only a little since 1950, the intensity of the strongest rainfall events rose by one third, the scientists found. In addition, the frequency of such potentially devastating extremes in the Balkans doubled over the past sixty years.