Extreme 2014 Balkan flooding linked to airstream slowdown

Extreme rainfall events increasing globally

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Flooding along the Sava River in May 2014, photographed from a NASA Earth-observing satellite. Visit NASA Earth observatory for more info.

Staff Report

Destructive floods that raged through parts of the Balkan region in 2014 are part of a global trend toward more extreme rainfall events — a pattern increasingly linked with changes in atmospheric circulation.

In new findings published this week, scientists found that disastrous floods in the Balkans two years ago are likely linked to the temporary slowdown of giant airstreams. Several other climate studies have explored whether climate change is causing the slowdown of giant atmospheric waves that carry storm systems from west the east across the northern hemisphere. The decline of Arctic sea ice has been eyed as a factor. Decreasing temperature and pressure gradients between high- and mid-latitudes may be causing more weather systems to get stuck, some scientists say.

UK scientists explored similar links between changes in the Jet Stream and extreme rain across the British Isles in 2013. Around the same time, University of Utah scientists looked at similar patterns across North America. Researchers have also explored if jet stream shifts are linked with extreme meltdown events across Greenland.

“Our findings provide more evidence that planetary waves cause extreme weather events,” said Potsdam Institute for Climate Research scientist Stefan Rahmstorf. “When these waves start to resonate this can have serious impacts for people on the ground. I am concerned that the ongoing climate change may be creating conditions more favorable for this kind of resonance,” said Rahmstorf, who co-authored the new study, published in Science Advances.

The 2014 storm poured record amounts of rain over Bosnia and Herzegovina, Serbia and Croatia. The study shows extreme rainfall events are strongly increasing in the Balkans, even more than the globally observed rise.

“We were surprised to see how long the weather system that led to the flooding stayed over the region – it’s like the Vb cyclone ‘Yvette’ was trapped there,” said PIK’s Lisa Stadtherr, the study’s lead author. “Day after day the rain was soaking the soil until it was saturated, which lead to the flooding that reportedly caused several dozen casualties and 3.5 billion Euro of damages.”

The mean daily rainfall in the Balkans has increased only a little since 1950, but the intensity of the strongest rainfall events rose by one third, the scientists found. In May 2014, daily rainfall amounts were locally bigger than ever before in the observed period. The frequency of such potentially devastating extremes in the Balkans, though they’re still rare, doubled over the past sixty years.

The changes seen in the Balkans are more than expected from simple warming of the air, said PIK researcher Dim Coumou.

“This is worrying, all the more because we’re seeing increasing extreme rainfall in many parts of the globe,” Coumou said. “Regional temperatures rose by one degree since the middle of the past century, and the increased water holding capacity of warmer air intensifies heavy rainfall by about 7 percent per degree of warming. Yet the observed rainfall changes in the Balkans are roughly five times that much … hence other factors must have come into play,” Comou added.

The study linked the unusual trapping of the weather system over the Balkans to a slowdown in the planetary waves.

“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 flooding,”according to Coumou. “We provide evidence that the near stationarity of the waves was linked to a subtle phenomenon we call resonance.”

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