'Frozen' atmospheric waves trigger extreme weather events

Posted by Kate Taylor

Another report has linked man-made climate change with extreme weather events around the world, including recent heat waves in the United States and Russia and the unprecedented Pakistan flood.

According to scientists from the Potsdam Institute for Climate Impact Research (PIK), climate change repeatedly disturbs the patterns of atmospheric flow around the globe's northern hemisphere through a subtle resonance mechanism.

"An important part of the global air motion in the mid-latitudes of the Earth normally takes the form of waves wandering around the planet, oscillating between the tropical and the Arctic regions," says explains lead author Vladimir Petoukhov.

"So when they swing up, these waves suck warm air from the tropics to Europe, Russia, or the US, and when they swing down, they do the same thing with cold air from the Arctic."

The team found that during several recent extreme weather events, these planetary waves almost froze in their tracks for weeks.

"In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves," says Petoukhov.
 
Climate change caused by greenhouse-gas emissions from fossil-fuel burning does not mean uniform global warming - indeed, temperature differences between the Arctic and, for example, Europe, are narrowing. But it's these temperature differences are a main driver of air flow.

In addition, continents generally warm and cool more readily than the oceans.

"These two factors are crucial for the mechanism we detected," says Petoukhov. "They result in an unnatural pattern of the mid-latitude air flow, so that for extended periods the slow synoptic waves get trapped."

The team developed equations to describe the wave motions and show under what conditions they can grind to a halt and get amplified. They tested their assumptions using standard daily weather data from the US National Centers for Environmental Prediction (NCEP).

And, they found, during recent periods in which several major weather extremes occurred, the trapping and strong amplification of particular waves was indeed observed. The data show an increase in the occurrence of these specific atmospheric patterns, which is statistically significant at the 90 percent confidence level.

"Our dynamical analysis helps to explain the increasing number of novel weather extremes. It complements previous research that already linked such phenomena to climate change, but did not yet identify a mechanism behind it," says Hans Joachim Schellnhuber, director of PIK and co-author of the study.

"This is quite a breakthrough, even though things are not at all simple -- the suggested physical process increases the probability of weather extremes, but additional factors certainly play a role as well, including natural variability."