Observational data confirms pattern changes
By Summit Voice
FRISCO — Increasing global temperatures are “freezing” atmospheric waves, resulting in more frequent weather extremes, including the 2011 U.S. heat wave and a 2010 heat wave in Russia that coincided with unprecedented flooding in Pakistan.
Scientists have surprised by how far outside past experience some of the recent extremes have been. The new data show that the emergence of extraordinary weather is not just a linear response to the mean warming trend.
“What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks,” said Vladimir Petoukhov, lead author of a study to be published this week in the US Proceedings of the National Academy of Sciences. “So instead of bringing in cool air after having brought warm air in before, the heat just stays. In fact, we observe a strong amplification of the usually weak, slowly moving component of these waves,” Petoukhov said.
The findings suggest that man-made climate change repeatedly disturbs the patterns of atmospheric flow around the globe’s Northern hemisphere through a subtle resonance mechanism, he explained.
In the context of impacts on people, time is critical. Two or three days of 30 degrees Celsius are no problem, but twenty or more days lead to extreme heat stress. Since many ecosystems and cities are not adapted to this, prolonged hot periods can result in a high death toll, forest fires, and dramatic harvest losses.
The new study builds on previous work by Rutgers climate scientist Jennifer Francis, who explained that climate change caused by greenhouse-gas emissions from fossil-fuel burning does not mean uniform global warming. In the Arctic, the relative increase of temperatures, amplified by the loss of snow and ice, is higher than on average. This reduces the temperature difference between the Arctic and, for example, Europe.
Those temperature differences drive the circumpolar flow of air. Additionally, continents generally warm and cool more readily than the oceans.
“These two factors are crucial for the mechanism we detected,” said 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 authors of the study developed equations that describe the wave motions in the extra-tropical atmosphere and show under what conditions those waves can grind to a halt and get amplified.
To test their models, the researchers compared their modeling outputs against standard daily weather data from the US National Centers for Environmental Prediction, finding that the trapping and strong amplification of particular waves was 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,” said 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.”
Also, the 32-year period studied in the project provides a good indication of the mechanism involved, yet is too short for definite conclusions, Schellnhuber added.