February 26, 2013
Wave Patterns In The Atmosphere Help Explain Weather Extremes
April Flowers for redOrbit.com - Your Universe Online
In recent years, the world has suffered from severe regional weather extremes such as the heat wave in the US in 2011, or the one in Russia in 2010 that coincided with the unprecedented Pakistan flood. There is one common physical cause behind these individual events, according to a new study from the Potsdam Institute for Climate Impact Research [PIK].
"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. 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," explains Vladimir Petoukhov.
"What we found is that during several recent extreme weather events these planetary waves almost freeze in their tracks for weeks. 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," says Petoukhov.
The study shows time is critical to this process. Two or three days of 86-degree Fahrenheit weather are no problem, but twenty or more days lead to extreme heat stress. prolonged hot periods can result in a high death toll, forest fires, and dramatic harvest losses for the many ecosystems and cities not adapted to such temperatures.
Global warming caused by greenhouse gas emissions from burning fossil-fuels is not uniform. In the Arctic, for example, the relative temperatures, amplified by the loss of snow and ice, is higher than on average. As a result, the temperature difference between the Arctic and, for example, Europe is reduced, yet temperature differences are a main driver of air flow. It should be noted that continents 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."
To describe the wave motions in the extra-tropical atmosphere and show under what conditions those waves can grind to a halt and become amplified, the team developed equations which were then tested using daily standard weather data from the US National Centers for Environmental Prediction (NCEP). The trapping and strong amplification of particular waves — like "wave seven" (which has seven troughs and crests spanning the globe) — was observed during recent periods of major weather extremes. The data collected by the researchers 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. "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."
The project spans a 32-year period, which provides a good indication of the mechanism involved, yet is too short for definite conclusions.
The study significantly advances the understanding of the relation between weather extremes and man made climate change, however, surprising the scientists with how far outside past experience some of the recent extremes have been. The emergence of extraordinary weather is not just a linear response to the mean warming trend, the study shows, suggesting the proposed mechanism could explain that.