News•June 22, 2014
Waves in the Atmosphere Fueling Extreme Weather
By Brian Kahn
The pattern of a wavy jet stream was a recurring theme in U.S. weather forecasts this winter as a particularly jagged one essentially split the country in two. While there is a debate over whether climate change causes that pattern, new research shows that the waviness does exacerbate extreme weather.
The research, published in Nature Climate Change on Sunday, looked at planetary waves on a monthly timescale. Waves are essentially the ridges and troughs left as the jet stream, a fast-moving river of air, cuts it way across the middle of the northern hemisphere. The jet stream essentially helps drive weather patterns around the northern half of the globe by pushing around storm systems and sometimes impeding their progress.
Animation of the jet stream as it moves over North America, illustrating its troughs and ridges.
James Screen, a climate scientist at the University of Exeter who co-authored the study, said he wanted to examine how planetary waves influenced persistent weather patterns, such as drought or extreme heat or cold.RELATEDHot West, Cold East May be the Norm as World Warms
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He examined the timeframe from 1979-2010, looking for 40 months that exhibited the most extreme precipitation, and for 40 months that showed the most extreme temperature departures from the norm. And the data showed that more wavy waves overwhelmingly accompanied months with temperature or precipitation extremes. Only a small percentage of months with extreme weather corresponded with a more relaxed series of waves.
In the U.S., Screen found that increased waviness made the western part of the country more susceptible to heat waves and the eastern part more likely to experience extreme cold. Droughts in the central part of the country as well in as Europe and central Asia, and wet spells in western Asia were also much more likely when waviness increased.
The regional differences largely stem from geographical features on the ground that influence planetary waves.
“The locations are tied to things like mountain ranges and the temperature contrast between the land and ocean. These are factors that don’t change,” Screen said.
They ensure that while there might be some month-to-month differences, the waves generally follow a similar pattern. And because they’re separated by the jet stream, that helps determine what impacts each region will see.
Kevin Trenberth, a senior scientist at the National Corporation for Atmospheric Research, said the study quantified a fairly well known pattern, though one he said climate scientists often take for granted. Climate researchers have started to look at these waves more closely, from how to use them to predict heat waves to how climate change could alter them.
A commentary in Science last month argued that climate change was at least in part to blame for the pattern that set up over the U.S. this past winter by making waves more common. That commentary is based on research published in 2012 that made the case for why rapid Arctic warming is increasing the odds of wilder planetary waves.
The Arctic is warming twice as fast as areas around the equator because of unique feedbacks involving ice cover in the region. The research argues that as the temperature gradient between the poles and the equator decreases, planetary waves are getting out of whack and becoming even more extreme, though other research has challenged those findings.
Screen’s study, however, only looked at the relationship between waves and extreme events rather than any long-term shift in trends.
“It’s still hotly debated whether we see any change in these waves,” Screen said. “I’m currently sitting in the middle, thinking it’s a plausible hypothesis, but currently the evidence is inconclusive at this point.”
Jennifer Francis, a researcher at Rutgers University who proposed the hypothesis, said there’s a ways to go toward understanding how climate change could affect planetary waves, and the meanderings of the jet stream.
“This is a complicated problem, and finding answers is further challenged by the short time period over which those regional temperature changes have emerged as clear signals from the highly variable atmosphere,” she said in an email. “New approaches to this question are underway, however, and I'm confident that a clearer picture will come to light in the next few years.”
Francis also stressed that understanding waves is just one component of understanding the larger category of extreme weather. Natural fluctuations in ocean temperatures, such as El Niño, and human-caused deforestation and air pollution, can all have an impact. Smaller fluctuations in the atmosphere can also lead to sudden, shorter-scale extreme events.
Trenberth said that putting aside the impact climate change could have on waves, it can also alter the water cycle because a warmer atmosphere can hold more water, increasing the odds of heavy precipitation and extreme dryness.