Melting sea ice in the Arctic has left vast expanses of dark open water available to absorb the sun’s energy. In the late autumn and early winter, when sea ice is at a minimum and temperatures begin to cool, the ocean releases that extra heat and moisture back into the atmosphere. Those fluxes help drive a positive feedback effect, further intensifying warming in the region.
One result of this Arctic amplification is that there’s less of a temperature difference between the Arctic and the lower latitudes. Some scientists have suggested that the lower temperature gradient is weakening the winds that circle the globe, particularly the polar jet stream. In the wintertime, the idea goes, a weaker, wavier jet stream could promote more and longer bouts of frigid air reaching farther south, leading to extreme snowfalls such as those that struck the eastern United States in the past few years.
But winter cold spells are just one part of a possibly larger story. The jet stream is weaker in summertime, too—and it “has been weakening over the last 36 years,” says Dim Coumou, a climatologist at the Potsdam Institute for Climate Impact Research in Germany and lead author on the new paper. While many of his colleagues have plugged away at the higher profile winter extremes debate, Coumou has focused on summer extremes. “Of course, heat extremes are on the rise almost everywhere on the globe, purely because we’re warming the air,” Coumou says. “But on top of that, changes in atmospheric circulation can favor particular weather conditions associated with heat waves.”
Coumou has examined the waviness of the jet stream in previous work and has suggested that its large twists and turns, slow-moving undulations called Rossby waves, promote atmospheric “blocking”—a kind of stagnation of weather patterns that he says can exacerbate heat waves. But in the new study, he and his colleagues took a different tack: Instead of focusing on blocking and slow-moving Rossby waves, they turned to smaller and quicker undulations called “free, transient Rossby waves.” The winding path of these waves sketches out storm tracks across the midlatitudes.
What Coumou and his team wanted to know, he says, was how the energy of these weather systems has changed over time, from 1979 to present. The kinetic energy of large-scale weather systems—including cyclones and anticyclones—is closely tied to the overall strength of the jet stream, Coumou says. “If the jet is strong, the wind shear will be strong.” And shear is important in generating “eddies”—cyclonic or anticyclonic swirls of energy—within the overall wind flow. “So if you see a weakening of the jet, it’s logical that you’ll also see a weakening of the kinetic energy of these systems.”
Coumou and colleagues analyzed observational data on daily winds for June, July, and August from 1979 to 2013, focusing on the Northern Hemisphere’s midlatitudes (between 35° north and 70° north). They found a slow decline in the overall “eddy kinetic energy”—a drop of about 8% to 15% in summertime energy during that time period, they report online today in Science. In other words, there were fewer, or less intense, summertime storms.
“That is very important for weather conditions on the continents,” Coumou says. “In summertime, these systems bring cool and moist air from oceans to continents and have a moderating effect on the continental weather.” In the absence of such storms, he adds, there’s a greater likelihood of drought and lingering heat. The researchers also noted a statistically significant relationship between times of low eddy kinetic energy and extremely high temperature anomalies—for example, the sweltering Russian heat wave of 2010.
The paper is “an important contribution” and “a new perspective on mechanisms linking Arctic amplification with midlatitude weather patterns,” says Jennifer Francis, a climatologist at Rutgers University, New Brunswick, in New Jersey. “When these small [free Rossby] waves get weaker in summer, they tend to be associated with more stagnant weather conditions, particularly heat waves.”
The finding of an overall decrease in summer storminess is “reasonably convincing,” says James Screen, a climatologist at the University of Exeter in the United Kingdom. But, he says, the difficulty is linking this effect to Arctic amplification. There are many factors that can affect the number and intensity of storms—not just the temperature gradient between pole and equator, but also east to west gradients and vertical gradients at both larger and local scales. “It’s unclear to what extent changes in storminess are … part of the large-scale Arctic amplification or [are] due to more local factors.”
Coumou has taken the story forward, analyzing existing climate models to track the future of eddy kinetic energy over the 21st century. “In those you see a clear weakening; it’s a robust signal in those models”—and that, the authors suggest, means that recent bouts of extreme heat might be just the beginning.