Visualization of a very wavy northern hemisphere jet stream. (Credit: NASA)
By Chris Mooney March 12 2015 Washington Post
Is the rapid melting of the Arctic paying us back for our greenhouse gas emissions by messing with the jet stream — which carries weather through the northern hemisphere? And could that, in turn, explain recent breakouts of extremes all around the northern half of the world — including recent snowfall in the east coast? That’s what Rutgers University’s Jennifer Francis has argued in a series of papers going back to 2012 — but there has been quite a lot of criticism. Several distinguished climate researchers even wrote to Science magazine in early 2014 contesting the notion, saying that “we do not view the theoretical arguments underlying it as compelling.” And yet stubbornly, more published research keeps appearing and seeming to add support to the idea that the warming Arctic is changing the jet stream. That statement comes with an exclamation point on Thursday in particular, with a new paper out in Science that confirms many of Francis’s ideas and applies them not just to extreme winter weather but, in some ways even more troubling, to extremes of summer heat. The new paper, by Dim Coumou and two colleagues at the Potsdam Institute for Climate Impact Research and the University of Potsdam in Germany, finds that the melting Arctic is indeed messing with the jet stream (as well as the broader atmospheric circulation) and our weather. But it also goes further by asserting that there’s a strong effect in the summer in particular. The progress of weather is slowing during the summer, the authors assert, and the result could be a very deadly one — including “more persistent heat waves in recent summers.” Or as the researchers put it, a weaker jet stream and atmospheric circulation in the summer, caused by a reduced differential in temperature between the equator and the north pole as the Arctic warms faster than the mid-latitudes, “has made weather more persistent and hence favored the occurrence of prolonged heat extremes.”… he Potsdam researchers looked at an atmospheric feature called “eddy kinetic energy,” which, as Francis explained, basically refers to the winds swirling around regions of high and low pressure. Those winds have decreased, the paper finds. “That’s why they’re saying that it’s more likely to have summer extreme events,” Francis said. “Because the weather just is not changing as much, and the weather systems themselves are just more stagnant and lethargic.” The new study points in particular to the devastating 2010 summer heat wave in Russia. “By late July and early August, numerous cities witnessed a crescendo of record breaking daily readings near 40ºC, more than +10ºC warmer than what would normally have been experienced at this warmest time of year,” noted the National Oceanic and Atmospheric Administration at the time. The resulting death toll could have been as high as 55,000. So how did such an extreme come about? The new paper notes that “the record breaking July temperatures over Moscow were associated with extremely low [eddy kinetic energy].” In other words, there was just not enough circulation of air to bring in cooler temperatures. “If this whole circulation slows down and there’s less energy in these storms, then basically we get more persistent weather situations, which can lead to some extreme heat waves,” said Stefan Rahmstorf, also a researcher at the Potsdam Institute for Climate Impact Research but not one of the study’s authors. So what’s the upshot for the ongoing debate over whether the Arctic is, indeed, messing with weather in the mid-latitudes all over the globe? “I think the balance of evidence is kind of moving towards confirming that there is this influence of the Arctic,” said Rahmstorf….
Rapid warming in the Arctic could influence mid-latitude circulation by reducing the poleward temperature gradient. The largest changes are generally expected in autumn or winter but whether significant changes have occurred is debated. Here we report significant weakening of summer circulation detected in three key dynamical quantities: (i) the zonal-mean zonal wind, (ii) the eddy kinetic energy (EKE) and (iii) the amplitude of fast-moving Rossby waves. Weakening of the zonal wind is explained by a reduction in poleward temperature gradient. Changes in Rossby waves and EKE are consistent with regression analyses of climate model projections and changes over the seasonal cycle. Monthly heat extremes are associated with low EKE and thus the observed weakening might have contributed to more persistent heat waves in recent summers.