In their paper, Francis and Vavrus suggested that a rapidly warming Arctic should interfere with the jet stream, the river of air high above us that flows eastward around the northern hemisphere and brings with it our weather. Sometimes, the jet stream flows relatively directly from west to east; but other times, it takes long, wavy loops, as in the image above. And according to Francis and Vavrus, Arctic warming should make the jet stream more wavy and loopy on average – some have called it “drunk” -- with dramatic weather consequences.
Here's the atmospheric physics behind the idea: Warm air expands, and naturally there is much more warm air at the equator than at the poles. Thus, the atmosphere is thicker at the equator, and the jet stream's motion is driven by the decline in atmospheric thickness as one moves in a poleward direction -- in effect, its atmospheric river flows "downhill," in Francis’s words. However, if the Arctic is warming faster than the mid-latitudes, then the difference in thickness as you move in a poleward direction should decrease. And this should slow the jet stream, leading to more loops and turns -- and consequently, weather of all types getting stuck in place for longer. There's a nice video explanation of this by Francis here:
According to Francis, the extreme U.S. winter of last year and now, the extremes at the beginning of this season, fit her theory. "This winter looks a whole lot like last winter, it’s a very amplified jet stream pattern," she says. "We know that when we get these patterns, it tends to be very persistent. And it is definitely the type of pattern that we expect to see more often as the Artic continues to warm so fast."
To be sure, Francis acknowledges that our recent bout of extreme cold was kickstarted most directly by Typhoon Nuri, which swerved up into the mid-latitudes and exploded into an atmospheric bomb over the Bering Sea. "That had the downstream effect of basically taking the jet stream and giving it a whip, whipping a wave into it," says Francis. But she also suspects that the jet stream is more susceptible to these kinds of dramatic influences because it is weaker now. In general, her theory does not say global warming caused any particular weather event, only that it is shifting the overall pattern of jet stream behavior, making certain kinds of persistent weather extremes more likely to occur.
Francis isn't the only one to suggest this. The widely read weather blogger Jeff Masters mused yesterday on whether the extreme snowfall in western New York this week might be due to "jet stream weirdness." "We've seen an unusual number of extreme jet stream patterns like this in the past fifteen years, which happens to coincide with the period of time we've been observing record loss of summertime Arctic sea ice and record retreat of springtime snow cover in the Arctic," noted Masters -- although he refrained from fully embracing the theory, noting that it still has its detractors. Capital Weather Gang's Jason Samenow also just discussed the evidence behind Francis's idea, which he calls "controversial."
Francis argues, however, that the evidence in her favor is mounting -- she cites no fewer than five (1, 2, 3, 4, 5) scientific papers published in the last year or so that she considers supportive, and hints that more are coming. "We’ve got 5 papers that all look at that particular mechanism in different ways -- different analysis, different data sets, observation and models -- and they all come to the same conclusion and they all identify this mechanism independently," she says.
You can't call Francis's idea fully established. You can't say there's a "scientific consensus" on it. And you can't say that the august U.N. Intergovernmental Panel on Climate Change embraces it. Not yet. But it's certainly a very serious idea and one of the most discussed theories in climate science. Call it a contender. And if it's right, well...then we all know, already, what global warming feels like.