Superstorm Sandy satellite image from Oct. 29, 2012. The largest Atlantic hurricane on record, Sandy is part of a spate of off-the-charts extreme weather events in recent years.
CREDIT: AP Photo/NOAA
by Joe Romm Posted on January 15, 2015 at 4:22 pm
We have seen a quantum jump in extreme weather events in the Northern Hemisphere in the last several years. Droughts, deluges, and heat waves are increasingly getting “stuck” or “blocked,” which in turn worsens and prolongs their impact beyond what might be expected just from the recent human-caused increase in global temperatures. A growing body of research ties that unexpected jump to a weakening of the jet stream — in particular to “more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns,” as a new study puts it. Much of this new research ties the weakening jet stream to “Arctic amplification (AA) — defined here as the enhanced sensitivity of Arctic temperature change relative to mid-latitude regions,” in the words of the new study, “Evidence for a wavier jet stream in response to rapid Arctic warming” by Jennifer Francis and Stephen Vavrus. But that is no by no means a universally accepted explanation. I’ll review some of the evidence in this post….
….Not every study comes to the same conclusion as NOAA, PIK, and Francis (see, for instance, here). One 2014 study claims to “disconfirm the hypothesis that deep tropospheric warming in the Arctic during OND [October, November. December has resulted substantially from sea ice loss.” But as Francis explained to me, the authors of that 2014 study “state that the first link in the ‘chain’ connecting rapid Arctic warming with a wavier jet stream, as proposed in our 2012 paper, is sea-ice loss — but in fact it is Arctic amplification (Arctic warming faster than mid-latitudes). While sea-ice loss is one of the factors contributing to Arctic amplification (AA), it is certainly not the most important factor — only 20% according to this study.” Francis also points out “their modeled response to sea-ice loss is presented as time-averages, so any signal of jet-stream wave amplification will not be detected unless the ridge/trough system occurs in the same place every time, which it often does not.”
Clearly the interactions between global warming and Northern Hemisphere weather are complex. We still have much more to learn about “Recent Arctic amplification and extreme mid-latitude weather,” as made clear in a recent Nature Geoscience paper (with that title) written by several of the leading researchers in the field, including Francis.
But the evidence is mounting that we have entered a new regime of extreme weather thanks to our as-yet unrestricted emissions of greenhouse gas. The latest 2015 study, by Francis and Vavrus, concludes:
These results reinforce the hypothesis that a rapidly warming Arctic promotes amplified jet-stream trajectories, which are known to favor persistent weather patterns and a higher likelihood of extreme weather events. Based on these results, we conclude that further strengthening and expansion of AA in all seasons, as a result of unabated increases in greenhouse gas emissions, will contribute to an increasingly wavy character in the upper-level winds, and consequently, an increase in extreme weather events that arise from prolonged atmospheric conditions.
Jennifer A Francis and Stephen J Vavrus 2015 Environ. Res. Lett.
10 014005 doi:10.1088/1748-9326/10/1/014005
Received 4 November 2014, accepted for publication 11 December 2014 Published 6 January 2015
New metrics and evidence are presented that support a linkage between rapid Arctic warming, relative to Northern hemisphere mid-latitudes, and more frequent high-amplitude (wavy) jet-stream configurations that favor persistent weather patterns. We find robust relationships among seasonal and regional patterns of weaker poleward thickness gradients, weaker zonal upper-level winds, and a more meridional flow direction. These results suggest that as the Arctic continues to warm faster than elsewhere in response to rising greenhouse-gas concentrations, the frequency of extreme weather events caused by persistent jet-stream patterns will increase.