Three blockbuster winter storms have bombarded the Northeast this month. Meanwhile, Arctic sea ice fell to record low winter levels. Temperatures, in the dead of winter, rocketed to the melting point at the North Pole, nearly 60 degrees above normal.
The study, titled “Warm Arctic episodes linked with increased frequency of extreme winter weather in the United States,” shows that severe winter weather, late in the season, has increased over the eastern United States since 1990 as the Arctic has dramatically warmed, faster than any other part of the world.
When the Arctic is warm, the study finds, cold weather and heavy snowfalls in the eastern United States are two to four times more likely than when it is cold.
“This paper argues that the weather was cold not in spite of climate change but likely because of climate change,” said Judah Cohen, lead author of the study.
Connecting what’s happening in the Arctic with weather in the mid-latitudes is a relatively new area of study and has, at times, proved controversial. But Cohen, director of seasonal weather forecast at Atmospheric and Environmental Research, said the statistical significance of the connection found in this analysis was very robust. “I think that’s an important outcome of the work,” he said.
The study investigated changes in an index of the severity of winter weather in different U.S. cities and how they were related to Arctic weather patterns. In short, the authors found that when the Arctic region was cold and air pressures were low, it was cold in the western United States and mild in the eastern United States. But when the Arctic was warm and pressures were high, it was mild in the West and cold in the East.
As Arctic temperatures have warmed in recent decades, late winter weather severity has increased in the East while decreasing in the West, the study found.
Because the increase in winter weather severity in the East has been most pronounced in February and March, when the biggest winter storms tend to form, major East Coast cities have seen an uptick in the frequency of crippling snowstorms. “We found a statistically significant increase in the return rate of heavier snowfall in Boston, New York and Washington,” Cohen said.
In fact, from D.C. to Boston, the majority of the top 10 biggest snowstorms on record have occurred since 1990, while weather records date to the late 1800s.
The study also shows that in cities in the West, such as Salt Lake City and Seattle, big snowstorms have become less frequent.
The study found that episodes of severe winter weather in the East were typically preceded by a sudden warming of the Arctic from the surface through the stratosphere as well as a split in the polar vortex, the zone of frigid air encircling the Arctic high altitudes. Such a vortex split shifts cold air normally centered over the Arctic over eastern North America and/or Europe.
Cohen said the vortex split led to the recent parade of nor’easters as well as the severe outbreak of cold and snow in Europe, dubbed the “Beast from the East.”
Daniel Swain, a climate scientist at the University of California at Los Angeles not involved in this work, said the Cohen study was consistent with work with which he has been involved, that has shown an increasing tendency for a cold-East, warm-West pattern during winter in recent decades.
“It’s also striking that these changes have coincided with substantial climatic upheaval in the Arctic, where accelerated warming has recently caused sea ice to drop to record lows,” Swain said in an email. “While I think the jury’s still out regarding whether these Arctic changes are actually causing this particular instance of extreme variability in the mid-latitudes, a growing chorus of recent evidence suggests that the Arctic can indeed influence weather in distant regions (at least under some circumstances).”
Michael Mann, director of the Earth System Science Center at Penn State University, and not involved in the study, called the overall findings “sound.” But he pointed out that other factors, besides Arctic warming, are probably behind some of the trends in winter severity.
“For example, increased snowfalls in the northeastern U.S. (and Mid-Atlantic) are in part reflective of warmer ocean temperatures and stronger coastal storms,” Mann said via email, “which produce stronger nor’easters like we’ve seen this season, with larger snowfall totals.”
Cohen, separately, said he agreed that warmer oceans should boost snowfall totals but that the study shows polar vortex disruption in the Arctic is the biggest trigger for cold and snow in the East.
The next knot for scientists to untangle is to figure out exactly how the warming Arctic causes polar vortex disruption. Cohen and others have proposed conceptual models that link the disruption to declining Arctic sea ice and other factors, but this is still a matter very much under investigation.