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STUDY: ‘Global warming’ causing ‘cold snaps and snowstorms’

Snow comes down in Hagerstown, Maryland, on Tuesday, as the fourth nor’easter in less the three weeks hit the Northeast. The storm could break records for spring snowstorms. (Colleen McGrath/The Herald-Mail via AP)

At a Glance

  • The study offers a potential explanation for some noteworthy U.S. winter episodes in the past decade amid global warming.
  • But several climate experts are not yet convinced that changes in the Arctic are leading to an increase in extreme U.S. winter weather.

Episodes of winter warming in the Arctic are increasingly likely to be followed by severe winter weather in the eastern U.S., including cold snaps and snowstorms, a study published in Nature Communications this month finds.

The new study is the latest salvo in a longstanding debate on the links between midlatitude winter weather and conditions in the Arctic, where climate change has led to dramatic warming.

The study’s authors include Judah Cohen and Karl Pfeiffer (Atmospheric and Environmental Research) and Jennifer Francis (Rutgers University). Cohen and Frances are two leading proponents of Arctic-midlatitude linkage. Their work offers a potential explanation for some noteworthy episodes of bitter cold and heavy snow that have occurred across the eastern U.S. over the past decade in the midst of overall national and global warming.

Although winters in the NWS Eastern Region have warmed by more than 2°F since 1900, there has been little trend since the late 1990s, with frequent sharp swings between record warmth and bitter cold from month to month and winter to winter.

Globally, surface temperatures have warmed in all seasons and virtually all latitudes over the last 20 to 30 years, with the most notable exception being winters across much of the United States, northern Europe, and northern Asia. Unusually cold winters have been especially prominent over central Siberia.

Average winter temperatures (December through February) grew colder from 1990 to 2013 across the central and eastern United States, northern Europe, and northern Asia. Image credit: Judah Cohen, via James Overland, “A difficult Arctic science issue: Midlatitude weather linkages,” Polar Science 2016.

In their latest study, Cohen and colleagues examine U.S. winter weather at 12 U.S. cities from 1950 to 2016 via the Accumulated Winter Storm Severity Index, or AWSSI. The index uses temperature, snowfall, and snow-depth thresholds to calculate how rough a winter has been at each location. The researchers compared AWSSI values to temperature and air pressure in the Arctic, north of latitude 65°N and up through the stratosphere to about 100,000 feet.

“A strong relationship between a warmer Arctic and increased frequency of severe winter weather is apparent for all stations east of the Rockies, with the strongest association in the eastern third of the US,” the scientists reported. The links were stronger for cold temperatures than for snowfall, although the team found a robust correlation between Arctic warming and increased heavy snow in eastern cities.

The increasingly strong correlations between AWSSI and a warm Arctic do not prove cause and effect, as the authors acknowledged. It’s possible that some third factor, such as changes in tropical oceans, could be driving both Arctic warming and midlatitude winter weather. To get at this question, the authors examined whether Arctic warming preceded or followed U.S. winter weather events. They found that AWSSI values tended to be highest about five days after the Arctic warming, which suggests that the Arctic plays at least some role in triggering the U.S. cold snaps and snowstorms.

Several experts in climate change and midlatitude weather are not yet convinced that changes in the Arctic are leading to an increase in extreme U.S. winter weather. “The basic findings are sound, but I would take issue with some of the interpretation in the study,” Michael Mann (Pennsylvania State University) told the Los Angeles Times.

“The link between the warm Arctic and cold in midlatitudes is an obvious one: the cold air has to go somewhere. The question is where, and what is the cause. This study reaffirms the relationship, but not its cause,” said Kevin Trenberth of the National Center for Atmospheric Research (NCAR). “Although there have been cold outbreaks in winter in the northeast U.S. [in recent years], the winters as a whole have been among the warmest on record.”

Pointing to another factor in play, Trenberth and Mann assert that warmer ocean temperatures just off the U.S. East Coast could be making nor’easters more intense and increasing their snowfall. All else being equal, warmer offshore waters would lead to more moisture entering the storm and releasing more energy, as water vapor condenses into raindrops and snowflakes. The warmer waters could also beef up the dynamical contrast between mild marine air and frigid continental air. Warmer oceans would enhance snowfall only up to a point, though, since enough warming would eventually push the storm as a whole toward rain.
A group led by Gary Lackmann (North Carolina State University) has been using high-resolution computer models to examine changes in U.S. coastal storms, including nor’easters. In a 2015 study led by Christopher Marciano and published in the Journal of Climate, the team found that the amount of climate warming possible by the 2090s (roughly 5-7°F) would reduce snowfall in all areas affected by a nor’easter. The study found that future nor’easters may also have lower surface pressures and stronger winds.