A pattern of warm and dry winters in the West, paired with frigid conditions in the East, has become more frequent since 1980, a trend that reflects the influence of global warming on the atmosphere in the Northern Hemisphere.
The pattern, described in a new study in the journal Atmospheres as a "North American winter temperature dipole," has been costly, including a multi-year drought in California and economically disruptive snowstorms in the Northeast, according to the scientists who analyzed climate trends in North America.
"Our historical analysis finds robust changes in the warm-West/cool-East pattern over North America within the last 35 years, " the authors wrote. "We show that the observed positive trend in the warm-West/cool-East events is attributable to historical anthropogenic emissions including greenhouse gases."
The trend will continue the rest of this century but then level off, as the East becomes too warm for extreme winter conditions, said Stanford University climate scientist Noah Diffenbaugh, one of the study's co-authors.
The past two years—Earth's warmest years on record—brought the greatest East-West temperature contrasts in the observational record. The coolest Eastern winter on record and the warmest Western winters since at least 1980 have all been recorded since 2013, the study found.
After establishing the increasing frequency of the warm-West, cold-East pattern, the scientists used a global climate model to determine that the probability of that increases with the rise of heat-trapping greenhouse gas emissions.
The research, which involved scientists from Stanford, Columbia University and NASA, adds to a growing body of work that links the buildup of greenhouse gases with major changes in atmospheric circulation. A subset of the same research group published a study in the journal Science Advances last April showing "statistically significant changes in the occurrence of atmospheric patterns associated with seasonal precipitation and temperature extremes."
Many recent studies suggest the meltdown of the Arctic ice cap is at least partly driving the changes.
"I think this is becoming increasingly apparent from the balance of available evidence," said University of Sheffield climate researcher Edward Hanna, who has published a study on how those changes are affecting northern Europe. Amplified warming in the Arctic has changed the pressure balance of air masses at high latitudes, which can affect the jet stream, "at times making it weaker and more meandering," said Hanna, who was not involved in the recent study. That, he said, steers weather patterns like the warm-West/cold-East phenomenon.
Hanna said the findings by Diffenbaugh corroborate his research that found an increase in a "blocking" weather pattern over Greenland that can lead to wet weather extremes in England. But Hanna cautioned that most of the data comes from the past few decades and in addition to the Arctic meltdown, changes in the tropics and natural variability are probably also factors in the complex climate equation. That means there is not yet a smoking gun to link Arctic melting with extreme weather, but the circumstantial evidence is growing. "Ten years more data in a strongly warming climate will help," he said.
During that time, many researchers will be trying to figure out if these patterns will be more persistent. If they are caused by the Arctic warmup, they should continue, said Rutgers University scientist Jennifer Francis, who has been investigating the link between dwindling Arctic ice and changes in the climate of the Northern Hemisphere.
Francis said several recent papers have shown that ice loss in the Barents and Kara seas influences the climate in Asia. "It's turning out to be a pretty solid link. When sea ice is reduced in those areas, it leads to very cold winters in central Asia," she said.
Other studies suggest a weakening of the jet stream in summer over the continents of the Northern Hemisphere has led to weaker surface winds and stagnant weather patterns, resulting in floods, droughts and heat waves, she added. For example, devastating floods in the Balkans have been attributed to a slowdown in the jet stream by scientists from the Potsdam Institute for Climate Impact.
"We're in this new world that's much much warmer with much less sea ice and that's changing the way the atmosphere behaves," said Francis. "It's an interesting time to be studying this, but the bad news is, we're watching this planet fall apart."