The overarching weather pattern responsible for the contrasting extremes between the coasts is known as the North American Winter Dipole. It is fancy term to describe abnormally warm conditions in the West and cold conditions in the East. Under such a pattern, the jet stream, the super highway for storms that divides cold and warm air, surges north in the western half of the nation, and crashes south in the eastern half.
Such a pattern is developing over the United States right now. It is the same pattern that was responsible for California’s historic drought from 2013 to 2016. UCLA climate scientist Daniel Swain coined the term “ridiculously resilient ridge” to describe the bulging jet stream along the West Coast. It blocked rain-bearing storms from penetrating inland and was associated with a pool of warm water known as the “blob” — which drew north a host of sea creatures seldom or previously never seen along parts of the Pacific Coast.
On the downhill side of this relentless ridge, where the jet stream plunged in the eastern U.S., the polar vortex was unleashed, particularly in the winters of 2013-2014 and 2014-2015. Brutal blasts of frigid air punished cities from Minneapolis to New York.
This dipole pattern has shown a tendency to happen more frequently in recent decades as the climate has warmed. Swain, author of the popular California Weather Blog, wrote, “There has indeed been an increase in the number of days each winter characterized by simultaneously very warm temperatures across the American West and very cold temperatures across the East.”
Climate warming due to human influence may well be playing a role in the recent prevalence of this “warm west-cold east” pattern.
“Using climate model simulations, we further found that an increase in extreme temperature dipole days like those we’ve observed in recent years is considerably more likely in a climate with rising greenhouse gas concentrations than in a hypothetical climate without human influence,” Swain wrote, citing a study he contributed to last year.
Swain’s blog post notes that the increase in this dipole pattern is related, in part, to faster warming in the western United States relative to the east. But he wrote that eventually, once warming accelerates across the entire country, this dipole pattern may fade.
“It will warm so much everywhere, you just don’t get that kind of cold in the East,” he said in an interview. “You start to lose that Arctic air pool to generate that cold winter air in the first place.”
In a separate study that explored how this dipole pattern develops, Swain and colleagues linked its formation to a particular configuration of temperatures in the Pacific Ocean. Specifically, Swain found warmer-than-normal waters in the western tropical Pacific Ocean and cool waters in the eastern Pacific are most conducive to such a pattern.
Perhaps not coincidentally, as the pattern relates to climate change, the western Pacific Ocean has been warming faster than any other region, Swain said. But why may that be?
An independent study, published in Nature Communications on Tuesday, relates Arctic sea ice loss — due to climate change — to this warm, dry pattern in the western United States and to the arrangement of weather systems in the Pacific Ocean that helped it form.
“In a two-step teleconnection, sea-ice changes lead to reorganization of tropical convection that in turn triggers an anticyclonic response over the North Pacific, resulting in significant drying over California,” the study’s abstract says.
Swain, along with the authors of this new Nature study, are careful to point out that links between the Arctic and mid-latitude climate are still novel, but a growing body of work is making connections.
“While more research should be done, we should be aware that an increasing number of studies, including this one, suggest that the loss of Arctic sea ice cover is not only a problem for remote Arctic communities, but could affect millions of people worldwide. Arctic sea ice loss could affect us, right here in California,” said Ivana Cvijanovic, lead author of the Nature study, in a news release.
Judah Cohen, an atmospheric scientist at Atmospheric and Environmental Research not involved this research, has published studies connecting a warming Arctic and decreasing sea ice to increasing cold and snowy weather patterns in the eastern United States. He said he agrees with the general idea of this new research.
“A warm Arctic favors cold weather in the East, and so it would make sense for it to also lead to warmth in the West,” he said.
Meanwhile, the forecast is for this warm west-cold east pattern in the United States to persist for at least the next two weeks. This means a prolonged period of critical fire danger and expanding drought conditions in Southern California, while the eastern United States shivers in an early spell of mid-winter-like chill.