Climate

What The Arctic Outbreak And Last Year’s ‘Polar Vortex’ Have In Common

CREDIT: NOAA

The weather forecast across the United States from November 16 through November 20.

The Arctic Outbreak, an unseasonable bout of frigid air that’s sweeping across the U.S. this week, is not the same as last winter’s so-called “Polar Vortex” event. Though both gave Americans the rare experience of breathing crisp, cold Arctic air, both were caused by different things: last winter’s “Polar Vortex” event seemingly random, and the current Arctic Outbreak caused by Super Typhoon Nuri in the Western Pacific.

But the two events do have at least two things in common. One is that that they’re both due to deep dips in the jet stream. In both cases, Arctic air from the Polar Vortex has been displaced to the south by a wavy jet stream, which brings coldness down to the temperate United States and leaves Alaska and the Arctic relatively warm. Sure enough, right now it’s warmer in Alaska than it is in Texas, and next week the National Weather Service predicts temperatures in Alaska will be 70 percent higher than average, with temperatures in Texas about 50 percent below average.

The jet stream for November 14, 2014.

The jet stream for November 14, 2014.

CREDIT: Accuweather.com

The second thing is that both events were unusual, catching Americans by relative surprise, and raising questions as to whether climate change played a role.

With both cases, the answer has been “maybe.” Though it may seem contradictory that extreme cold events could be linked to global warming, it’s been shown time and again that bizarre and unpredictable things can happen to the weather when heat accumulates in the atmosphere and ocean. A warmer atmosphere holds more water vapor, which in turn can make precipitation events — including snow — more extreme. And some scientists think a warmer ocean can make tropical storms more intense, which can drive big dips in the jet stream.

The latter is why some scientists, like bioanthropologist Greg Laden, think that climate change helped make this year’s Arctic Outbreak more intense.

“We can’t rule out climate change here,” said Laden, who writes for National Geographic’s Scienceblog. “[Super Typhoon Nuri] was in the top 2 or 3 hurricanes, maybe the top 2 of the year, in terms of overall strength. That’s because the Pacific has been really warm, creating a lot of extra hurricanes and extra strong hurricanes.”

While no weather event, including hurricanes, can be attributed directly to climate change (it’s really the wrong question — “All weather events are affected by climate change because the environment in which they occur is warmer and moister than it used to be.”), Laden is correct that Nuri was one of the two most intense storms on Earth so far this year. And Nuri, as Mashable’s Andrew Freedman explains, increased the chance of extreme weather across the U.S.

Typhoon Nuri, in purple, has dragged up warm air, in red, and pushed cold air down.

Typhoon Nuri, in purple, has dragged up warm air, in red, and pushed cold air down.

CREDIT: GFS/Levi Cohen

“Nuri really created this bomb effect,” Laden said. “It pushed up over in the west, and then we get this trough of cold weather coming down from the Arctic.”

Meanwhile, the Polar Vortex event last year was also speculated to be made more likely by climate change. A growing body of research led by Jennifer Francis, a research professor at Rutgers University’s Institute of Marine and Coastal Science, suggests Arctic warming causes less drastic changes in temperatures between northern and southern climates, leading to weakened west-to-east winds, and ultimately, a wavier jet stream like the one that caused last year’s Polar Vortex event.

“This kind of pattern is going to be more likely, and has been more likely,” she said at the time. “Extremes on both ends are a symptom. Wild, unusual temperatures of both sides, both warmer and colder.”

Francis’ research is not yet settled science. It’s disputed by some well-respected climatologists, including Dr. Kevin E. Trenberth, a distinguished senior climate scientist at the National Center for Atmospheric Research, who thinks more work needs to be done to show how climate change causes a wavy jet stream.

But now, with this autumn Arctic event, Francis’ prediction during last winter’s Polar Vortex seems to be coming true.

“The pattern that we’re in right now looks very similar to the pattern that we were in all last winter,” she told ClimateProgress on Thursday. “It’s hard to say that this particular pattern in connected with any particular cause in terms of climate change, but what we do know is that we’re seeing a very wavy pattern over the U.S. right now.”

Meteorologists currently predict that there won’t be a repeat of last year’s Polar Vortex event this winter. A recent report from the Climate Prediction Center said they think the western part of the U.S. should stay relatively warm throughout the season.

But Francis says that forecast could be changed — particularly if an El Nino, a phenomenon which has been predicted to occur for the last year, finally shows up. And who knows, she said: the jet stream could remain wavy, but set up in a different place in the U.S., making some areas abnormally warm.

“All we can say is that we think these wavy patterns are going to increase,” she said. “We can’t say where they’re going to set up — but there’s always other factors in the climate system that have the potential to impact weather as well.”