The second is a tropospheric polar vortex: a west-to-east flow that encircles the pole in middle (between about 30 and 60 degrees N) or high latitudes (above 60 degrees N). Also called a circumpolar vortex, it resides in the Earth’s troposphere, the region of our atmosphere directly below the stratosphere. The circumpolar vortex is also known as the jet stream—a term you’re probably familiar with. The jet stream is what helps guide weather masses and makes flights from Los Angeles to New York City take less time than the reverse.

We should all be thankful for the stratospheric polar vortex. It’s what keeps bone-chilling air confined to the polar region… most of the time.

Because the Arctic is warming faster than the mid-latitudes, things are getting dicey. The stratospheric polar vortex’s winds (called the polar night jet) are driven by the difference in temperature (the gradient) between the pole and the mid-latitudes. When this difference is not as stark, some scientists believe that the winds weaken and break continuity, which can lead to disturbances in the otherwise compact vortex. This January, for example, the stratospheric polar vortex split in two (an event known as “sudden stratospheric warming”), forcing cold air to move downward into North America and Siberia.

Of course, the Arctic is warming at a rate almost twice the global average in part due to anthropogenic (human-caused) climate change, perhaps making vortex splits more likely. And there’s observational evidence that such weak polar vortex conditions are happening more frequently.

Scientists aren’t exactly sure what the ripple effect of such Arctic events will be on the rest of the globe. Jennifer Francis, a senior scientist at Woods Hole Research Center, and Stephen Vavrus, a senior scientist at the University of Wisconsin-Madison’s Nelson Institute Center for Climatic Research, co-authored a seminal 2012 paper that outlined a simple hypothesis for the effect that Arctic warming will have on the troposphere, for starters.

They suggested that a reduction in the temperature gradient between the pole and the mid-latitudes actually weakens or slows the jet stream, causing it to become wavier. In other words, Francis and Vavrus linked global warming directly to changes in the amplitude of the jet stream—and, by extension, its ability to pull cold air south.

Some scientists thought the hypothesis was elegant. Others found it flawed. While the temperature gradient between the poles and mid-latitudes does determine the strength of the westerly jet stream winds (“the relationship is solid,” Francis says, even if it doesn’t happen everywhere or all of the time), critics wondered whether or not the jet stream has, in fact, gotten wavier in recent years. So, they asked why a decrease in gradient should necessarily lead to an increase in waviness.

“There is no particular reason to believe that that one thing leads to a wavier jet stream,” Martin says, though he mentions that he has a paper in review that might help solve the problem.

Vavrus says that since he and Francis published the 2012 paper, research has emerged that shows more of an indirect linkage between climate change and the shape-shifting jet stream. Now, there’s evidence to support the idea that a weakened stratospheric vortex propagates downward into to the troposphere over the course of a few weeks.

“The troposphere and the stratosphere are working somehow in concert, but exactly how is yet to be determined,” Martin says.

In essence, mini vortices—called tropopause polar vortices—in the upper troposphere appear to form in the high Arctic during the Arctic night (when the region above the Arctic circle receives no direct sunlight) through radiative cooling off of Arctic clouds. “Those are relatively new discoveries,” Martin says. Because the jet stream has been migrating poleward just slightly, he and others think, these mini vortices—and the low-level cold air beneath them—are perhaps more likely to get caught in the jet stream flow, which can usher them southward.

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Martin describes it as a possible feedback loop: A warming Arctic in the troposphere feeds disturbances in energy to the polar night jet (the stratospheric vortex’s winds). Sometimes, the winds absorb the energy, thereby weakening the polar night jet’s structure—and potentially splitting the vortex in two. The troposphere rearranges itself as a result, sometimes putting these mini cold vortices in closer proximity to the jet stream. And if the jet stream continues to creep northward, as some scientists have posited, that effect could be exacerbated.

“It’s a new piece that I think a bunch of us are beginning to wonder about,” Martin says.

For Francis and Vavrus (whose 2012 paper doesn’t even mention the stratosphere), these new ideas show that the scientific process is working. The two researchers' initial hypotheses sparked debate within the scientific community, encouraging many other researchers to "try to get to the bottom of this question" by conducting their own studies, Vavrus says.

"When Jennifer and I put forth this paper, things kind of got ahead of us," he explains, adding that they had done some research to back up the hypothesis, but not enough to test their ideas in a comprehensive way. "Both Jennifer and I are fine discovering that a lot of what we put forth may not be valid or at least not as simple as we proposed. I think that’s been gratifying.”

In the nearly seven years since publishing the paper, "the story has become much more nuanced," Francis says. She and her team are learning that there are different ways a warming Arctic can affect the jet stream and weather patterns, depending on where in the Northern Hemisphere you’re looking—and that there might be a connection between more frequent splitting of the polar vortex and climate change. "It’s not just one story anymore," she says. "It’s actually many different stories."

David Holland, a professor of mathematics and atmosphere/ocean science at New York University’s Courant Institute of Mathematical Sciences, wants to see more data on why these dramatic cold air outbreaks happen.

“I think it’s going to take more data to be able to separate out the cause of this—I would argue, decades of data,” Holland says. “It’s a small planet, but it’s a big planet.”

Meanwhile, Vavrus says he is glad that the questions are centered on this topic, in particular, as opposed to larger issues.

“The debate going on about the role of the Arctic in shaping events, and whether climate change is playing a role—I think that’s exactly where the debate should be,” Vavrus says. “We shouldn’t be debating whether the climate’s changing or whether humans are responsible.”

Martin echoes that refrain. “This winter is still among the warmest ever, as measured over the whole hemisphere,” he says. “What’s going on outside your door does not tell you what’s going on outside the whole hemisphere’s door.” He compares it to traffic conditions: Understanding what the traffic is like on your street won’t actually tell you if traffic is congested elsewhere in the metro area.

Francis compares the difference between climate and weather to the difference between personality and mood. Your mood, like weather, is fleeting—but your personality, like climate, is a larger, more all-encompassing set of tendencies. There’s no question that Earth’s climate is warming, she says, even if Lake Michigan is completely frozen over.

The uncertainty, then, doesn’t lie in whether or not the climate is changing, nor whether or not humans are driving this change, Francis says. “The biggest uncertainties about the future are what we’re going to do.”

Caitlin Saks contributed reporting.