This past year, the U.S. Midwest and Northeast suffered through a long, cold winter. The West, in contrast, saw a warm, dry season. Yet both were symptoms of the same cause—a leaky polar vortex. This situation gets even more counterintuitive, too. A contentious idea that's been kicking around for a while posits that, as backwards as it seems, a weak polar vortex (and the resulting cold winter) might actually have been a sign of global warming. And now a new study adds evidence that this may, in fact, be the case.
Here's how it works. Normally, cold Arctic air is confined to the polar region by a strong atmospheric circulation around the pole. (That's the polar vortex.) Last year the polar vortex weakened, and the cold Arctic air spilled south, flash freezing the eastern half of the continent.
The key to making sense of how a leaky polar vortex could be tied to global warming lies in the rapid recent decreases in Arctic sea ice cover. As the sea ice melts, some researchers think, it changes how the jet stream crosses North America. The warming Arctic can mean the jet stream blows more slowly, and when that happens, the jet stream can get wobbly and “trap weather systems — including both heat waves and cold snaps — in place for an unusually long time,” says Michael Lemonick for Climate Central. (A wobbly jet stream could have consequences not just for the polar vortex, says Seth Borenstein for the Associated Press, but for a whole range of extreme weather events.)
Last winter Andrew Freedman wrote for Climate Central about the potential climate change connection of the polar vortex. Though, as Freedman and others noted, the connection is mostly correlation: there seems to be a link between melting sea ice and cold winters in the U.S., but scientists aren't really sure about how, exactly, they might be connected.
In their new study, an international team of scientists lay out what they think could be a physical mechanism that could explain the link between melting Arctic sea ice and a weakening polar vortex.
The lack of sea ice, the scientists say in their study, strips the Arctic ocean of the ice's insulating properties. This causes more heat to move from the water to the air. This change in energy flow affects the air pressure in the Arctic and changes how air moves around the region. That, in turn, could cause the polar vortex to weaken and the cold air to spill south.
Whether the recent spate of extreme weather can be tied conclusively to global climate change is still a matter of considerable scientific debate, but this new study is just another piece of evidence in the pile.