The return of the 'polar vortex'

The Point Everyone Is Missing About The Return Of The ‘Polar Vortex’

Think Progress,

14 July, 2014

It will be unseasonably chilly in the eastern part of the United States this week, due to a peculiar weather pattern that’s causing deep waves in the jet stream. One of those big waves is bringing cool air down from the northeast Pacific and the Arctic. This will cause nighttime temperatures to be, on average, in the 50s or 60s on Tuesday and Wednesday.

So is it the polar vortex, or isn’t it? That’s been the big debate among meteorologists and news outlets. But according to at least one scientist, that debate misses a more important point about the unusual weather pattern sweeping the United States — that it’s causing extreme weather in other parts of the country.

“We’ve got this cool air coming down over the eastern half of the country, and that’s gonna just be kind of nice,” said Jennifer Francis, an atmospheric scientist and research professor at Rutgers University’s Institute of Marine and Coastal Sciences. “But along the east coast, we’re looking at storms and floods. On the west coast, we’re looking at heat and fires. And it’s all part of this jet stream pattern.”

The jet stream is set to make a huge dip into the eastern U.S. this week.

CREDIT: ACCUWEATHER

A particularly wavy jet stream is what is causing the so-called “polar vortex,” or cold air from the Arctic, to travel down to the United States, Francis said. The dramatic and unusual southward swoop, shown in the map on the right, allows air from the cold north to travel south. The same thing happened this past January, when a dramatic southward swing of the jet stream brought increased cold air to an already-freezing region. That was big news, extreme weather-wise.

Now, the “polar vortex” is making for fairly mild weather. But at the same time, that same wavy jet stream is swinging northward in the western United States, bringing increased heat to an already-dry and wildfire-stricken region. Extreme weather-wise, Francis says, this is a bigger deal.

“They’re going to be suffering out there,” she said. “That part of the story is more important in a way because they’re already dealing with such a drought that this is just going make the drought and the fires and everything much worse, while in the east it’s just going to be a nice couple of cool days to break up the hot summer.”

As the jet stream dips upward following its southward travels, the east coast will also see some more newsworthy weather, Francis said. “That’s where it’s really stormy,” she said “You see storms and flooding on the east side of those southward dips.”

Weather forecasts are predicting thunderstorms and flash flooding up the East Coast this week, from Atlanta to New York.

This particularly wavy jet stream has been largely attributed to Japan’s typhoon Neoguri, the immense energy of which likely caused a swift acceleration of the North Pacific jet stream. Francis says to think of it like taking a jump rope and giving it a whip with your hand — except the jump rope is the jet stream, and your hand is a typhoon. The whip, intensified by your strength, travels along the rope with a ripple effect.

In addition to being powered by storms, though, the jet streams movements may be impacted by a warming Arctic. Francis’ research suggests that as the Arctic warms due to climate change, less drastic changes in temperatures occur between northern and southern climates. This leads to weakened west-to-east winds, and ultimately, a wavier jet stream.

“Our research suggests that these kinds of patterns where the jet stream is taking these big northward and southward swings is going to become more frequent in the future, at least partly related to the fact that the Arctic is warming so fast,” she said. “We can’t say that this particular example is related to the Arctic. That being said, the Arctic is very warm right now, and so it could be contributing to making this pattern more wavy than it would have been otherwise.”

Francis’ research on Arctic warming being connected to a wavier jet stream is still disputed. Dr. Kevin E. Trenberth, a distinguished senior climate scientist at the National Center for Atmospheric Research, told ClimateProgress in January that he was still skeptical of the connection. “[It] shows a correlation, but correlation is not causation,” he said at the time. In a recently published update on the linkage between Arctic warming and its impact on jet stream patterns, The National Research Council was optimistic about the connection, but noted it was still too early to draw concrete conclusions.

The weather the U.S. is experiencing now is the result of a similar offshoot of the Arctic jet stream that brought down the colder temperatures in January, though this week’s weather will include a good deal of northeast Pacific air, rather than just Arctic. As for whether both instances should really be called the “polar vortex,” Francis says no, saying it’s a mischaracterization of the term. Really, she said, the polar vortex refers to the whole Arctic jet stream taken together, not the actual cold air inside of it.

Some still feel comfortable using the term, though, since the flow of air that’s coming down is — at least partially — coming from the Arctic. Jason Samenow has a piece in the Washington Post defending the term, detailing three graphs that he says prove this week’s weather patterns are a “textbook case of polar vortex influence on mid-latitude weather.”

“In my view, discussing the polar vortex’s significant role in mid-latitude cold air outbreaks is a great way to engage the public in the intriguing dynamics of meteorology,” Samenow wrote. “Some critics are being too literal about its definition and/or burying their heads in the sand, blinding themselves from a fascinating weather reality.”