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Cold Snap Shakes Up Winter Weather Outlook


This is SCIENCE FRIDAY. I'm Ira Flatow. I was having a déjà vu this week. Looking back at our news archives, I discovered that almost exactly two years ago to this day, in 2011, we were hit by a frigid cold snap in the same parts of the country, the same parts of the U.S. that were hit this week. We're seeing freezing temperatures stretching from Midwest to the Northeast, snowstorms across the pond in England.

But what makes this weather weirder is that usually cold-weather Arctic cities like Oslo and Anchorage are actually warmer. They're warmer than the Lower 48. What the heck is going on? It turns out unusual activity over the Arctic Circle is to blame for the dropping temperatures. It's a phenomenon called Sudden Stratospheric Warming. And it could bring weeks of brutally cold weather. Better bundle up.

Jeff Weber is a climatologist and atmospheric scientist at the University Corporation for Atmospheric Research in Boulder, Colorado. He joins us by phone. Welcome to the program.

JEFF WEBER: Thank you very much, Ira, it's an honor to be on your show.

FLATOW: Well, you're welcome. Tell us: What is this Sudden Stratospheric Warming? It sounds like it's getting warmer, should not be getting colder.

WEBER: Yes, the stratosphere and the troposphere are two layers of our atmosphere, and they're kind of inversely related. So as the stratosphere warms, the troposphere can cool. The real dynamics of this situation, though, is that as the stratosphere has gone through this Sudden Stratospheric Warming event, as the stratosphere warms, it tries to then, in the air in the stratosphere, it descends back into what we call the polar vortex, which is where all of the cold air over the Arctic kind of usually kind of polls up and sits during the winter months.

Now as that bubble of stratospheric air comes down and descends into the polar vortex, it disrupts the circulation, and all that very cold Arctic air is then able to spill out from the Arctic down into the Lower 48 or into Europe.

FLATOW: And how long can you expect this to last for?

WEBER: Well, these events are very hard to forecast. What they do is they kind of disrupt the circulation around the planet, what we call the jet stream or the planetary wave. And that's one of the key factors that we use to do our forecasting. And so that jet stream being disrupted, it's very hard to forecast how long this is going to persist.

Generally, the Sudden Stratospheric Warming events tend to disrupt the polar vortexes for at least a month.

FLATOW: At least a month?


FLATOW: So the jet stream is dipped way down?

WEBER: Exactly right. The jet stream - if you remember last year, when we kind of had a winter with no winter, we had a jet stream that was very zonal, meaning it came across the time zones from west to east, and it kind of stayed right along the Canadian-U.S. border, and that's kind of the separation between the cold air and the warm air. So we were on the southern part of the jet stream, and so we were very warm all winter long.

Now with this Sudden Stratospheric Warming event and the disruption of the circum-polar vortex, our jet stream has started to buckle and meander quite a bit. And so now instead of going primarily from west to east, we have a big north and south component of that jet stream, allowing cities like Anchorage to be very warm while New York City or even areas in Georgia are much colder than Anchorage.

Georgia was 32 degrees when Anchorage was 48.

FLATOW: Wow, is this the same thing that happened a couple of years ago, or am I just imagining this?

WEBER: Now, we've had actually more stratospheric warming events in the past decade than we generally have in the past. This is somewhat of a new science, we've only been able to study the stratosphere since the '50s. So we don't really have any statistically significant time periods to be looking at. But we are seeing that these are occurring now almost every year. We're seeing them about every two years in the past. Now we're seeing them almost every year for the past decade.

And so this is becoming a more common event. So I'm not exactly sure what timeline you were looking at, but we did have a stratospheric warming event in 2011.

FLATOW: Yeah, is there some event that actually triggers this to happen?

WEBER: Yes, there is. There needs to be some sort of either dynamic, thermal dynamic or aerographical forcing down here in the troposphere to get this energy to go up into the stratosphere. Generally there's almost no interchange between the troposphere and the stratosphere, and so there needs to be some sort of forcing.

And so there was a huge storm, very similar to Hurricane Sandy, off of the east coast of Japan in the Kamchatka Peninsula of Russia, and it was actually a little bit more intense than Hurricane Sandy. It had a central pressure of 932 millibars, whereas Sandy was 942. So it was a very deep system, and it was a very broad system. It was about 1,400 miles in length.

And so you had this huge system off of the northeast coast of Japan that allowed that atmospheric wave to propagate upward into the stratosphere.

FLATOW: And so once you get this sort of conveyor belt going, well, you don't know when it's going to stop.

WEBER: Yeah, well, I think at this point in time the actual flux or the flow into the stratosphere has kind of been diminished. And so now we need to have the stratosphere kind of get back into its healthy state and re-radiate all that energy back out into space and quit disrupting our circumpolar vortex.

FLATOW: You tell it to do that for us, would you?

WEBER: I'll pull the lever.


FLATOW: So once you change the flow, and you don't need the stimulus, you don't need that storm anymore, you just try to wait for it to rebound, and that rebound could take weeks is what you're saying.

WEBER: Exactly right. Right now we have a very meandering, very non-definitive jet stream, and so that's allowing a lot of this cold air to just kind of flow out of the poles down into the continental United States. And so until that circumpolar vortex re-establishes itself, that cold air is going to be free to spill out all across the planet. And depending upon where these dips in the jet stream occur, that's where the cold air will flow out.

And so I see a new cold event for the East Coast around the end of January, into February. It's going to be preceded by a very warm event, a very warm rain event, up into the 60s, and then as that wraps around that storm, we're going to get that cold snap behind it. So you're going to have a very dramatic change from February 1st, from like January 31st we'll be like 60 and rainy and then February 1st it's going to be in the single digits again and very cold.

So we're going to have a very warm, cold, kind of a very diverse pattern for the next month.

FLATOW: You know, you sound like an early Punxsutawney Phil here.


WEBER: (Unintelligible) February 1st and that's very close to Groundhog Day.


FLATOW: Well, one last question about this because there is speculation that this weather may be due to the loss of ice during the summertime at the polar region. Is there any evidence for that being the trigger?

WEBER: No, that has a lot of other dramatic effects on our climate, but for stratospheric warming events, what we're looking at are very large systems, such as the one that was off of the northeast coast of Japan. Also it's been correlated to sunspot activity. So that's kind of a factor that doesn't affect the troposphere so much, but it does involve some (unintelligible) content action in the stratosphere. So we're seeing a correlation between a solar activity and then these big, very unstable systems late in the season like we saw off the northeast coast of Japan.

FLATOW: All right, well, people always blame stuff on sunspots, so...

WEBER: That's right, it's hard to prove them wrong.


FLATOW: Thank you very much for taking time to be with us, Jeff.

WEBER: My pleasure, Ira.

FLATOW: Jeff Weber is a climatologist and atmospheric scientist at the University Corporation for Atmospheric Research in Boulder, Colorado. Transcript provided by NPR, Copyright NPR.