The Polar Vortex: Myth and Reality

The media was transfixed by a meteorological term this week:  the Polar Vortex.  Article after article in the print and online media described this feature as an unusual menace to be feared.  Groups like Climate Central had a field day connecting the errant vortex with global warming and the skeptic sites like Watts Up With That (WUWT) had even a bigger romp making fun of the global-warming-causes-cold-wave crowd.  Even the White House got into the act with a two-minute video and even an online meeting about this topic.

The trouble is that most of these groups made serious mistakes either in describing the polar vortex (also known as the circumpolar vortex) or its implications.  Let's set the record straight here.

Before I do so, consider that the term vortex has a lot of scary connotations for many people.   In the old sailing days, warnings about sea vortices that would suck in unsuspecting sailing ships were extant (see image)

while in the movie Wizard of Oz, Dorothy was sucked into a tornado vortex and was "no longer in Kansas."

More recently, terrifying vortices in movies such as Twister have sucked cows and cars into the sky, others like Sharknado have brought down man-eating sharks, and in the recent movie Thor:  Dark World a vortex associated  with "dark energy" almost ended the universe as we know it.

The Universe almost ended in Thor:  Dark World due to a dark energy vortex

So using the term vortex has a lot of scary connotations with it.    And the Polar Vortex of last week was scarier than most because it was heading our way.   Flights were being cancelled prior to its arrival, trains were stuck on their tracks.  There was no way to escape.  A chill runs up my spine just thinking about it.

Several of the media outlets were suggesting or hinting that this was something new or at least very, very unusual. They were wrong.

Let's begin by noting that many of media folks, White House "experts", and other authorities did not even get the basic description right.  The term Polar Vortex (or circumpolar vortex)  has been around for over a half century, something noted recently by one of my favorite weathercasters, Al Roker.  Here is the "official definition" in the authoritative Glossary of Meteorology produced by the American Meteorological Society:

 The planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere.

I was taught about the polar vortex in graduate school decades ago and it is described in a number of textbooks and hundreds of papers.  Several faculty in my department are experts in various aspects of its structure and origin.  I have attended dozens of seminars, where its structure and origin have been examined.  So I think I know enough to give you the basic description.

In the scientific literature, the polar vortices are shown to be strongest in the winter season and are associated with cooling air near the poles.  These features are most evident in the upper troposphere and the stratosphere, but have some reflection above and below those levels.  Here is a weather map for the 100 hPa pressure level (around 53,000 ft) in the lower stratosphere during the middle of the cold wave.  You see the L?  That is the low center...where the pressure level buckles down (you can

 also think of it as being where the pressure is lowest at a constant height)  The solid lines are height lines (the height of the pressure level above sea level).  The winds flow parallel to those lines and are strongest where the lines are closest together.  The wind blow with higher heights to the right, so the air is flowing counterclockwise around the low.

Folks, this is it.  THE POLAR VORTEX.  And you can see the low center is over the polar regions and NOT over the U.S.   You will notice a trough of lower height over eastern North America. THAT was associated with the cold wave.   Getting less scary isn't it?

As ones goes down lower, the wavy nature of the flow tends to increase.  So let's go down to the level that was most frequently shown by the vortex scaremeisters:  500 hPa (above 18,000 ft).  This figure show the height above sea level of this pressure level.  Lowest heights are shown in blue.  The global circulation is larger at this level (very normal) and you notice the circulation is much wavier, with each of the waves associated with colder air.  There are four main waves of roughly equal magnitude (although the eastern U.S. one is the strongest).   Instead of one big low as higher up, there is a modest low near the pole and four modest lows associated with each trough. 

 So there is no monster polar vortex that headed right for the U.S.!  Rather at this time and level the circulation has divided into a pattern of four troughs (a trough is a region of relatively lower heights).  If the trough position over the U.S. had been shifted 50 degrees east its location above, the cold air would have been over the Atlantic and would not have gotten the attention of the U.S. media.

Let me put this in context another way.  Check out the image below.  On the left is the average heights of the 500 hPa pressure surface from Jan 4-10th.  You can see the low heights over the Arctic region (purple colors are lowest heights) and the lobe of low heights over eastern N. America.  On the right, is the height anomaly (difference) from climatology (or normal conditions).  Yes, there is an area of lower height than normal over the eastern N. America, but an even more unusual trough was found over the eastern Atlantic (lighter purple).  And there are other areas of higher than normal heights.   We could call those the antivortices; that would really terrify folks.  Like antimatter.

 The polar vortex occasionally breaks down during events called stratospheric warmings, when the stratosphere over the poles rapidly warms as energetic waves propagate upwards from low levels.

So I think you get the picture.  We had a strong upper level trough over central/eastern N. America that brought cold air into the region, for some the coldest in about a decade.  No polar vortex moved over the U.S, just a cold extension of the mother circulation.    Other locations around the Northern Hemisphere experienced similar conditions.

Some of the media, politicians, and others did not accurately portray the nature of what happened last week.  Some suggested that the event was somehow "consistent" or "may" have been forced by global warming, a claim that has very little support from either observations or theory (see previous blog).  Global warming skeptic folks who claimed this feature was somehow proof that global warming was nonsense were equally culpable for playing with the truth.  As they said in Sharknado:  "enough said".

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