What is the Polar Vortex?

The recent blasts of very cold Arctic air which has invaded a large chunk of the United States, giving areas some of the coldest weather they've experienced in years has driven the media to discover the term "Polar Vortex" and run absolutely wild with the term.  With that,  I thought this would be a great time to dedicate a post about the polar vortex and explain exactly what the polar vortex is as well as discuss it's role in meteorology and in weather forecasting.  While this post will get a bit technical, everything will be explained as clear as possible and as simple as possible.  

What is the Polar Vortex?

As defined by the American Meteorological Society (AMS), the Polar Vortex is, The Planetary-scale cyclonic circulation, centered generally in the polar regions, extending from the middle troposphere to the stratosphere.  In other words, the polar vortex is an area of low pressure which is centered over the polar regions of the Northern Hemisphere and the polar vortex ALWAYS exists.  In the Northern Hemisphere, the polar vortex typically has two main centers of circulation with one typically located near Baffin Island and the other center of circulation typically located over northeast Siberia.  

The polar vortex (which remember is an area of low pressure with two circulation centers) varies in intensity/strength throughout the course of the year and the location of the two main centers also changes throughout the course of the year.  The polar vortex is typically more "active", meaning it's typically a more dominant signal, during the Northern Hemisphere winter as this is when the temperature gradients between the Equator and the North/South Poles is the greatest.  This leads to stronger wind gradients along with stronger pressure gradients.  

How is the Polar Vortex Measured?

We measure the strength of the polar vortex by the phase and strength of the Arctic Oscillation (AO).  The National Climatic Data Center (NCDC) defines the AO as, a large scale mode of climate variability, also referred to as the North Hemisphere annular mode.  The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55N latitude.  The Arctic Oscillation is measured in three different phases; negative, neutral, and positive.  Each of these three phases define the state of the AO and each of these phases influence the weather pattern not only across North America but other areas of the world as well.  Below we will take a brief look into each phase and what they mean in terms of weather.  

• Negative AO phase -  When atmospheric pressures up at 500mb (about 18,000ft AGL) associated with the Polar Vortex circulation are weaker than normal (meaning HIGHER atmospheric pressure values compared to the average) and zonal wind circulation around the vortex are weaker than normal, this indicates a negative phase of the AO.  During this phase, it becomes much easier for colder Arctic air to invade the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most negative AO January's along with the temperature anomalies for the month of January for each of the top most negative AO January's:

 

• Neutral AO Phase - The phase of the AO is termed neutral when there is no clear phase signal.  Typically, you're dealing with height anomalies in the Arctic regions which are much more neutral (not much above or below normal).  The graphic below displays 500mb height anomalies for the 6 January's with the three highest negative values and the three lowest positive values.  Looking at the height anomalies over the Arctic regions we see more neutral heights: 

• Positive AO Phase - The positive phase of the AO is a mere opposite of the negative phase.  500mb heights associated with the positive phase are lower than normal and the zonal wind circulation around the vortex is stronger than normal, meaning stronger westerlies.  These stronger westerly winds work to keep colder air bottled and locked up across far northern latitudes and increases the likelihood of above-average temperatures across the northern tier of the United States.  Below are the monthly averaged 500mb heights/zonal winds for the Top 10 most positive AO January's along with the temperature anomalies for the month of January for each of the top most positive AO January's:

Now that we have taken a brief look into what the polar vortex is and how the strength of the polar vortex is measured let's do a quick review.  

• Polar Vortex is an area of low pressure centered over the polar regions of the Northern Hemisphere and ALWAYS exists.  
• The strength of the polar vortex is measured by the Arctic Oscillation (AO).
• The Arctic Oscillation had three distinct phases; negative, neutral, and positive.  
• Each phase influences the weather pattern across North American and across other parts of the world as well.  

We've discussed what the polar vortex is and how the strength is measured but we will now take a look into other aspects of the polar vortex, such as movement.  While the polar vortex is normally situated over the polar regions of the Northern Hemisphere, certain atmospheric phenomena can actually cause the polar vortex to shift locations or even split into two separate areas of low pressure.  None of this is that uncommon either.  

In the case of this past Arctic Outbreak which plagued the United States east of the Rocky Mountains the first week of January, the weather pattern across the Northern Hemisphere allowed and favored for the polar vortex to actually sag southward into southern Canada and northern-tier of the United States, bringing down the brutally cold air we had.  Below are the average 500mb heights across the Northern Hemisphere, along with the average 500mb zonal winds from January 2nd, 2014 to January 6th, 2014.  The main areas of interest are all numbered.  Numbers 1-3 are showing the area of higher heights, indicating the ridging, while number 4 shows the very low heights, indicating troughing, associated with the polar vortex.  Number 5 shows the negative, or easterly zonal winds which allows the polar vortex to sink southward:

 What we all can take from this is, the polar vortex is nothing new and it's not some brand new creation caused by "Global Warming" or "Climate Change".  The polar vortex has always been in existence and was first discovered as early as the year 1853.  Despite alot of the talk about it within the media as well as social media, this isn't scary and it does not signal anything about the future.  It is not uncommon at all to have the polar vortex sag southward towards or even into the United States and this has happened many times before.  With all of this, hopefully there is now a better understanding of what the polar vortex is, how we measure it, what it does, and how it affects the weather pattern across North America.