Let’s take a brief look at what the Polar Vortex is and tools you can use to measure and forecast impacts to our weather patterns here in the US.
What is the Polar Vortex?
The term ‘polar vortex’ is the name given to the strong Stratospheric winds that surround low pressure found at the North and South Poles. They are a constant, always there phenomenon, supported by the Earth’s rotation and tilt. Since there are multiple, the term “polar vortices” might be the more accurate way to describe them collectively. However, we usually talk about them independently. Here in the Northern Hemisphere, we are closest to the North Pole and are mostly impacted by behavior of the “Arctic Polar Vortex”. We usually avoid the need to distinguish from the Antarctic Polar Vortex (South Pole), and just refer to ours as the “Polar Vortex”. I’ll be doing the same for the remainder of this page.
The Polar Vortex strengthens during late summer when sunlight near the North Pole begins to decrease and temperatures drop. It’s at its strongest during winter, when the North Pole can go months without sunlight. These lower temps cause stronger winds. And, due to The Coriolis Effect, the Earth’s rotation causes these stronger winds to spin counter-clockwise, or west to east. This is also why, in the Northern Hemisphere, most of our wind and weather travels west to east. The circular, spinning winds of the Polar Vortex act like a vortex (hence the name) drawing the surrounding air in towards the center and keeping cold air contained near the pole.
The Polar Vortex is a massive phenomenon. It can sometimes reach 6,000 miles in diameter, which is wider than all of Canada.
Layers of the Atmosphere
The Troposphere (where we live and our weather exists) starts at the surface, around 1000mb (or hPa) pressure, and continues up about 10 miles, around 100mb. (Going forward I will use mb and hPa interchangeably). As you go up the Troposphere, temperatures (red line) plummet. This air is part of the frigid arctic air that is held north by the Polar Vortex.
At around 100mb, the Stratosphere begins and continues up to about 30 miles above the surface, around 1 mb. The best place to analyze the behavior of the Polar Vortex in the Stratosphere is a height around 10mb (hPa).
How does the Polar Vortex impact our weather?
By now you should realize the Polar Vortex does not mean “a sudden burst of cold air from the north”. A more accurate name for that condition might be an Alberta Clipper. The Polar Vortex is always there.
When the Polar Vortex is “stable” and at its strongest, it keeps arctic air up away from the US, causing temperatures in the US to be more average for the season. However, when the Polar Vortex is “unstable” or weak, it releases its hold and can “leak” that frigid cold air into the Northern Hemisphere.
Disruptions to the Polar Vortex
It’s been hinted at several times already, certain atmospheric conditions can disrupt the Polar Vortex’s normally stable state. To explain this, we look at the jet stream. The jet stream is a large flow of strong westerly winds which support a lot of characteristics of our weather. The jet stream is not static either. Heat from the warm seas near the equator generate waves in the atmosphere (called Rossby waves) that usually take on a northern and eastern migration. These waves are also exaggerated during El Niño events.
The best way to see these waves’ effects are in northern tracks (ridges) of the jet stream. On the right side of Image 3 above, it shows what this might look like These waves are constantly crashing on the Polar Vortex, like tapping on a plate which is spinning on a stick.
These waves can get big enough to cause the Polar Vortex to wobble or even stop and change directions. This is called a Sudden Stratospheric Warming (SSW) event. When this happens, warm air from the outside rushes into the Artic, causing a rapid rise in temperature. Consequently, cold arctic air leaves the pole and moves down into the Northern Hemisphere. These major SSW events usually only happen once or twice a year, and are more common during winter. They typically last only a few days to a week, but they can have a major impact on our winters.
The biggest challenge with these SSW events is that we can see them coming, but we don’t know where the cold air will end up across the Northern Hemisphere.
Polar Vortex and the Polar Jet Stream
A quick point of clarification, while they have a relationship with one another, the Polar Vortex should not be confused with the Polar Jet Stream; they are two separate structures within our atmosphere. Both have winds that typically flow west to east (counter-clockwise), and they both change with the seasons. However, the jet stream exists in the Troposphere and plays a much bigger role in our day-to-day weather. Whereas, the Polar Vortex exists in the Stratosphere and plays almost no role in our day to day weather. However, certain events can cause it to have an impact, and when it does, the impact can be extreme. If you’re interested, read more on Jet Streams.
Analyzing the Polar Vortex
A great way to measure and forecast the Polar Vortex is to look at charts that forecast temperatures and zonal wind speeds (west to east) in the Stratosphere at the elevation where these vortex winds commonly occur. The most popular charts for this are the Temperature and Zonal Winds for 60°N (latitude) at 10mb (hPA). Below is an example of the Zonal Wind chart for 2023/2024.
Red: Actual, Green: Ensemble Member Forecasts, Yellow: Mean
How to read the chart? The blue dashed line represents 0 m/s, or no wind. For the Polar Vortex to be operating in “normal” or stable condition, winds would be plotted above this line, with a positive (west to east) value. A plot directly on the blue line (at 0 m/s) would represent a stall in the Polar Vortex. Anything below 0 (negative) would represent opposite: east to west winds or “abnormal” winds. The red line represents the actual measured values. The green lines represent the individual GEFS members’ forecasts, and the different shading and yellow line represent the modeling forecasts’ ranges and mean (respectively).
As you can see, the Polar Vortex operates with some clear seasonality. Winds start picking up in the fall, peak around mid-winter, and dip back down over spring. For anyone located in the upper midwest of the US , reading this in 2024, you will remember the week of frigid cold temperatures this past January. You can see that SSW event, where the red line dipped below 0.
Historical SSW Events
We can see these Sudden Stratospheric Warming (SSW) events from the past by looking at a historical charts of the zonal winds and temperatures at 10 hPa. For example, let’s take a look at some historical averages for zonal wind. Image 5 shows the average zonal wind for the month of January, for each year going back to 1980. Image 6 shows the same, but for the month of February.
Now let’s compare that to a table of documented historical SSW events. You can see the 1:1 correlation between the images above and below. I’ve color coded to help match the dates.
Forecasting the Polar Vortex and SSW Events
As mentioned before, we can forecast Polar Vortex events by looking at the Zonal Wind and Temp plots for ~60° N @ 10hPa elevation. Thankfully, most of the major models (ECMWF, GFS, etc) and Ensembles (GEFS, CMC-GEPS) have parameters for this. Let’s look at some examples of these using the Stratobserve plots.
The yellow lines are marking the signals that forecast models think the Polar Vortex might be impacted in the upcoming weeks. In the top of Image 8, there is fairly good agreement amongst the models/ensembles that winds will slow and even possibly reverse during the later part of February. In the bottom of Image 8, you can see the correlated rising temperature.
Of course, like any weather model or prediction…they are just that… a prediction. Even if this becomes a SSW event, knowing whether or not the US or any particular state will be impacted is another prediction in itself. But at least we know it is happening and can plan for it as a variable in our upcoming forecasts.
Resources
If you’re interested in monitoring the Polar Vortex to get a bit of an edge on what our winter weather might look like, I’d recommend looking at the following resources. Remember, when the Polar Vortex is strong (positive measured winds), cold air is held tight to the Arctic. However, when the Polar Vortex weakens, winds will drop near zero (or negative) and this likely will have a strong correlation to a multi-day plummet in temps across the Northern Hemisphere.
StratObserve – Time Series
There are some other great visualizations on this site.
GFS 16 Day Forecast – SSW Monitoring
Here you can find the CFS, GFS, GEFS Plots. Just make sure you’re looking at the NH (Northern Hemisphere) section, then find the 10mb column and look for rows TMP and U Wind.
NOAA Ozone Watch – Current and Historical
Similar to the SSW Monitoring page above, you can find plots for 60°N 10-nPa. You can select between wind and temp on the right.
ECMWF – Zonal Wind at 10-hPa
ECMWF – Mean Anomaly Temp at 10-hPa