By: AuthorAndrej Flis
Posted onPublished: 06/02/2022
The Polar Vortex is starting to power up unusually in the late Winter Season, reaching record cold levels in the lower stratosphere. This is not without consequences, as it will strengthen the polar circulation into late winter and will be ready to continue into early Spring, influencing the weather in the United States and Europe.
The Polar Vortex is a powerful short-to-medium term weather component during the cold season. It is strongly connected all the way from the ground up into the higher levels of the atmosphere.
For this reason, we always take great notice of the activity high above in the stratosphere and monitor it all the time. In the next days, the Polar Vortex will behave a bit unusual, as it will kick into overdrive, reaching near record-high power values for this time of year.
First, we will quickly and simply learn what the Polar Vortex really is and how is it so influential. We try to explain this in most of our winter articles, as this is an important part of every winter season.
Such powerful yet simple knowledge really helps to understand the bigger picture of how the weather works in the large picture.
NORTH HEMISPHERE POLAR VORTEX
The Polar Vortex can be simply explained as a very large cyclonic circulation, covering the whole north pole, down to the mid-latitudes. It has a strong presence at all levels, from the ground up into the middle atmosphere, having different shapes at different altitudes.
The Polar Vortex is so large that we have to divide it into two altitude parts. One is the lower (tropospheric) part and the second is the upper (stratospheric) part. The stratospheric polar vortex plays an important role in weather development, while the lower tropospheric polar vortex actually circulates the weather that we experience.
But what is this stratosphere? Well, the atmosphere has different layers. Our weather is found in the lowest layer of the atmosphere called the troposphere. It reaches up to around 8 km (5 miles) altitude over the polar regions and up to around 15 km (9-10 miles) over the equator.
Above it, we have a much deeper layer called the stratosphere. This layer is around 30 km/18.5mi deep and is very dry. The Ozone layer resides in the stratosphere. You can see the layers of the atmosphere on the image below, with the troposphere and the weather on the bottom and the stratosphere with the ozone layer above it.
The image below shows a typical example of the upper Polar Vortex at around 30km/18.5miles altitude in the middle stratosphere during winter. It has a very nice circular shape, with the temperature dropping quickly towards its inner core.
A strong Polar Vortex usually means strong polar circulation. This usually locks the cold air into the Polar regions, creating a milder winter for most of the United States and Europe.
As a contrast, a weak (wavy) Polar Vortex can bring very dynamic weather. It has a much harder time containing the cold air, which can now escape out of the polar regions, into the United States and/or Europe. Image by NOAA.
In the next image below, we have the polar vortex at a much lower altitude, around 5km/3miles. The closer to the ground we go, the more deformed the polar vortex gets because it has to interact with the mountains and overall terrain and also with the strong weather systems.
Be aware of the cold “arms” extending out of the polar vortex. They bring colder air and snowfall into the mid-latitudes. These arms pack a lot of energy and can create strong winter storms, like for example Nor’easters in the United States or very strong wind storms across the North Atlantic.
For an even better idea, we produced a high-resolution video for you, which nicely shows the Polar Vortex spinning over the Northern Hemisphere at the 30mb level, around 23km/14miles altitude.
Video shows the NASA GEOS-5 analysis for late January. Notice how the polar vortex covers a large part of the Northern Hemisphere. You can nicely see how it spins over the Northern Hemisphere, driving also the winter weather with its circulation.
Polar Vortex circulation (click to play)
Hurricane Marie moving across the Eastern Pacific – SWE / MK
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But why (and how) does the polar vortex even form every winter season?
Every year as we head into autumn, the polar regions start to receive much less sunlight. This way, cooling begins over the north pole. But as the polar temperatures drop, the atmosphere further south is still relatively warm as it continues to receive energy from the Sun.
You can see the winter solstice on the image below when the polar regions receive little to zero solar energy, compared to regions further south.
So, as the temperature difference towards the south increases, this also means pressure changes. A large low-pressure (cyclonic) circulation starts to develop across the Northern Hemisphere from the surface layers, far up into the stratosphere. This is known as the Polar Vortex.
While the stratospheric polar vortex is spinning high above our weather, it is still directly connected to the lower levels and can shape our daily weather in many ways, driving it as one large circulation over the entire hemisphere.
POLAR VORTEX POWERING UP
When looking at the polar vortex in the stratosphere, we tend to use the 10mb level. That is around 28-32km (17-20 miles) in altitude. This layer is considered to be in the mid-stratosphere and provides a very good representation of the general strength of the stratospheric polar vortex and its downward connection.
The strength of the polar vortex is most often measured by the power of the winds that it produces. This is typically done by measuring the zonal (west to east) wind speeds around the polar circle (60°N latitude).
On the image below we have the seasonal average zonal wind speed for the Polar Vortex at 10mb level. The black line is the long-term average. Winer season last year is the red line, and the blue line is the current winter season 2021/22. The yellow area shows the daily historical min/max wind speeds. Image from weatheriscool.com
It is obvious that this season, the polar vortex is having a near-constant power increase. It was fluctuating up/down but kept a steady uptrend in power. Looking at the black “average” line, the polar vortex typically starts the seasonal weakening in mid-January.
Below we have an extended ensemble forecast for the 10mb winds. It shows the polar vortex currently being quite stronger than average. The forecast keeps it at a very strong level, reaching unusually high power for this time of year.
Looking at the pressure anomalies over the polar regions in the past three months, we can notice an interesting progression. The next image below shows pressure anomalies from the surface into the upper stratosphere.
You can observe the strong low-pressure buildup in the stratosphere in late November. That was a strong polar vortex, connecting easily down to the surface levels in early December. But strong high-pressure anomalies have emerged over the polar circle in December, which pushed back against the stratosphere, “disconnecting” the upper and the lower polar vortex.
In late January, we can see the strong buildup of low-pressure anomalies in the stratosphere. That corresponds to the high power and circulation of the stratospheric polar vortex. It is currently not fully connected down on a hemispheric level but has a more local connection.
Looking at the current polar vortex development, we can see that the vortex is in a good shape. It has a slightly oval shape, but a strong and stable wind field. There is a semi-persistent high-pressure area in the North Pacific and East Asia pressing against it, creating its oval shape.
Taking a look at the temperature profile at the 50mb level (19km/12mi), we see its cold-core over the Arctic regions and Greenland. This altitude is considered to be the lower stratosphere and is more connected to the weather circulation in the lower levels.