The Polar Vortex is facing a stratospheric warming event, which will be strong enough to cause a temporary split of its outer core. In the meantime, the same dynamics will also shape the weather circulation in the lower levels, as cross-polar ridging dominates into the official start of the meteorological spring season.
The Polar Vortex is a powerful short-to-medium term weather driver during the cold season. It has a strong connection all the way from the ground up into the higher levels of the atmosphere.
For this reason, it is crucial to monitor the activity high above in the stratosphere. In the next few days, the Polar Vortex will experience a first disruptive warming wave, influencing the weather as well.
First, we will quickly and simply learn what the Polar Vortex really is and why is it so important. We tend to explain this in most of our cold season articles, as this is an important part of the global weather system.
NORTH HEMISPHERE POLAR VORTEX
The Polar Vortex is simply explained as a very large cyclonic circulation, covering the whole north pole, down to the mid-latitudes. It is present 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 atmospheric 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 forcings, while the lower tropospheric polar vortex actually is the weather circulation that we experience.
But where is the 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 is found in the stratosphere.
You can see the layers of the atmosphere in the image below. The troposphere (blue layer) and the weather are on the bottom and the stratosphere (green layer) 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 the cold season. 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. 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 can bring colder weather 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 a more visual idea, we produced a high-resolution video, which nicely shows the Polar Vortex spinning over the Northern Hemisphere at the 30mb level (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, helping to drive the weather below with its circulation.
But why (and how) does the polar vortex even form in the cold season?
As we head into autumn, the polar regions naturally receive much less sunlight and thermal energy. 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 a change in pressure. 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 the weather, it is still directly connected to the lower levels and can shape our daily weather in many ways, driving it in one large circulation over the entire hemisphere.
POLAR VORTEX IN TROUBLE
When looking at the polar vortex in the stratosphere, we typically 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 is a very good representation of the general strength and status of the stratospheric polar vortex.
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, and the blue line is the polar vortex strength in the 2021/22 season. But the red line is the forecast, which shows the rapid weakening of the polar vortex is coming in the next few days.
Overall, 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 a closer look at the ensemble forecast for the 10mb winds. It shows the polar vortex currently being quite stronger than average. But a weakening/deceleration event is coming, shaking the polar vortex down to its core.
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, and now in February, we can see the strong buildup of low-pressure anomalies in the stratosphere again. 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 connection in the core.
Looking at the current polar vortex development, we can see that the vortex is already being under pressure from the European and the North Pacific sector. It has a slightly oval shape, but a strong and stable wind field.
Taking a look at the temperature profile at the 10mb level (30km/18.5mi), we can see a warming wave developing from the North Atlantic into the Siberian sector. This is a result of wave activity from below in the Euro-Atlantic region.
Taking a look at the temperature profile lower down 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.
Below we have a zonal mean (average) temperature for this 50mb level over the polar circle, from NASA. You can see a steady cooling, continuing with a strong pace in the past weeks, reaching record cold levels in mid-February. But now it’s slowly rising back with incoming dynamics.
The vertical pressure anomaly profile reveals a connection between the stratosphere and the lower levels. We can observe the stratospheric low-pressure anomalies extending down in the -60 to -90 Longitude area. That is exactly where the low-pressure system over eastern Canada and Greenland can be found.
We can nicely see that if we look at the latest 5-day forecast of the pressure anomalies across the Northern Hemisphere. Notice the large area of lower pressure and strong circulation in the blue oval. The Greenland/Canada region is the connected core of the polar vortex, also being driven by help from the stratosphere.
At the same time, a strong ridge is rising from western into northern Europe, which is the source of wave activity and warming energy in the stratosphere. A ridge is also rising over western Canada up into the Arctic circle, creating another high-energy area in the stratosphere.
This is a special pattern of the lower level polar circulation. As the core circulation is stronger, it can also pull more cold air from the northern regions. On the forecast below we see a new cold wave spreading down now into the northeastern United States.
As the cold air has moved out of the western Arctic Ocean, it was replaced by warmer air, creating that strong warm anomaly in the Arctic.
Taking a closer look at the end of this period over North America, the coldest air is moving out of the northeastern United States by mid-next week. Warmer air will move in from the west and south, as a high-pressure system will expand across the continental United States.
STRATOSPHERIC POLAR VORTEX SPLIT
Going ahead into late next week, we can see the polar vortex being pressed more strongly from the North Pacific as well. The pressure from both sides will be enough to elongate the polar vortex and even split its core into two zones. As a whole, the polar vortex will survive, but its core will be split apart.
Looking at the temperature profile of the 10mb level (30km/18.5mi), we can see a warming wave now developing on the other side of the polar vortex. We can also see the split outer core. This is also a result of the strong dynamics at the surface.
Looking at the lower stratosphere levels at 50mb, the core here is also elongated. We can see a push from both sides of the hemisphere, but it is not strong/coordinated enough to completely split the polar vortex in half.
The polar vortex joint ensemble forecast for 10mb level (30km/18.5mi) shows the increasing wind speeds as the first warming wave presses in. But it quickly starts weakening as the second wave presses from the Pacific, splitting apart the outer core of the polar vortex and changing its shape.
On the vertical pressure anomaly profile, we have the best view possible, of this activity affecting the polar vortex. Marked with the red square is the high-pressure buildup from the surface up, into the stratosphere, splitting the outer core of the polar vortex.
Going into the second week ahead, we can see this happening in the lower levels. We have the strong double ridge cutting the polar circulation across the polar regions. That leaves two main low-pressure areas in Greenland and over Siberia. This translates upwards into the stratosphere.
The temperature forecast for this period shows the massive warm anomaly pooling over the Arctic, as the high pressure takes over. Colder air is spilled out, into the northwestern United States, eastern Canada, and Europe.
As the main core of the polar vortex moves further out towards Greenland, it leaves the door open for high pressure and warmer temperatures over the eastern United States. Low pressure in the west pulls down colder air into the west/northwest parts of the United States.
NOAA OFFICIAL 8-14 DAY WEATHER OUTLOOK
This is all nicely captured below on the official NOAA/CPC 8-14 day forecast for the United States. Colder than normal temperatures will prevail over much of the western half of the country. Warmer temperatures will return in this period to the east/southeast, under the influence of the high-pressure system.
Looking at the official NOAA precipitation forecast, we have more precipitation over much of the northern and eastern parts of the United States, in the most southerly flow. Less precipitation is expected under the drier cooler northerly flow in the west and southwest.
POLAR VORTEX FROM BOTTOM TO TOP
Below we have a wind speed forecast for the atmosphere up to around 50k/31mi altitude. We can see strong wind speeds in the stratosphere in the final days of February. But we have marked the obvious upcoming disruption, which starts at the bottom and affects all the atmospheric layers up into the Mesosphere.
Looking at the same graphic, but at the anomalies, it shows us where the wind speeds are weaker/stronger than normal. We can see the obvious weakening with the upcoming warming/disruption, affecting all the layers high up into the atmosphere.
In the next graphic below, we have the pressure anomalies in the atmosphere shown over time in the Northern Hemisphere. Higher numbers (blue colors) mean low pressure and stronger polar vortex strength.
We have had a very strong polar vortex in February. But the forecast part shows the lower high-pressure buildup, sending wave energy into the upper levels of the Stratosphere, deflecting it back down.
THE FUTURE OF THE POLAR VORTEX
Going deeper into the second week of March, we can see that the polar vortex will recover its shape and circulation. But some of its power is lost, and a high-pressure area will remain over North America in the stratosphere, preventing the polar vortex from going into overdrive again.
At this point in time, the temperatures are steadily increasing over the North Pole, as the Sun has returned and is getting stronger each day. That is slowly eroding the polar vortex, which feeds on the temperature difference between the cold north pole and warmer regions further south.
Looking at the temperature profile of the 10mb level (30km/18.5mi), we can see that the warming wave is mostly gone, but overall temperatures in the stratosphere are rising. The cold core of the polar vortex is very weak and will get even weaker as temperatures are rising.
At this time, the lower stratospheric polar vortex still retains its cold-core, as the lower stratosphere is not as affected as the upper levels. We can also see the remaining warming wave at this level, circulating around the cold-core.
The vertical pressure anomaly profile reveals the “standoff” between the polar vortex and the high-pressure area over North America (black box). But at the lower levels, the picture is quite different, showing a disconnect between the direct circulation influence (violet box).
This connection has been a regular feature in the past weeks, helping to drive the cold winter weather down into the eastern United States. A strong polar vortex core over northern/eastern Canada has been pumping down colder air in the northerly flow.
Looking at the lower pressure patterns towards mid-March, the ridging is still connected over the polar regions. But the polar vortex core is reorganizing over Canada again, as the ridging also strengthens in the North Pacific.
Thanks to that ridge in the North Pacific, that means lower pressure and northerly flow over the western United States. That, in turn, means southerly flow and high pressure over the southeastern United States.
Looking at temperature anomalies for this period, we can see the return of the cold pooling over Canada. Temperatures drop as the circulation strengthens over Canada, also aided by the ridge in the North Pacific, together bringing more cold air out of the Arctic regions.
The ensemble forecast gives an example scenario, where surface temperatures remain warmer than normal over much of the southeastern United States. But due to the pressure pattern, colder than normal air is spilling down from the north into much of the western and northern United States.
The snow depth forecast towards mid-month still shows snow cover in the Rocky Mountains in the west and over most of the northern and far northeastern regions. The warmer southerly flow will erode any remaining snow cover across the south-central United States.
SPRING 2022 BEGINS
Meteorological seasons cover 3-months, divided into periods based on the temperature cycle. So Winter covers the 3 coldest months, Dec-Jan-Feb and Summer covers 3 warmest months, Jun-Jul-Aug.
In-between we have the transitional seasons of Spring and Fall. The meteorological spring season covers the Mar-Apr-May period. So with March, the meteorological Spring officially begins.
The latest extended-range forecast from ECMWF was released on Thursday, so we can look at some March trends. First, we need to look at the state of the Polar Vortex. Below is the forecast for the stratospheric polar vortex, which shows warm anomalies over the polar regions in the second half of March.
This shows continued weakening, temperature-wise, as the sunlight returns to the polar regions, increasing the temperatures in the stratosphere.
The extended-range polar vortex wind forecast below shows a sharp drop in wind speeds in the stratosphere, resulting from the current warming and the core split. But there will be little recovery and a slow gradual power drop towards the end of the month.
The corresponding lower pressure pattern for mid-March shows the higher pressure remaining over the North Pacific. A broad low-pressure zone remains over Canada and the northern United States. Low-pressure extends into the North Atlantic, building high pressure and warmer temperatures over Europe.
The temperature forecast for North America shows the colder temperatures extending over most of the northern United States. A low-pressure area over the north usually means a warmer zonal (westerly) flow into the southern half of the United States, as seen in the ECMWF forecast below.
Towards the late month, we are starting to see the low-pressure zone over Canada pulling back to the north. This will promote high-pressure expansion over the southern United States. Low pressure remains over southern Greenland, keeping the high pressure over Europe.
Temperature trends for this period show continuation of expanding warmer anomalies over much of the central and southern United States. But this pattern however still permits the occasional drops of cold air from the north into the midwest and the east, but not to a degree that it would show cold anomalies on the forecast.
As we go into early April, the sunlight now gets much higher over the North Pole, further increasing the temperature in the stratosphere. This means that the polar vortex is slowly dissolving, as the temperature difference with the south decreases. The polar vortex is seen shifting to the Siberian sector as warmer than normal temperatures appear over the polar regions.
It remains to be seen how the polar vortex slows down this season, and if there will be more warming events. But it can have a huge impact on the weather development over the course of the spring season, so we will monitor it closely.
We will quickly look at the latest seasonal forecast trends for the 2022 Spring season, from one of the main global long-range models.
SPRING SEASONAL OUTLOOK
Our seasonal model of choice this time is the UKMO model, from the United Kingdom Met-Office. This has typically been a good performer in the past seasons, so we also tend to use it in our standard “suite” of long-range model forecasts.
UKMO has a similar pattern as the ECMWF, with a stronger low-pressure zone over Canada and the North Atlantic. It shows the same strong blocking high-pressure in the North Pacific.
This shows a much stronger positive NAO pattern, creating an amplified jet stream into northern Europe and a stronger high-pressure zone over Europe.
The global temperature pattern forecast also looks similar. Over North America, we have the cold pool over western Canada and Alaska, but reaching down further into the northern United States. The southern and eastern United States is forecast to be warmer than normal.
Europe features mostly warmer than normal conditions, with a storm track further up over northern Europe on the northerly jet stream.
Looking closer at Europe, we see warm anomalies over most of the continent. The pattern that UKMO suggests leaves little room for cold air intrusions. But there are cold air outbreaks every spring. What usually changes each year is the number of these cold outbreaks. UKMO also has a colder signal for southeastern Europe.
Over North America, the UKMO has a much colder signal than the EMCWF, due to the stronger low-pressure system in Canada. We see the cold anomalies extending down from Canada further into the northwestern United States and the Midwest.
Warm anomalies are a certainty for the southern half of the United States. The pressure pattern that the UKMO shows for the southern United States, is in agreement with other seasonal forecasts.
The precipitation forecast shows the drier conditions over the southwestern United States. Northern and eastern parts of the United States have a higher chance of wetter conditions under the jet stream. Europe features higher precipitation over northern regions, due to the higher frequency of low-pressure systems moving over this area.
Below is the latest official Spring temperature forecast for the United States by NOAA. It shows the temperature probability, with colder to equal chances in the northwestern/western United States. The southern and eastern United States have a higher probability of warmer than normal weather, as seen in the models above.
The official precipitation forecast is also quite similar to the model forecast. We see an equal-to-higher probability for more precipitation in the northwestern and the eastern United States. The southern United States however is forecast to have a drier than normal spring season.
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