A strong Stratospheric Warming event is ongoing over the South Pole. It has disturbed the southern Polar Vortex and will help to create strong weather anomalies at the surface. But the latest forecast data suggests that its impact might be far more reaching, likely even into the Winter of 2024/2025, over the Northern Hemisphere.
We wrote about this event as it started, around mid-July. Since then, we have discovered a study that suggests a connection between the southern and northern hemispheres and these strong events.
The Southern Hemisphere does not experience a stratospheric warming event very often, with only a few known events in the past decades. But, as you will see, these events can have a far-reaching effect despite unfolding on the other side of the world.
POLAR VORTEX CIRCULATION
The driving force behind these events is the Polar Vortex. We wrote about it many times now. But as we have a lot of new readers, we like to include a brief overview/explanation of what it is and why it matters.
To understand the Polar Vortex, it is usually best to simply visualize it. Basically, the Polar Vortex is just a name that describes the broad winter circulation over the northern (and southern) hemispheres.
Below, you can see a 3-dimensional image of the polar vortex. The vertical scale is greatly enhanced for better visual presentation. It nicely shows the otherwise hidden structure of the polar vortex, connecting down to the lower levels, the pressure systems, and the winter weather patterns.
The Polar Vortex extends high into the atmosphere. The lowest level is called the troposphere, where all weather events occur. But above that, we have the Stratosphere, a deeper and drier layer that is also home to the ozone layer.
Overall, the Polar Vortex behaves like a very large cyclone, covering the whole north pole down to the mid-latitudes. It is connected through all atmospheric levels, from the ground up, but can have different shapes at different altitudes.
Because of this, we separate the entire Polar Vortex into an upper (stratospheric) and a lower (tropospheric) part. They both play their role differently, so we monitor them as two separate systems, but how they are connected is also very important. Image by NOAA-Climate.
We must monitor the state of the Polar Vortex, as it can have a profound impact on our daily weather. We mainly separate the Polar Vortex into two different modes:
A strong/stable Polar Vortex usually means strong polar circulation and jet stream. This contains the colder air into the Arctic Circle, creating milder conditions for most of the United States and Europe.
In contrast, a weak/disrupted Polar Vortex creates a weak jet stream pattern. As a result, it has a harder time containing the cold air, which can now escape from the polar regions into the United States and Europe. Image by NOAA.
So, a weak Polar Vortex creates a more disrupted weather pattern, regardless of which hemisphere we are talking about.
To even better understand the polar vortex, we created a 3D video animation below, that shows the polar vortex in motion. You can see how it spins over the Northern Hemisphere, connecting down into the lower levels.
The Polar Vortex can influence weather development across the Northern Hemisphere, especially during Winter in North America and Europe. Its effects can also reach far into the Spring season.
That is usually achieved by increasing the pressure or temperature in the polar Stratosphere. We call such events “Sudden Stratospheric Warming,” as the name suggests, a rapid temperature increase in the Stratosphere.
STRATOSPHERIC WARMING EVENTS
A stratospheric warming event (SSW) is like a sickness of the polar vortex. You can imagine it as a fever, weakening and disrupting the polar vortex. And since it is connected to the lower levels, it can also change the weather below over time.
Below is an image of the stratospheric temperature in early January 2021. You can see a strong warming wave in the stratosphere crawling over the pole. It separated the core of the polar vortex into two parts, disrupting the atmospheric circulation.
One part of the broken polar vortex has moved over North America and one over the European sector. At this point, this did not have a direct impact on the surface weather, as it is at a 30km altitude. But the polar vortex was collapsing like a house of cards at this point, with effects eventually reaching the ground.
The next image below shows an atmospheric pressure index. Negative values indicate lower pressure (blue colors), and positive values indicate higher pressure (red colors). We have altitude from the ground up to the top of the stratosphere (~46km), covering Winter 2020/2021 and early Spring 2021.
Strong positive values were seen in the stratosphere in early January, associated with high pressure buildup during the stratospheric warming event. The event and its influence slowly descended over time, reaching lower levels by mid- and late January.
This persisted well into February, influencing the weather circulation even when the stratospheric warming was already over at the higher levels. So you can see just how big of an impact such an event can have on the weather patterns.
The event above unfolded over the Northern Hemisphere, but the dynamics are exactly the same over the Southern Hemisphere. Of course, the direct initial impacts are limited to the Southern Hemisphere, but as you will see later, its effects can extend into the Northern Hemisphere.
ONGOING STRATOSPHERIC WARMING
The Southern Hemisphere has opposite weather seasons to the Northern Hemisphere. So, while we are going towards late Summer, there is still Winter in the Southern Hemisphere, meaning it has an active Polar Vortex circulation.
As mentioned, the current event began around mid-July. The image below shows the temperature analysis of the stratosphere over the south pole in the early days of the SSW event. A very strong warming wave was pressing against the core of the southern polar vortex, expanding deeply into the polar circle.
Below is the latest pressure anomaly in the stratosphere over the south pole. You can see the polar vortex is now being compressed by two high-pressure anomalies, not just one. These are strong high-pressure anomalies that are changing the very shape of the polar vortex. Images by weathermodels.com
The latest temperature anomaly analysis also shows a strong temperature anomaly at this level (10mb ~ 30km/18miles). You can see that the stratospheric warming wave has expanded and surrounded the polar vortex, raising temperatures much above the normal values.
But the stratospheric warming has expanded not just around the polar vortex but also in the vertical levels. The image below shows the temperature anomaly in the lower stratosphere, where you can also see the warming anomaly around the polar vortex. This shows that a strong event is unfolding, expanding on all levels.
Looking at the wind and temperature analysis/forecast graph for this level of the stratosphere, you can see the winds reducing as the polar vortex weakens. Also, you can see a strong peak in temperatures as the stratospheric warming event develops to its maximum anomaly in the next few days.
This is a strong stratospheric warming event that, as we will now see, is already starting to influence the weather in the lower levels. It can also play a role in the Winter of 2024/2025 over the Northern Hemisphere.
FIRST WEATHER CHANGES BELOW
In the image below, you can see the atmosphere by altitude, and the colors indicate pressure anomalies over the south pole. We can see a large amount of high-pressure anomalies above in the stratosphere, starting in mid-July with the first warming wave.
With progress ahead, you can see how the anomalies moved downwards and connected with the lower levels. In the forecast part, you can see a very strong (sigma 5) anomaly in the stratosphere, indicating a very strong disturbance.
Looking at the lower pressure patterns for the medium range, you can see a strong high-pressure anomaly over the south pole, and the displaced low-pressure areas towards Australia and South Atlantic. This indicates a disruption in the circulation, connected with the developing pressure anomalies in the stratosphere.
Also, looking at the surface temperature forecast for next week, you can see a very strong warm anomaly over Antarctica, with the cold air displaced towards South America and the Atlantic. This is the expected result of a strong stratospheric warming and a polar vortex disruption, as the high-pressure anomalies push the cold air out of the pole.
We can track the lower-level pressure changes with a simple index called the Antarctic Oscillation (AAO). It basically shows the strength of the lower-level polar vortex over the south pole. We also have such an index for the northern hemisphere. Positive means a strong circulation, and negative means a weak circulation.
As you can see, the AAO has been very negative since the second half of July, following the stratospheric warming event, and goes even lower in the coming two weeks. This shows a major disruption of the low-level polar vortex circulation and a buildup of high pressure over the pole under the stratospheric warming event.
But this index also has another important use. It will help us determine the potential impact of this southern stratospheric event on the Winter of 2024/2025 over the Northern Hemisphere.
WINTER 2024/2025 IMPACTS?
It is natural to think that, after weeks/months, an event over the South Pole can hardly impact the weather over the Northern Hemisphere. But the atmosphere is global, without boundaries, and it is both fluid and in constant motion.
If you look at the image below, you can see a simple schematic of the global air circulation between the hemispheres and the poles. There is much more flow and dynamics in the Winter hemisphere, while the Summer hemisphere is calmer generally.
This is called the “Brewer-Dobson circulation“, and connects the circulation from both Hemispheres higher up in the atmosphere. In reality, it is far more complex, but this simple schematic shows the main idea of airflow and energy transfers.
With the current stratospheric warming event, we can try to track the upcoming lower pressure changes and find connections to those changes in the Northern Hemisphere. It might sound complex, but it’s not.
Remember the AAO index that we showed you? As a result of the stratospheric warming event, it is going strongly negative over Antarctica. Below is an image showing the pressure anomalies in the August-October period with a negative AAO index.
You can see a high-pressure anomaly over Antarctica and a belt of low-pressure anomalies around it. This covers more than half of the southern hemisphere, so it covers a large part of the atmosphere.
Below is the actual ECMWF pressure anomaly forecast for the same period, which shows the exact same pressure pattern. This means a negative AAO is expected to be the main mode over the following weeks and months, likely amplified by the stratospheric warming.
Based on this, we can try to find a connection between weak south pole circulation during August-October and the anomalies in the following Winter period of December-February in the Northern Hemisphere.
In the image below, you can see a simple connection/signal between the south pole pressure anomalies from August to October and the following Winter pressure anomalies over North America and Europe from December to February.
This graphic shows a signal that prolonged high-pressure anomalies over the South Pole in the August-October period can indicate low-pressure over the United States Midwest, the Northeast, and Southwest the following December-February. A high-pressure anomaly is indicated for Europe.
If we look at the actual ECMWF pressure forecast for the November-January period, we can see a very similar pattern. You can see a low-pressure area positioned over southern Greenland and into eastern Canada and the northern United States, and a high-pressure over Europe.
This is really close to the connection data we showed above, but this forecast was issued in early July when the stratospheric warming event was not yet fully resolved in the forecast data. It will be really interesting to see what the latest forecast will look like, which will also cover the whole Winter 2024/2025 period of December to February.
Looking at the same data but for temperature, we can see the Winter temperature signal following a negative AAO in August-October. It indicates colder temperatures over the United States Midwest, the Northeast, and the South. But a warmer signal is seen over Europe under high pressure.
These two basically show the connection/correlation that there is actually some weather signal in the data. They connect the Autumn high-pressure anomalies over the south pole with the following Winter weather patterns over the northern hemisphere.
But, there is a large caveat here: This correlation does not mean that the high-pressure anomaly over the South Pole is directly responsible for the lower temperature signal over the United States the following winter. It just indicates some connection but doesn’t explain what exactly it is.
As we know, a large-scale event like Stratospheric Warming can cause a prolonged high-pressure anomaly over the South Pole. With several weeks to months of time, some effects of the SSW event could perhaps manifest in the Northern Hemisphere through the large-scale atmospheric motion.
A study made in 2018 by a group of authors focused directly on the connection between the north and south hemisphere pressure anomalies. The study found conclusive results for events over one hemisphere affecting another, likely through the stratosphere and polar vortex dynamics.
We also examined the anomalies of the polar vortex during Winter over the Northern Hemisphere, following negative AAO and high pressure anomalies over the South Pole.
First is the stratospheric pressure anomaly for the Winter period (December-February). You can see a tendency for higher pressure, which indicates a weaker polar vortex. This signal is not strong, but it exhibits almost a stratospheric warming signature with a polar vortex core split into two parts.
The stratospheric temperature data for the same period show the same signal: an above-average temperature anomaly over the north pole. This is in line with a stratospheric warming event signature and the weakening of the northern polar vortex.
Both these images again show hints that there is some connection between the polar vortex over the south and over the north. Strong changes or a stratospheric warming event over the southern polar vortex can impact the northern polar vortex the following Winter and, consequently, the winter weather below.
Currently, the northern hemisphere is in its summer mode, so we do not have an active polar vortex. The image below shows the stratospheric wind forecast over the north, currently in negative mode as we do not have a polar vortex active.
But, you can see zonal winds turning westerly in early September, indicating the slow pressure drop in the northern stratosphere and the eventual return of a new polar vortex, which will last through the next Winter 2024/2025.
So, if you remember the strong high-pressure anomalies forecast over the south pole for the August-October period, that is directly in time when a new northern polar vortex will emerge, and both hemispheres will briefly have an active polar vortex with an active connection.
This opens a potential window of connection and feedback that is indicated in studies and the data.
As we shared a lot of information, we will add a recap below of all that is going on right now:
- A strong stratospheric warming event is ongoing over the South Pole. It will disrupt the southern polar vortex and help to create and sustain high-pressure anomalies on the surface.
- The south pole pressure forecast for August-October shows a prolonged period of high-pressure anomalies, creating a negative circulation pattern.
- Past data and studies show that prolonged high-pressure anomalies over the South Pole in the August-Oct period can influence the weather over the Northern Hemisphere in the following weeks/months.
- The most likely cause of the weather impacts is the stratospheric connection between the northern and southern polar vortex that helps to transfer the atmospheric dynamics.
- The prevalent signal in the data shows that strong high-pressure anomalies over the South Pole in the August-Oct period correspond to lower pressure anomalies over the United States and Canada in the Winter season. The data also shows colder-than-normal temperatures over much of the central and eastern parts of the United States.
- Over Europe, the prevailing signal is a high-pressure anomaly and warmer temperatures during the Winter season.
- The main final point: These signals and connections are not a fixed rule, as the signal is weak enough to indicate it is not a main (large) driving force of the Winter weather patterns. But it is still visible and does show some form of a role in the overall large-scale atmospheric circulation.
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Learn more: Stratospheric Warming Event in January 2024