A stratospheric warming event is about to unfold over the Southern Hemisphere, which is currently in its Winter season. The latest forecasts show strong temperature and pressure waves in the Stratosphere above the South Pole. These events are rare in the Southern Hemisphere but are usually very powerful and are known to have a global impact, potentially even on the upcoming northern Winter season.
The Southern Hemisphere does not experience these winter stratospheric warming events so often, with only a few known events in the past decades.
We will look at the current stratospheric warming event and how it is forecast to develop. We will also look at some of the unusual effects a full stratospheric warming event can have on the Northern Hemisphere, or even on the northern winter season, in special cases.
THE POLAR VORTEX
The main player or the driving force behind these events is the Polar Vortex. To understand it, it’s best to try to visualize it. But to keep it simple, it 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 invisible structure of the Polar Vortex, connecting down to the lower levels, the pressure systems, and the winter weather patterns below.
Overall, the Polar Vortex behaves similarly to a very large cyclone, with some even (falsely) calling it a stratospheric hurricane. It covers the whole polar area down to the mid-latitudes. It is connected through all atmospheric levels, from the ground up, but it can have different shapes at different altitudes.
For this reason, we separate the entire Polar Vortex into an upper (stratospheric) and a lower (tropospheric) part. They both play their roles differently, so we monitor them as two separate systems. But it’s very important how both parts are connected. Image by NOAA-Climate.
The upper (stratospheric) part is more circular and symmetrical as it spins higher above the ground, with fewer obstacles in its flow, like the complex terrain and mountains in the lower levels.
To simplify, all you need to know is that the upper and lower parts of the Polar Vortex are connected. Changes in the upper part can significantly impact weather anomalies in the lower parts of the atmosphere and the weather we experience at the surface.
Below is an image that shows the Polar Vortex at different altitudes. The closer we go to the ground levels (on the left side), the more deformed it becomes due to the complex terrain and the strong weather fronts and systems that can deform it.
This process is exactly the same in the Southern Hemisphere. But there is much less terrain there, so there are fewer disruptions of the stratospheric Polar Vortex or warming events.
In this article, we will analyze a new Stratospheric warming event over the Southern Hemisphere. The event itself occurs exactly the same way as over the Northern Hemisphere and can grow in strength over time.
NORTHERN STRATOSPHERIC WARMING EVENT
This year, in mid-March, a strong Sudden Stratospheric Warming event (SSW) began over the Northern Hemisphere. As the name suggests, this event brings a rapid rise in pressure and temperature in the stratosphere. This can collapse the Polar Vortex circulation, impacting the weather patterns below for up to several weeks.
Below is the mid-stratosphere (31km/18.5miles) analysis from mid-March over the North Pole. It shows the stratospheric high-pressure system (H) taking over the polar regions and splitting the Polar Vortex into two separate cores (L). This is a stratospheric warming even at its peak power.
Such a strong, warm anomaly is only seen during stratospheric warming events. This event was unusually strong for this time of year, compared to previous early spring SSW events.
This had a big impact on the atmosphere. Below you can see an image with pressure anomalies in the lower half of the atmosphere. Negative values indicate lower pressure (blue anomalies), and positive values indicate higher pressure (red colors). You can see a strong high-pressure anomaly in the stratosphere from early March all the way into mid-April.
That large mass of high pressure is the stratospheric warming anomaly, and you can see how its impact slowly moved downwards to the weather layer (below 100-200hPa). It impacts the lower levels in waves, playing a role in the surface weather patterns.
EARLY SIGNS OF CHANGE
The Southern Hemisphere has opposite weather seasons to the Northern Hemisphere. So, while we are now in Summer, there is Winter in the Southern Hemisphere. This means there is an active Polar Vortex circulation over the South Pole.
Below is the latest analysis from the NASA stratospheric monitoring system. It shows the zonal wind around the core of the southern stratospheric Polar Vortex at the 10 mb level (31 km/18.5 miles). The red and purple lines indicate the current season and show us below-normal winds and the strength of the south Polar Vortex.
If we look at the actual latest temperatures in the mid-stratosphere, you can see that the core over the South Pole really is colder than normal, but it has been moved out of place. A strong warming wave is pushing against the cold core, raising temperatures over 30°C above normal.
If we look at the pressure analysis, it shows a good shape of the Polar Vortex, with a high-pressure anomaly area building and pushing against it. This high-pressure anomaly is associated with the warming wave seen in the previous image and is responsible for displacing the whole cold core of the Polar Vortex.
But looking ahead, we have the extended range stratospheric wind forecast below for the 10mb level. Wind is used as a good estimate of the polar vortex strength, and it can tell us a lot about the dynamics in the stratosphere.
You can see that the southern Polar Vortex is running at a strong level. But a period of rapid weakening is coming and already starting. You can see that in about two weeks, the Polar Vortex will have its strength almost cut in half. Further ahead, we can see a continued seasonal reduction in power.
At this point, we have to add one important fact. Not every warming in the stratosphere is officially classified as a “Sudden Stratospheric Warming” event. For it to be a major official event, it needs to have the stratospheric winds reversed (below 0 on the image above).
But even without a full reversal, any stronger changes in the stratosphere can reflect downwards and impact the daily weather.
So, what are these strong waves that will greatly reduce the strength of the southern Polar Vortex?
A NEW STRATOSPHERIC WARMING EVENT
Usually, the early warming and pressure waves can be a prelude to stronger events that follow behind, with a proper Stratospheric Warming event unfolding that can disrupt the whole Polar Vortex.
Below is the pressure anomaly for next week in the mid-stratosphere over the South Pole. You can see the high-pressure anomaly is forecast to intensify, pushing/displacing the core of the polar vortex away from the central area.
Looking at the temperature anomaly for early next week at the same level, you can see a strong warming wave developing under the high-pressure anomaly. The core of the Polar Vortex remains cold, but the warming wave will start with temperatures 25-30°C above normal for this time of year.
The pressure forecast for the peak of the event in late September shows a continued high-pressure anomaly around the Polar Vortex. In this forecast, we can see the above-normal pressure spreading a bit further over Antarctica, pushing the Polar Vortex far off center, disrupting its dynamics.
If we look at the temperature anomalies in the mid-stratosphere for the same period, you can see a much more evolved warming wave, spanning the center of the polar region. Above normal temperatures are taking over and are forecast to displace the main cold core of the Polar Vortex, with temperatures near 50°C above normal.
We can also look further down in the stratosphere for late September, at a lower 50mb level (19km/12miles), where you can still clearly see the stratospheric warming anomaly. It’s not that often that a warming wave in the upper levels of the stratosphere reaches further down. So this is an indication of a strong warming event, despite not having the full wind reversal.
We also produced a video from the forecast images provided by WeatherBell (using a commercial license) to show what this whole process looks like in the forecast. Video shows the stratospheric pressure anomalies from the NCEP GFS model.
You can see the spinning of the Polar Vortex, and the attack of the frequent stratospheric pressure waves around it, getting stronger over time. But what can we expect in terms of any connection and weather changes at the surface?
WINTER WEATHER IMPACTS
Below is the GFS forecast for the South Pole pressure index. A negative value means a weaker polar circulation and a more disrupted flow. You can see a fixed forecast on a negative pressure index. Usually, after the stratospheric warming events, this index goes and stays negative, indicating a prolonged weather disruption.
An example of what a negative pressure index means can be seen below, in the temperature forecast for tomorrow over Australia. You can see a very broad colder air anomaly, which is a result of the pressure patterns being disrupted, allowing the colder air to escape into the mid-latitudes.
Looking ahead at the pressure forecast after the stratospheric warming, you can see a quite disrupted pressure pattern. High-pressure areas are taking over the polar regions, pushing the low-pressure and colder air out into the mid-latitudes.
Of course, this is just an example forecast from a single model, but it best shows the idea of what you would expect after stronger stratospheric events.
Looking at the farther ranges following the stratospheric warming event, you can see a strong cold blast is forecast for Australia, with a very cold air mass anomaly. This is again just one forecast, but it does show what is possible after the stratospheric events.
But the impacts of a southern Stratospheric Warming event can reach much further. As the data below will show, it can reach even into the northern Stratosphere.
FROM ONE WINTER TO ANOTHER
So, should you care about a stratospheric warming event over the South Pole if you live in the Northern Hemisphere? The answer is yes, at least in some cases. In the image below, you can see a simple schematic of the global air circulation between the two hemispheres. There is much more flow and dynamics in the Winter hemisphere, while the Summer hemisphere is usually calmer.
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.
This nicely demonstrates that the northern and southern stratospheres are interconnected on many different levels and can “communicate”, so to speak.
Below are interesting graphics from a study by Goncharenko, L. & Harvey, V.L. & Greer, Katelynn & Zhang, Shun-Rong & Coster, Anthea & Paxton, Larry. The graphics show the change in Total Electron Content (TEC) in the Ionosphere over the United States, following the 2019 Stratospheric Warming over the South Pole.
The image shows a large change in the Ionospheric anomaly over the United States following the strong stratospheric warming event over the South Pole. Strong 40-80% anomalies in electron charge were observed in the ionosphere over the United States.
The same (but reversed) change was also observed over Europe, as the energy wave from the strong Stratospheric warming event had already reached the Northern Hemisphere.
Overall, this study found both positive and negative disturbances in the thermospheric Oxygen/Nitrogen ratios and total electron content over the United States and Europe.
Direct weather changes were observed following this strong event in the higher and lower atmospheres. Signs have also emerged that the event was strong enough to have a weather effect on the following Winter 2019/2020 in the Northern Hemisphere.
It is not easy to track these changes and their impacts, as a significant amount of the signal gets lost in the complex global weather circulation. But in the image below, you can see a simple connection between the south pole pressure anomalies from August to October and the following temperatures over the Northern Hemisphere in December to February.
This shows a signal between prolonged high-pressure anomalies over the South Pole during the August-October period and colder temperatures over the central and eastern United States the following December-February. Europe shows a reversed signal with a warmer anomaly.
A stratospheric warming event can greatly enhance prolonged high-pressure anomalies over the South Pole in this period, having a role in any connection that does exist between the hemispheres.
There is a large caveat here: This correlation/signal 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 it can’t explain what exactly it is.
A large-scale event like a stratospheric warming can cause a prolonged high-pressure anomaly over the South Pole. After several weeks to months, some effects of the SSW event could manifest in the Northern Hemisphere, via the global weather system.
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