The concept of Sudden Stratospheric Warming (SSW) has seen increasing attention in recent years. For meteorologists, it is always a very exciting time to forecast such an event during winter, but it is often difficult for the general public to understand exactly what it means. There is often a kind of euphoria when the term pops up. Some clickbait media respond to this with headlines like 'Beast from the East' and 'extreme winter weather expected due to an SSW', but does an SSW actually create winter weather? In this background article, I describe the phenomenon, provide more information on why we can't cheer right away, but why it can be interesting to follow the developments around this event. Time to get up into the atmosphere and take a look at the stratosphere. In this background article, I try to answer the following questions:
To avoid confusion, let's go back to basics. Weather takes place in the lowest layer of the atmosphere: the troposphere. This layer is about 6 to 16 kilometres thick and is lower at higher latitudes and generally thicker in the tropics. The layer above the troposphere is called the stratosphere. While the temperature in the troposphere drops the higher you get, the opposite is true for the stratosphere. With increasing altitude, the temperature here increases. This is due to the ozone layer within the stratosphere, which also acts like a shield for us. The incoming UV radiation from the sun is absorbed, a process which releases heat. The increasing temperature with altitude creates stable conditions. Even the mightiest thunderstorm clouds with considerable updrafts are largely stopped at the transition from the troposphere to the stratosphere (the tropopause).
In the background article on the jet stream I briefly explained that there are several jet streams. It is important not to confuse the polar jet streams now. In fact, there are two different polar jet streams: the stratospheric polar vortex and the tropospheric jet stream. The stratospheric polar vortex is located roughly between the tropopause and about 50 kilometres altitude (the transition to the mesosphere). Meteorologists often use weather maps of the so-called 10 hPa pressure plane, which roughly corresponds to an altitude of 30 kilometres. Below that, at about 8 to 10 kilometres (in the higher parts of the troposphere), you have the tropospheric jet stream, which has a huge influence on weather at our latitudes. I described this jet stream here. Here, we focus on the stratospheric Polar Vortex.
The Polar Vortex in the stratosphere is a large low-pressure area over the pole with enormously strong westerly winds that can easily reach speeds of 200 to 300 kilometres per hour in winter. We encounter this low-pressure area with strong westerlies around the pole only during the winter months, because the sun's energy cannot reach the polar region during this period. For several months there is 24/7 darkness (polar night) here, allowing a lot of outgoing radiation to escape, which cools the atmosphere considerably. A huge area with cold air gets trapped by a strong vortex, the Polar Night Jet. When this Polar Vortex is strong and we thus find high wind speeds from the west, the tropospheric jet stream is usually also strong and zonal (west to east). In the southern hemisphere, the polar vortex is usually very strong and effective in trapping this cold. In the northern hemisphere, the polar vortex is weaker and more mixing takes place between northern and southern regions.
One event that could cause more mixing with lower latitudes to take place is a Sudden Stratospheric Warming (SSW). This warming is impressively strong. It involves tens of degrees within a span of a few days, sometimes as much as 50 to 70 degrees within a week! The sudden warming during the winter season is accompanied by a pronounced weakening of the westerlies in the stratosphere. In an SSW, the low-pressure area over the Arctic is driven out and a blocking high-pressure area replaces it. Westerly winds are weakened or even transformed into easterly winds. We can divide the events into two categories: minor warming and major warming. The difference lies in the wind reversal. We speak of a major warming when the strong westerlies of the Polar Night Jet not only weaken, but even reverse, so that easterlies prevail.
The occurrence of an SSW is very complex, yet I will try to explain it in the simplest possible way. Wave energy is the keyword in the formation of an SSW. Mountains (such as the Rockies) and certain blocking weather settings are responsible for the formation of so-called Rossby waves in the troposphere. This wave energy can sometimes propagate vertically and even enter the stratosphere. Here, dissipation can then take place where the waves break and the energy is converted into heat. This warming can then disrupt the stratospheric polar vortex, thus causing easterly winds.
It will take several weeks before the easterly winds (negative values) in the stratosphere can descend towards the troposphere. So at least before then, there is no impact on our weather.
How does an SSW event translate to the weather in the troposphere? Initially, this warming still has no effect on wind patterns in the troposphere, but in the following weeks, these high-altitude easterly winds can increasingly descend towards the troposphere. Wave breaking occurs at lower and lower altitudes, which causes the easterly winds from the stratosphere to gradually descend towards the top of the troposphere. Here, the easterly winds come into contact with the tropospheric jet stream. This may weaken the jet stream and move it southwards, favouring a negative NAO index with a possible Atlantic blockade paving the way for cold outbreaks at our latitudes. This coupling between the stratosphere and troposphere is not always the same.
There are two types of SSW events: a displacement, where the core of the Polar Vortex is shifted away from the pole and a split, where the core is even split completely. Sometimes we also see a mixed version occur. In general, we can only expect an impact on weather in the troposphere after about 2 to possibly even 6 weeks. For a shift, it can usually take a bit longer than for a split of the polar vortex.
I want to stress out that it’s all about the possible impacts on our weather. Cold outbreaks from the north are more likely due to a weakened polar vortex. While the likelihood of a blocking high-pressure area over the Atlantic and Greenland is higher, a SSW-event is definitely no guarantee of extreme winter weather! The coupling between the stratosphere and troposphere does not always take place during an SSW. In 2018, for example, it did, resulting in winter weather throughout Europe, but the following year an SSW event hardly affected the weather. Easterly winds in the stratosphere were not able to penetrate down into the troposphere, leaving the jet stream unaffected.
These days, you come across it more and more in various media. It is great to see widespread interest in such a meteorological phenomenon, but to clarify: a sudden stratospheric warming is nothing new. Back in the 1950s, measurements were made of the stratosphere observing sudden warming. Statistically, once every 2-3 years an SSW occurs. So don't get too caught up in the hype of some reports that promise you winter. Especially during an SSW, you have to be patient for a while. Weather models are still struggling to calculate the coupling between the stratosphere and troposphere and thus on the impact on our weather. All in all, it is a very interesting but also complex matter and, moreover, also still a hot topic for scientific research, as we do not fully understand all the mechanisms.
Sources: Stratosphere Troposphere Interactions - K. Mohanakumar, Metoffice, DWD, NOAA, Stratobserve.com