This article is adapted from a research brief by the ARC Centre of Excellence for Climate Extremes. Find the original here. The authors, Martin Jucker of ARC Centre of Excellence for Climate Extremes and Thomas Reichler of the University of Utah, are co-authors of the study.
In the Southern Hemisphere spring of 2019 the usually very strong winds in the stratosphere, about 186 mph (300 km/h) at a height of 18.6 miles (30 km) almost completely collapsed within a matter of days —a so-called Sudden Stratospheric Warming (SSW) was underway. These events high in the atmosphere are known to influence the wind structure at the surface and cause anomalous weather conditions at the ground. The stratosphere affects surface weather and climate on a global scale, and ongoing global warming leads to major changes of this influence.
That’s why in early September, the stratospheric research community argued that something very rare was happening and might make things much worse.
Unfortunately, the researchers were right.
The spring of 2019 was exceptional for Australia: Scorching heat, the culmination of a three-year drought, and bushfires raging much earlier in the season than any other time in recorded history. By summer, the burning and related loss of flora, fauna and human lives, as well as respiratory health impacts, were at a disastrous scale. While the reasons for this devastating natural catastrophe were many—and still are subject to intensive research—a new study in Geophysical Research Letters shows how rare one of the main contributors to that extreme fire weather was, and how in a future warmer world it will become even rarer to the point of being almost impossible.
Interestingly, the usual suspect for Australian fire weather, the El Niño Southern Oscillation (or ENSO) wasn’t doing much that year. But in 2019 there was another factor: a strongly negative Southern Annular Mode (SAM), which is known to be related to hot and dry conditions over Eastern Australia. The SAM can be driven by many different things, but in 2019 the obvious driver was a very rare one indeed.
Sudden stratospheric warming
SSWs are much more frequent over the Northern Hemisphere, however, than over the Southern Hemisphere. Over the North, SSWs occur at irregular intervals, but on average every second winter. The winters of 2017-18 and 2018-19 experienced SSWs, and an event in January 2021 lasted into February.
SSWs affect weather over the entire northern high latitudes. They create extremely cold conditions over the North Atlantic, Northern Europe and Asia, while the eastern Canadian Arctic tends to be warm. SSWs also bring more storms into Southern Europe, increasing the risk of flooding in the Mediterranean, while parts of Scandinavia and the British Isles tend to experience dryness. These conditions last for up to two months, which is of great practical importance as it helps scientists to make weather predictions at times beyond regular forecasts of 7–10 days.
It is the worldwide impacts that make SSWs so interesting. Scientists from around the globe research SSWs, creating opportunities for international collaborations. For example, this work represents a collaboration between the University of Utah, the University of New South Wales in Australia, and the John Hopkins University in Baltimore.
An SSW has only been recorded once before in the Southern Hemisphere, in the spring of 2002 (which was also followed by fires that summer). What we know from climate models is that events like these are usually followed by hot and dry conditions in Australia, and that these conditions prevail for many months (e.g. into the Southern Hemisphere summer).
But because this had only happened for the second time on record, it was impossible to say how rare this event was and, crucially, we didn’t know if this frequency would alter with climate change. Was this second event after 2002 a result of climate change? Does this mean we should expect more of those to happen? The only reliable measurements of the stratosphere come from satellites, and they have only been around since the end of the 1970s. Before that, balloon measurements give us some indication that nothing similar has happened since the 1950s. Before that, we have no idea.
Modeling the stratosphere
Running climate models which include a good representation of the stratosphere is very expensive, but people didn’t think it was important to run these investigations —at least compared to what the oceans and other things do to local climate and weather. As a result, the climate model simulations we did have of this part of the atmosphere were usually not reliable or simply too short.
This is why we ran a very good climate model for a long time—9,900 years to be precise (the simulated time, not the time it took us to run it) at the Center for High Performance Computing (CHPC) at the U.
Generous computing support by the U’s CHPC was absolutely crucial for this research. The climate model simulations required several years of compute time and created terabytes of data. While the data are now stored at an external computing facility, the analysis of the vast amount of data is entirely performed at the CHPC. These simulations also served as the basis for several other scientific publications.
With this simulation, we were able to determine how many times events like 2019 would have happened. We found they would have occurred about once every 22 years under conditions similar to the year 1990. That aligns well with the 17 years between the sudden stratospheric warming events in 2002 and 2019. However, we also found that in the future (a world similar to 2080), due to greenhouse gas emissions, these events tend to decline rapidly, happening about once every 300 years.
We are already a third of the way to 2080 (since 1990), so these stratospheric events in the Southern Hemisphere already happen less frequently than once every 22 years. So, what will the world look like in 300 years if that is the next time a Southern Hemisphere SSW event occurs? Probably nothing like 2021 (remember the year 1721?), but the chances are that until another such event happens, we will have to deal with lots of other catastrophic events related to climate change. In the balance of things, perhaps having a few more SSW events and learning how to deal with them would be better than dealing with the consequences of climate change. But the stratosphere still influences our weather, whether there are SSWs or not.
Find the full study here.