For the past month, a rare atmospheric phenomenon has been brewing over the Antarctic, raising the upper atmosphere's temperature by 40 degrees, threatening the direction of a reverse Only for the second time since records began has a powerful jet stream been created.
At the very first sign of this event, called the sudden warming of the stratosphere, climatic researcher Eun-Pa Lim of the Australian Bureau of Meteorology in Melbourne stopped rising temperatures. She had designed a model that simulated the short-term climate over the southern hemisphere predicts 1
The forecast has excited meteorologists because it shows how far the field has come in understanding the stratosphere – the second major layer of Australia – Earth's atmosphere – and its impact on the weather.
For decades, meteorologists thought that the weather mainly depends on what happens in the troposphere, the layer between the stratosphere and the Earth's surface. Then, in 2001, daily weather maps of the stratosphere showed how the two regions interact 2 .
These interactions are now flowing into models such as those developed by Lim to predict the short-term climate in regions around the world – conditions ranging between a 7-10-day weather forecast and the following three months. For example, meteorologists can now predict how the conditions in the stratosphere will affect a climatic phenomenon that causes heavy rainfall in the US during the winter.
"We have a much better understanding of how the stratosphere affects surface weather," says Adam Scaife, director of long-term forecasting at the Hadley Center for Climate Research and Services at Met Office in Exeter, UK.
] Improved accuracy and confidence in such predictions makes a huge difference to government agencies preparing for heat waves or fires, and to farmers, such Lim.
In the northern hemisphere, sudden stratospheric warming events are common, occurring on average every two years, but in the southern hemisphere are rare. The first event in the south in 2002 surprised the scientists.
Even though they knew it would come, the models at that time would not have been able to predict how the sudden warming in the stratosphere would affect the weather, says Harry Hendon, head of climate processes at the Australian Bureau of Meteorology.
Climate models have improved significantly over the last 15 years, partly due to faster, cheaper computers. They are also much better able to combine observation data sources such as satellite measurements of stratospheric temperature and humidity.
Such advances helped meteorologists predict the beginning of the current warming of the stratosphere about a week in advance. The events usually begin towards the end of winter, when mountains or the contrast between warm sea temperatures and cold land masses generate Rossby waves known as continental-scale atmospheric waves. When they are big enough, they can reach into the stratosphere and break like a wave over a beach, compressing and warming the air in the stratosphere above the pole.
This pressure can force the strong stratospheric winds that encircle the pole – the polar night-ray – to abruptly decelerate and reverse, from westerly to easterly winds, Scaife says.
A complete reversal does not occur yet in the current event, but wind speeds have already dropped. Scientists at the Bureau of Meteorology do not know exactly what triggered this year's event, but they predict it will be stronger than in 2002 – and thus have a bigger impact on the weather.
Lim's Model Teasing the Stratosphere Conditions that drain into the troposphere have helped to predict how this could impact. In addition to warming the weather in eastern Australia, the event will bring colder and wetter conditions in western Tasmania, on New Zealand's southern island and at the southern tip of South America.
The current warming has also triggered an influx of ozone-rich air to counter the thinning of ozone over the Antarctic, which normally occurs in spring. "Everything else is a bit depressing, but at least we will have a good coverage from the harmful UV radiation this spring," says Lim.
Meteorologists are now waiting to see if the forecast applies. Hendon hopes that in this case the office will begin incorporating Lim's model into its standard operations to deliver short-term climate forecasts each spring.
Similar forecasting tools are used to improve predictions for other weather systems. For example, in 2016, scientists discovered that wind fluctuations in the stratosphere influence a climate phenomenon called the Madden-Julian oscillation, which can bring heavy rainfall to the west coast of the United States during the winter 3 . Hendon and colleagues calculated that models that account for this wind fluctuation can extend forecasts for this phenomenon by eight days 4 .
"Most models from 2000 could not even simulate this tropical Madden-Julian oscillation," says Hendon, "and now we can predict it for three or four weeks."