For the past month, a rare atmospheric phenomenon has been brewing above Antarctica, raising temperatures in the upper atmosphere by 40 degrees and threatening to reverse the direction of a powerful jet stream for only the second time since records began.
At the first signs of this event, known as sudden stratospheric warming, Eun-Pa Lim, a climate scientist at the Australian Bureau of Meteorology in Melbourne, plugged the rising temperatures into a model she had designed that forecasts short-term climate over the Southern Hemisphere1. The model predicted that the warming above Antarctica will drive hot, dry winds across eastern Australia over the next three months.
The forecast has excited meteorologists because it shows how far the field has come in understanding the stratosphere — the second major layer of Earth’s atmosphere — and its effects on weather.
For decades, meteorologists thought weather was mostly driven by what was happening in the troposphere, the layer between the stratosphere and Earth’s surface. Then, in 2001, daily stratospheric weather maps revealed how the two regions interact2.
Now these interactions are being included in models such as the one designed by Lim to forecast short-term climate — conditions occurring between a 7–10-day weather forecast and the following three months — in regions around the world. For instance, meteorologists can now predict how conditions in the stratosphere will affect a climatic phenomenon that drives heavy rainfall in the United States in winter.
“We have a much better understanding of how the stratosphere affects the weather at the surface,” says Adam Scaife, head of long-range forecasting at the Met Office Hadley Centre for Climate Science and Services in Exeter, UK.
Improved accuracy and confidence in such forecasts makes a big difference to government agencies preparing for heatwaves or fires, as well as to farmers, such as those in drought-affected eastern Australia, when planning irrigation or herd-mustering schedules, says Lim.
Sudden stratospheric warming events are common in the Northern Hemisphere, occurring every second year on average, but they are rare in the Southern Hemisphere. The first event recorded in the south, in 2002, took scientists by surprise.
Even if they had known it was coming, models back then couldn’t have predicted how the abrupt warming in the stratosphere might affect the weather, says Harry Hendon, head of climate processes at the Australian Bureau of Meteorology.
Climate models have improved significantly over the past 15 years, partly driven by faster, cheaper computers. They’re also much better at combining sources of observational data, such as satellite measurements of stratospheric temperature and atmospheric humidity.
Such advances helped meteorologists to forecast the start of the current stratospheric warming about a week in advance. The events typically start towards the end of winter, when mountains or the contrast between warm ocean temperatures and cold land masses generate continental-scale atmospheric disturbances known as Rossby waves. If these are large 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 encircling the pole — the polar-night jet stream — to abruptly slow and reverse, changing from being westerly winds to flowing in an easterly direction, says Scaife.
A complete reversal has not yet occurred in the current event, but wind speeds have already plummeted. Scientists at the Bureau of Meteorology don’t know exactly what sparked this year’s event, but they predict that it will be stronger than in 2002 — and so have a greater effect on the weather.
Lim’s model, which teases out how stratospheric conditions bleed down into the troposphere, has helped predict how this might play out. Apart from bringing warmer weather to eastern Australia, the event will drive colder, wetter conditions to western Tasmania, New Zealand’s South Island and the southern tip of South America.
The warming so far has also sent an influx of ozone-rich air to counter the thinning of ozone over Antarctica that usually occurs in spring. “Everything else seems a bit depressing, but at least we will have good cover from the harmful UV this spring,” says Lim.
Meteorologists are now waiting to see whether the forecast holds. Hendon hopes that, if it does, the bureau will start incorporating Lim’s model into its standard operations, to provide short-term climate predictions every spring.
Similar forecasting tools are being deployed to improve forecasts of other weather systems. For instance, scientists discovered in 2016 that wind variation in the stratosphere influences a climate phenomenon called the Madden–Julian oscillation, which can bring heavy rainfall to the west coast of the United States in winter3. Hendon and colleagues calculated that models that take this wind variation into account can extend forecasts of this phenomenon by eight days4.
“Most models running in 2000 couldn’t even simulate this tropical Madden–Julian oscillation,” says Hendon. “And now we can predict it for three or four weeks.”
Nature 574, 160-161 (2019)