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07 October 2019

Rare warming over Antarctica reveals power of models

Improved comprehension of conditions in the stratosphere are currently helping to produce more-accurate shortterm climate forecasts.

Dyani Lewis

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Warmth of the stratosphere above the pole may influence weather in the Southern Hemisphere for several months. Credit: John Sonntag/Planet Pix through Zuma Wire/Shutterstock

For the last month, a rare atmospheric phenomenon has been brewing above Antarctica, raising temperatures at the atmosphere by 40 degrees and threatening to reverse the management of a strong jet stream for the second time since records began. At the first indications of this event Eun-Pa Lim, a scientist at the Bureau of Meteorology in Melbourne, sailed the climbing temperatures. The model predicted that the warming above Antarctica will drive hot, dry winds across eastern Australia on the following three months.The prediction has eager meteorologists as it shows how far the field has come in realizing that the stratosphere — that the next major stratum of Earth’s atmosphere — and its effects on weather.For decades, even meteorologists thought weather was mostly driven by what had been happening within the troposphere, the layer between the stratosphere and Earth’s surface. Then, in 2001, daily weather maps that were stratospheric showed the 2 regions interact2. Now these interactions are being included in models such as the one designed by Lim to predict shortterm climate — conditions occurring in regions round the world — between a 7 weather prediction and the three months. For instance, meteorologists are now able to predict a climatic phenomenon that compels rainfall in the United States will be affected by conditions within the stratosphere. “We have a lot better understanding of how the stratosphere impacts the elements at the surface,” says Adam Scaife, head of long range forecasting at the Met Office Hadley Centre for Climate Science and Technology Services at Exeter, UK.Improved confidence and precision at such forecasts makes a major difference to government agencies preparing for heat-waves or fires, and into farmers, such as the ones at drought-affected eastern Australia when planning irrigation or herd-mustering schedules, says Lim.Improved forecastsSudden stratospheric warming events are somewhat common in the Northern Hemisphere, occurring every single year on average, but they are infrequent at the Southern Hemisphere. The first event in the south, in 2002, took scientists from surprise.Even when they’d known it had been coming, models back couldn’t have predicted the sudden warming in the stratosphere could affect the weather,” says Harry Hendon, head of climate processes at the Australian Bureau of both Meteorology.Climate models have improved significantly over the previous 15 years, partially driven by faster, cheaper computers. They’re also far better at blending sources of valuable data, such as satellite measurements of stratospheric temperature and atmospheric humidity.Such advances helped meteorologists to predict the start of current stratospheric warming about weekly ahead of time. The events start near the end of winter, when the comparison between cold landmasses and warm ocean temperatures or mountains generate continental-scale atmospheric waves. If these are large enough, then they are able to hit in the stratosphere and divide warming and compressing the air. This pressure can induce the strong stratospheric winds surrounding the pole — that the polar night jet stream — to suddenly slow down and reverse, changing from being westerly storms to flowing in a easterly direction, says Scaife.A complete alteration hasn’t yet occurred in the present event, but end speeds have shrunk. Scientists at the Bureau of Meteorology don’t know what sparked this year’s event, but they predict that it will be more expensive than in 2002 — and so have a greater effect in the weather.Lim’s version, which teases out the way stratospheric conditions bleed down into the troposphere, has helped predict how this could play out. Besides bringing warmer weather to eastern Australia, the big 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 a influx of ozone-rich air to cancel the lack of ozone over Antarctica that always occurs in spring. “Everything seems somewhat gloomy, but at least we will have good cover by the harmful UV that spring,” says Lim. Meteorologists are now waiting to check perhaps the prediction holds. Hendon hopes that, if it will, the bureau will start incorporating Lim’s version into its standard operations, to present shortterm climate forecasts every spring.Similar forecasting tools have been set up to improve forecasts of other weather techniques. For instance, scientists discovered in 2016 that end variation within the stratosphere influences a climate phenomenon called the Madden–Julian oscillation, that may bring significant rainfall to the west coast of the United States. Colleagues and hendon calculated that all models that take this end variation may expand forecasts with this happening by eight days4. “Most models running in 2000 couldn’t even mimic this tropical Madden–Julian oscillation,” says Hendon. “And now we can predict it for four months”

Doi: 10.1038/ / d41586-019-02985-8

References1.Lim, E.-P., Hendon, H. H. & Thompson, D. W. J. J. Geophys. Res. Atmos. 123, 12002–12016 (2018).Article

Google Scholar2.Baldwin, M. P. & Dunkerton, T. J. Science 294, I581–584 (2001).Article

Google Scholar3.Yoo, C. & Son, S.-W. Geophys. Res. Lett. 43, 1392–1398 (20-16 ).Article

Google Scholar4.Marshall, A. G., Hendon, H. H., Son, S.-W. & Lim, Y. Clim. Dyn. 49, 1365–1377 (20 17 ).Article

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