First published October 31, 2019.
In late August 2019 the air high above the South Pole began to heat up at a pace rarely seen in the Southern Hemisphere. This “sudden stratospheric warming” grew into the strongest Antarctic event of its kind on record, and Australia’s Bureau of Meteorology warned that its effects would filter down to the surface over the following months, bringing a warmer, drier spring and early summer to much of eastern Australia, especially New South Wales and southern Queensland.
That forecast turned out to be part of a much bigger story. Below we explain what a sudden stratospheric warming is, why it happened over Antarctica in 2019, how it reached Australia, and why it also gave the ozone layer an unexpected break.
What is a sudden stratospheric warming?
The stratosphere is the layer of the atmosphere that begins roughly 10 to 15 kilometres above the ground and extends to about 50 kilometres. Each winter, as the polar regions sink into darkness, the air above the pole becomes extremely cold compared with the air over the surrounding oceans. That contrast drives a powerful ring of west-to-east winds circling the pole, known as the polar vortex.
Normally, the vortex weakens slowly as sunlight returns in spring. But large-scale waves in the lower atmosphere, created by weather systems, mountains and the contrast between land and sea, can travel upward and slam into it. When that happens the stratosphere over the pole can warm by tens of degrees within days, and the vortex slows sharply.
Meteorologists divide these events by strength. In a major warming the westerly winds actually reverse and blow from the east, often splitting or shoving the vortex off the pole. In a minor warming the winds weaken a great deal but do not fully turn around.
Why the Southern Hemisphere rarely sees them
In the Northern Hemisphere sudden warmings are fairly common, happening roughly every two to three years. Its big continents and mountain ranges, such as the Rockies and the Himalayas, generate strong atmospheric waves that regularly disturb the Arctic vortex.
The Southern Hemisphere is mostly ocean, so those waves are weaker and the Antarctic vortex is usually much more stable. Before 2019 the only well-documented major warming in the south happened in September 2002, when the vortex split in two. Weaker wobbles in the vortex have been recorded many times since reliable observations began, but nothing on the scale of 2002.
Bureau of Meteorology maps comparing stratospheric temperatures above Antarctica during the September 2002 event (left) and the September 2019 event (right).
Bureau of Meteorology chart of years with an unusually weak Antarctic polar vortex, set against other years from 1979 to 2016.
What made 2019 different
The 2019 warming started in the final week of August and intensified through September. Stratospheric temperatures over the pole climbed far above normal for the time of year, and the Bureau predicted it would beat the 2002 event as the strongest Antarctic warming on record.
One big difference from 2002 was the warning time. In 2002 scientists only understood what had happened after the fact. In 2019, thanks to better models and the Bureau’s newer supercomputer, forecasters saw the event coming almost three weeks ahead and could build it into their seasonal outlooks.
Scientists have since debated whether 2019 fully met the strict “major” definition, because the westerlies weakened dramatically without reversing as completely as in 2002. Either way, the temperature spike and the slowdown of the vortex were exceptional, and their effects on the weather below were real.
How the warming reached Australia
Changes in the stratosphere do not stay up there. Over several weeks the weakened vortex nudges the westerly winds of the lower atmosphere, which carry storms around the Southern Ocean, towards the Equator. Climate scientists describe this as a negative phase of the Southern Annular Mode (SAM).
The effects of that shift depend on where you live:
- Places in the path of the westerlies, such as western Tasmania, New Zealand’s South Island and Patagonia, tend to get more storms, more rain and colder conditions.
- Subtropical Australia, particularly inland New South Wales and southern Queensland, tends to get the opposite: fewer rain-bearing systems, clearer skies, more sunshine and higher temperatures, with a greater risk of heatwaves and fire weather.
The Bureau expected these effects to reach the surface within about a month and to linger possibly into January 2020. They came on top of another strong driver: a positive Indian Ocean Dipole, which was cutting off moisture that normally flows towards Australia from the Indian Ocean. Together they pointed to a hot and dry spring.
What actually followed
The forecast proved accurate. 2019 became Australia’s warmest and driest year on record in the Bureau’s data, and the spring was exceptionally dry across much of the east. Those conditions set the stage for the 2019–20 “Black Summer” bushfires, which burned millions of hectares in New South Wales, Victoria and beyond.
It is important not to oversimplify. The stratospheric warming did not cause the fires on its own. Long-term warming, the positive Indian Ocean Dipole, an extended drought and the negative SAM all combined. But the event is a clear example of how something happening tens of kilometres above Antarctica can tip the odds for weather on another continent.
A silver lining for the ozone layer
The warming did bring one good result. The Antarctic ozone hole forms each spring because extremely cold temperatures allow polar stratospheric clouds to form, and chemical reactions on those ice particles release chlorine that destroys ozone. A warmer stratosphere means fewer of those clouds, and a disturbed vortex also lets ozone-rich air from lower latitudes mix in.
In 2019 NASA reported that the ozone hole was the smallest observed since it was first discovered in the early 1980s. Scientists were careful to point out that this was a weather effect, not proof that the ozone layer had healed. Long-term recovery depends on the Montreal Protocol’s ban on ozone-depleting chemicals, which is gradually working but is expected to take decades.
Events like 2019 remain rare in the south. Better forecasting means the next one should come with weeks of warning, giving farmers, fire agencies and communities more time to prepare.
Sources: ScienceAlert, republishing The Conversation: Harry Hendon, Andrew B. Watkins, Eun-Pa Lim and Griffith Young (Bureau of Meteorology), October 2019; Wikipedia: Sudden stratospheric warming; Wikipedia: Ozone depletion; Bureau of Meteorology: Southern Annular Mode; Bureau of Meteorology: climate outlooks. Images: Australian Bureau of Meteorology.
COMMENTS