Record warm temperatures above Antarctica are likely to bring above-average spring temperatures and below-average rainfall across parts of New South Wales as well as southern Queensland.
The warming started in the last week of August 2019, when temperatures in the stratosphere high above the South Pole started rapidly heating in a phenomenon named "sudden stratospheric warming".
The warming is forecasted to intensify in the coming weeks, and its effects will expand downward to Earth's surface, affecting much of eastern Australia over the coming months.
The Bureau of Meteorology predicts the strongest Antarctic warming on record, likely to exceed the previous record of September 2002.
Every winter, westerly winds develop in the stratosphere high above the South Pole and circle the polar region.
September stratospheric warming from 2002 (left) and 2019 (right). (Australian Bureau of Meteorology)
Nine anomalous polar vortex years compared to other years between 1979-2016. (Bureau of Meteorology)
The winds develop due to the difference in temperature over the pole and the Southern Ocean.
As the sun shifts southward in the spring, the polar region begins to warm. The warming makes the stratospheric vortex and associated westerly winds weaken over a few months gradually.
However, in some years, that breakdown can happen faster than usual. Waves of air from the layers in the lower atmosphere warm the stratosphere above the South Pole and weaken or "mix" the high-speed westerly winds.
If the waves are strong enough, then, very rarely, they can rapidly break down the polar vortex, reversing the direction of the winds, so they become easterly. That's the technical definition of "sudden stratospheric warming."
While we've seen plenty of weak as well as moderate variations in the polar vortex over the past 60 years, the only other true sudden stratospheric warming event in the Southern Hemisphere was in September 2002.
In contrast, their northern counterpart occurs every other year or so during the late winter of the Northern Hemisphere because of the stronger and more variable tropospheric wave activity.
Impacts from the stratospheric warming are likely to reach the planet's surface in the next month and possibly extend through to January 2020.
Apart from warming the Antarctic region, a notable effect will be a shift of the Southern Ocean westerly winds towards the Equator.
According to Science Alert, for regions directly in the path of the strongest westerlies, such as western Tasmania, New Zealand's South Island as well as Patagonia in South America, that generally causes more storminess and rainfall, and colder temperatures.
However, for subtropical Australia, the shift results in reduced rainfall, clearer skies, and warmer temperatures.
The influence of the stratospheric warming has already been captured by the Bureau's climate outlooks, alongside with the influence of additional major climate drivers like the current positive Indian Ocean Dipole, leads to a hot as well as dry outlook for spring.
One positive note of sudden stratospheric warming is generally the reduction - or even absence - of the spring Antarctic ozone hole. That's firstly due to the rapid rise of temperatures in the upper atmosphere, which means the super cold polar stratospheric ice clouds. These are vital for the chemical process that destroys ozone, might not even form. Secondly, the disrupted winds tedn to carry more ozone-rich air from the tropics to the polar region, which helps repair the ozone hole.
Thanks to improvements in the related modeling and the Bureau's new supercomputer, such events can be forecast better than ever before.
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