Record warm temperatures above Antarctica over the coming weeks are likely to bring above-average spring temperatures and below-average rainfall across large parts of New South Wales and southern Queensland.
The warming began in the last week of August, when temperatures in the stratosphere high above the South Pole began rapidly heating in a phenomenon called “sudden stratospheric warming”.
In the coming weeks the warming is forecast to intensify, and its effects will extend downward to Earth’s surface, affecting much of eastern Australia over the coming months.
The Bureau of Meteorology is predicting the strongest Antarctic warming on record, likely to exceed the previous record of September 2002.
What’s going on?
Every winter, westerly winds – often up to 200 kilometre per hour (120 miles per hour) – develop in the stratosphere high above the South Pole and circle the polar region.
The winds develop as a result of the difference in temperature over the pole (where there is no sunlight) and the Southern Ocean (where the sun still shines).
As the sun shifts southward during spring, the polar region starts to warm. This warming causes the stratospheric vortex and associated westerly winds to gradually weaken over the period of a few months.
However, in some years this breakdown can happen faster than usual.
Waves of air from the lower atmosphere (from large weather systems or flow over mountains) warm the stratosphere above the South Pole, and weaken or “mix” the high-speed westerly winds.
Very rarely, if the waves are strong enough they can rapidly break down the polar vortex, actually reversing the direction of the winds so they become easterly. This is the technical definition of “sudden stratospheric warming.”
Although we have seen plenty of weak or 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 late winter of the Northern Hemisphere because of stronger and more variable tropospheric wave activity.
What can Australia expect?
Impacts from this stratospheric warming are likely to reach Earth’s surface in the next month and possibly extend through to January.
Apart from warming the Antarctic region, the most notable effect will be a shift of the Southern Ocean westerly winds towards the Equator.
For regions directly in the path of the strongest westerlies, which includes western Tasmania, New Zealand’s South Island, and Patagonia in South America, this generally results in more storminess and rainfall, and colder temperatures.
But for subtropical Australia, which largely sits north of the main belt of westerlies, the shift results in reduced rainfall, clearer skies, and warmer temperatures.