An unusual disturbance high above Antarctica is causing polar air to encroach on different parts of the Southern Hemisphere, including New Zealand.
Every winter, a ring of stormy, freezing weather - known as the polar vortex - encircles Antarctica. Typically, it keeps harsh, wintry conditions locked up near the south pole.
However, NIWA meteorologist Ben Noll said that on rare occasions, it becomes disturbed.
"Sometimes the polar stratosphere, a layer of atmosphere 10-20 km above Antarctica, warms rapidly, with temperatures rising by more than 25C in a week. This is called a sudden stratospheric warming (SSW) and causes the polar vortex to weaken or become stretched and displaced.
"This can influence the atmospheric layer where our weather happens. The icy air masses near the pole have an easier time escaping into the hemisphere’s mid-latitudes, like an ice cube tray being shaken loose," said Ben.
This phenomenon is currently being observed above Antarctica and will strongly influence the Southern Hemisphere’s weather patterns throughout August. It will contribute to a cold and frosty start to the month in New Zealand, followed by a moderating trend.
"Unusually cold air is forecast to be more persistent in southern South America, including Chile, Argentina, and even as far north as Paraguay, Uruguay, and southern Brazil. But while the chill seeps into the mid-latitudes, Antarctica will experience well above average temperatures," said Ben.
There have only been three SSWs in the satellite era: September 2002, September 2010, and August-September 2019.
After the SSW in 2002, New Zealand experienced its coldest October in 20 years with below average temperatures covering much of the country and frequent ground frosts.
For New Zealand, the weather-related impacts from the 2024 SSW look to feature cold, frosty conditions initially, followed by a trend toward warmer and wetter conditions later in August and into September.
NIWA meteorologists discussed the polar vortex in their just-issued Seasonal Climate Outlook for August-October: https://youtu.be/xjhdkrtyYpQ?si=7RNr-_H32SZBVmUc&t=227
