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photo: UnSplash/Aaron Burden
Cold air is rising from the polar vortex surrounding Antarctica.
NIWA says every winter a circle of stormy, cold weather – known as the polar vortex – surrounds Antarctica.
Meteorologist Ben Noll said it usually confined its harsh winter conditions to near the South Pole, but on rare occasions it could be disturbed and move across New Zealand.
“Sometimes, the polar stratosphere (the layer of atmosphere 10-20 km above Antarctica) can warm very quickly, with temperatures rising by more than 25 degrees Celsius in a week. This is called a sudden stratospheric warming and can cause the polar vortex to weaken or stretch and shift.”
Noll said it can affect the atmosphere and therefore the weather.
“Icy air masses near the poles can more easily escape to the mid-latitudes of the hemisphere, like a tray of ice cubes being shaken loose.”
He said the phenomenon has now been observed over Antarctica and will have a strong impact on weather patterns in the Southern Hemisphere throughout August.
This will result in cold weather early in the month followed by more mild temperatures.
There have been only three instances of abrupt stratospheric warming in the satellite era: in September 2002, September 2010, and August-September 2019.
Following the 2002 El Niño event, New Zealand experienced its coldest October in 20 years, with temperatures below average and frequent ground frost across much of the country.
Mr Noll said this year’s weather influences meant the cold, frosty weather from late August to late September was likely to be followed by warmer, wetter weather.
MetService meteorologist Lewis Ferris said temperatures dropped below freezing in many parts of the South Island on Friday morning.
“A very cold front from the south is bringing the potential for snow to the lower parts of the South Island and bringing cold air across the country, particularly those areas that are still being affected by southerly winds.”
The coldest place overnight was Mount Cook Airport, with a temperature of -10.3C, while Central Otago dropped to -4C and Christchurch experienced -1C.
Ferris said the cold temperatures will continue into the weekend, but much of the country will see sunny, settled weather.
Christchurch’s overnight low of -5C means people in the Garden City are in for a chilly start on Saturday.
Ferris said most locations will continue to experience chilly mornings followed by sunny afternoons.
He said the exception was Northland, which was currently under a heavy rain warning which could be upgraded to an orange warning from Saturday afternoon to Sunday afternoon, when the rain was expected to intensify and become more widespread.
“The cold snap that’s been sweeping across the country this week has been a boon to the ski resorts, so conditions will be perfect this weekend, but you’ll also be experiencing larger crowds.”
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