Rare weather phenomenon amassing in southern hemisphere - Sudden Stratospheric Warming (SSW)
A rare weather phenomenon for the southern hemisphere - Sudden Stratospheric Warming (SSW) - is amassing in the polar stratosphere. This may lead to unusual or extreme weather in parts of the southern hemisphere (potentially New Zealand) during September and October 2019. SSWs are much more common in the northern hemisphere. You are probably familiar with a major northern hemisphere's SSW event of February 2018 - dubbed the 'Beast from the East' - and odds are you'll be seeing many international headlines with 'September to Remember' in the days and weeks ahead.
SSW occurs when the temperature of the stratosphere (30 - 50 km / 18 - 31 miles above ground) over the South Pole rises by more than 25 °C (45 °F) and NIWA meteorologist Ben Noll says this looks likely to occur next week.
Such events are rare in the southern hemisphere, with only two in New Zealand in recorded times - one in September 2002 and the other in September 2010, NIWA reports. 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. In 2010 – which is classed as a minor event – a number of rainfall records were broken with well below normal sunshine and very cold temperatures in parts of the South Island.
The SSW is likely to peak between Thursday, August 29 and Monday, September 2.
"For up to about a month after the SSW, polar air masses, known as streamers, can break off from the weakened vortex and move towards New Zealand. It doesn’t guarantee unusual or extreme weather, but it can happen," Noll says.
SSWs are much more common in the northern hemisphere. The southern hemisphere is characterized by a cold Antarctic continent surrounded by relatively warm seas, leading to a more stable circulation around Antarctica than in the Arctic where a relatively warm Arctic Ocean is surrounded by cold continents in winter.
In the southern hemisphere's winter, a ring of stormy and freezing weather encircles Antarctica. Known as the polar vortex, it is usually very good at keeping harsh, wintry conditions locked up close to the pole. When a SSW occurs, it can help to weaken or displace the polar vortex in the stratosphere, which then filters down onto the tropospheric polar vortex and influences weather patterns.
Sudden Stratospheric Warming (SSW) event above South Pole to go down as the strongest on recordVery rare Sudden Stratospheric Warming (SSW) event continues unfolding above the South Pole.
This SSW could go down in history as the strongest warming event on record, NIWA said, adding there is increased risk for southerlies in New Zealand into October.
SSW events are rare in the southern hemisphere, with only two in New Zealand in recorded times - one in September 2002 and the other in September 2010
The Australian Bureau of Meteorology is also predicting it as the strongest Antarctic warming on record, likely to exceed the previous record of September 2002 when New Zealand experienced its coldest October in 20 years.
Below-average temperatures covered much of the country in 2002, resulting in frequent ground frosts.
In 2010 SSW event – which is classed as a minor event – a number of rainfall records were broken with well below normal sunshine and very cold temperatures in parts of the South Island.
"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," BOM meteorologists said.
"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.
"Past stratospheric warming events and associated wind changes have had their strongest effects in NSW and southern Queensland, where springtime temperatures increased, rainfall decreased and heatwaves and fire risk rose."
An unusual atmospheric phenomenon is amassing in the stratosphere above Antarctica, called a Sudden Stratospheric Warming.— NIWA Weather (@NiwaWeather) August 22, 2019
There have only been 2️⃣ such events on record & number 3️⃣ is possible next week!
It may contribute to unsettled, chilly weather across NZ in Sep-Oct ❄️ pic.twitter.com/LnZxMjVtUT
Temperatures have been below average month-to-date in New Zealand's South Island, NIWA said September 10.
Temperatures in central Otago plummeted to -5 °C (23 °F) today, forcing farmers to crank up frost fighting efforts with machines and helicopters, Newshub reports.
Tim Jones, chief executive of 45 South Orchard and Packhouse in Cromwell, says the cold is expected to stick around for another month.
"The last one in 2002 I do recall had an awful lot of frosts right throughout the spring," he said. "Particularly leading on into October and November. Who knows whether we're up for that again this year, but the key thing is that we're prepared for it."