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.
Featured image credit: NIWA