Sudden stratospheric warming underway – what does it mean for New Zealand?

Posted on the 16th July by Russell Voice

A relatively rare sudden stratospheric warming event is beginning to occur above Antarctica, and it may influence New Zealand over the next 6-8 weeks.

The term sudden stratospheric warming (SSW) refers to an abrupt increase in air temperature high above either of Earth’s polar regions, typically on the magnitude of tens of degrees Celsius in a few days. This warming occurs in a layer of the atmosphere called the stratosphere, roughly 30 to 40km above the surface.

While warming in the stratosphere does not immediately (or always) affect weather patterns near the ground, SSW events can filter down through the atmosphere and influence tropospheric weather in the weeks following the initial SSW.

If an SSW event does make its way down to the troposphere, it can cause the tropospheric polar vortex to weaken, which allows cold polar air to drift further away from Antarctica and spread towards the mid-latitudes.

Through this domino effect, SSW events can cause the Southern Annular Mode (SAM) to shift into a negative phase, which can have the following impacts in New Zealand during winter.

POSSIBLE IMPACTS HERE IN NEW ZEALAND

•This can lead to more cold fronts and low pressure systems over New Zealand. A slightly higher frequency of brief surges of colder weather; possibly our first wee indication may occur late July. This may be quite noticeable after a very calm June and first half of July.

•Increased rainfall and low snow potential over southern New Zealand. This may finally kick start our local ski field season, as we move into early August.

•Brief periods of stormier weather. Stronger southerly and south-westerly winds and brief periods of very cold weather for eastern areas.

•Note SSW is climate driver and influencer, it’s not a specific forecast as such. Ultimately, what this means is we may see a slightly higher chance of surges and bursts of colder air over the next 6-8 weeks. More to come.

•Keep up to date with Canterbury Weather Updates forecasts for all the finer details.