Anomalies of the recent SSW event begin downward propagation, potentially bringing a cold start to August

On July 30, 2024, NIWA reported that the anomalies from the recent stratospheric warming have started to propagate downward and are affecting tropospheric weather patterns in the southern hemisphere.

NIWA is forecasting very high pressure over Antarctica, which will culminate in a cold start to August for New Zealand and western Australia.

A ring of stormy weather called the polar vortex forms high above Antarctica in the stratosphere every winter. However this year it showed signs of warming and disturbance.

The rare Sudden Stratospheric Warming (SSW) event was detected in the stratosphere above the Antarctic region earlier this month. Forecast models had predicted it would start to filter down through the atmosphere later this month.

Anomalies from the recent stratospheric warming are propagating downward & affecting tropospheric weather patterns in the Southern Hemisphere

Very high pressure is forecast over Antarctica, which will culminate in a cold start to August for New Zealand & western Australia. https://t.co/CSLAqZWT21 pic.twitter.com/z1w99LtSXi

— NIWA Weather (@NiwaWeather) July 30, 2024

The downward propagation of these anomalies means that the tropospheric polar vortex will weaken, which would allow cold polar air to drift further away from Antarctica and spread towards the mid-latitudes.

The Southern Annular Mode (SAM) started shifting into its negative phase about 1 week ago and was likely caused by the SSW event over Antarctica, which is expected to bring colder and wetter conditions to southern Australia.

The negative phase of SAM brings notable changes in atmospheric conditions, including increased rainfall and cooler temperatures in southern Australia and parts of New Zealand.

Meteorologists had warned that there were signs that another period of stratospheric warming could occur above Antarctica in the last week of July. Which could increase the likelihood of more negative SAM phases in late winter and early spring.

SSW events are rare in the southern hemisphere, with only a few known events in the past decades, but they can be powerful and are known to affect the entire globe.

Major SSW events can cause prolonged high-pressure anomalies over the South Pole, potentially affecting the Northern Hemisphere after several weeks to months.

References:

¹ Impacts of Sudden Stratospheric Warming (SSW) on southern hemisphere’s weather patterns – The Watchers – July 24, 2024

² Rare Sudden Stratospheric Warming event detected over Antarctica – The Watchers – July 15, 2024