Impacts of Sudden Stratospheric Warming (SSW) on southern hemisphere’s weather patterns

The Southern Annular Mode (SAM) has started shifting into its negative phase this week, likely caused by a recent Sudden Stratospheric Warming (SSW) event over Antarctica, which is expected to bring colder and wetter conditions to southern Australia.

• This week, the Southern Annular Mode (SAM), a key climate phenomenon in the southern hemisphere, has started shifting into its negative phase. The SAM, which describes the north-south movement of the westerly wind belt around Antarctica, plays a significant role in shaping the weather patterns across regions such as Australia, New Zealand, and parts of South America and Africa.
• 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. This shift can have wide-reaching effects on agriculture, water resources, and marine environments. Understanding and monitoring these changes are crucial for preparing and adapting to the upcoming weather impacts.
• Meteorologists warn there are signs that another period of stratospheric warming could occur above Antarctica in the last week of July. If this does happen, it will increase the likelihood of more negative SAM phases in late winter and early spring.

Abnormally warm air has disrupted the shape of the southern hemisphere’s stratospheric polar vortex over the past two weeks, causing a phenomenon known as Sudden Stratospheric Warming (SSW). Occurring roughly 30 – 40 km (19 to 25 miles) above the Earth’s surface, this warming event is now filtering downwards, affecting weather patterns closer to the ground.

SSW is characterized by an abrupt increase in air temperature above Earth’s polar regions. When this warming occurs during winter and spring, it can cause the tropospheric polar vortex to weaken and distort.

This weakening typically triggers a negative Southern Annular Mode (SAM) phase, resulting in westerly winds and polar air drifting further from Antarctica towards the mid-latitudes.

The shift to a negative SAM phase during winter can lead to several weather changes in Australia:

• Increased cold fronts and low-pressure systems over southern Australia.
• Higher rainfall and snow potential in southwest and southeast Australia.
• Reduced rainfall in parts of eastern Australia.
• Stronger winds in the southern half of Australia.

Additionally, a weakened polar vortex can cause the southern hemisphere’s polar jet stream to adopt a more wavy pattern, known as meridional flow.

This pattern allows cold air to move away from Antarctica in large blobs, leading to wintry outbreaks in mid-latitude areas under these polar air pools.

WeatherZone’s Ben Domasino reported that the SAM has begun shifting into a negative phase this week, likely in response to the recent stratospheric warming.

Forecast models predict this negative SAM will strengthen through the end of July and persist into early August. Concurrently, a wavy polar jet stream is anticipated to meander around the southern hemisphere.

Australia’s weather forecast over the next fortnight reflects these changes:

• At least three cold fronts will sweep over southern Australia in the next 10 days, causing increased wind and dropping temperatures in the region.
• Heavy and frequent rain is expected in southwestern Western Australia and western Tasmania.
• Dry and warm continental air will flow over eastern Australia, leading to a series of dry and warmer-than-average days on the east coast.

There are indications that another period of stratospheric warming could occur above Antarctica in the last week of July. If that happens, it will increase the likelihood of more negative SAM phases in late winter and early spring, potentially extending the current weather patterns.

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 have an effect on the entire globe, said Andrej Flis of Severe Weather Europe.

“These stratospheric warming events pack a lot of energy and can affect the entire hemisphere. We have had several cases where that happened in the Northern Hemisphere, affecting the winter weather patterns,” Flis said.

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

This connection is facilitated by the Brewer-Dobson circulation between the northern and southern stratospheres. A study of a significant 2019 SSW event over the South Pole identified ionospheric anomalies in the United States, with electron charge anomalies of 40-80%, while Europe experienced opposite changes.

The study found disturbances in the thermospheric Oxygen/Nitrogen ratio and Total Electron Content (TEC) over both regions, influencing Earth’s geomagnetic activity. These disturbances led to observable weather changes, affecting the winter weather of 2019/20 in the Northern Hemisphere.

References:

¹ Negative Southern Annular Mode (SAM) emerging – WeatherZone – July 23, 2024

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

³ A rare Stratospheric Warming event has begun over the South Pole, with unusually strong anomalies now developing – Severe Weather Europe – July 15, 2024

⁴ Impact of September 2019 Antarctic Sudden Stratospheric Warming on Mid-Latitude Ionosphere and Thermosphere Over North America and Europe – Larisa P. Goncharenko et al. – AGU Geophysical Research Letters – July 2021 – https://doi.org/10.1029/2021GL094517