Sudden Stratospheric Warming Event Underway above Antarctica: What does this mean for us?
10hPa Temperature Anomaly (c) - Showing Temperatures high in the Stratosphere 25-35c above normal.
A Sudden Stratospheric Warming (SSW) event above Antarctica is a rare and dramatic increase in temperature within the stratosphere, the atmospheric layer roughly 10 km to 50 km above the Earth's surface. Normally, during the Antarctic winter, a strong "polar vortex" — a circulation of very cold, fast westerly winds — keeps frigid air trapped over the South Pole.
During an SSW, large atmospheric waves from the lower atmosphere propagate upwards, disrupting this polar vortex. This causes the stratospheric winds to weaken significantly, sometimes even reversing to flow from east to west. This disruption leads to a rapid descent of the very cold air, which compresses and warms dramatically. Temperatures in the polar stratosphere can increase by more than 25∘C in just a week, and sometimes even by 50∘C over a few days.
This current warming event, at this stage, is not near the level of our last SSW event in 2022, and significantly weaker than the major event in 2002 which resulted in a particularly cold October with below-average temperatures and frequent ground frosts. The SSW caused a weakening of the prevailing westerly winds, leading to a shift towards easterly winds and dramatic changes in weather patterns. October 2002 was the coldest October in New Zealand in 20 years.
Meanwhile, the event in 2022 did not result in a coupling with the troposphere around the New Zealand area, which could have led to significant cold impacts.
Unlike the Arctic, where SSWs occur about once a year, they are much less frequent above Antarctica, typically happening only once every five years or so. While the warming occurs high up, these events can have "knock-on" effects on surface weather several weeks later, influencing the jet stream and potentially leading to outbreaks of cold air in mid-latitude regions of the Southern Hemisphere, including New Zealand.
While this event is not expected to become severe, like in 2002, we are keeping a close eye on developments over the coming next few days to weeks - one crucial indicator to watch during these sorts of setups will be the progression of the Southern Annular Mode next Month (positive generally leads to more settled weather, while negative can lead to intense cold outbreaks, and typically more unsettled weather)
Bottom line: A Sudden Stratospheric Warming Event is currently underway above Antarctica and is likely to remain so for the remainder of July. Whether this event can be coupled with the troposphere (the weather on the ground) in terms of a negative Southern Annular Mode in August/September remains to be seen based on current modeling.
