Sudden Stratospheric Warming and its effects this Australian Spring
[image by NASA Stratospheric chemistry in the Southern Hemisphere]
Sudden warming like this has only been recorded twice before. The first event occurred in September 2002, and another weaker event occurred in September 2010. No other events have been recorded in 60 years.
These events are called Sudden Stratospheric Warming events. They occur high above the South Pole and can lead to unusually cold springs here in Australia.
The last Sudden Stratospheric Warming in 2010 preceded New Zealand’s coldest October in 20 years. The country experienced below average temperatures and frequent frosts.
Last year, in the Northern Hemisphere, the upper atmosphere above the Arctic warmed suddenly, leading to the infamous “Beast from the East”. The United Kingdom and Ireland experienced unseasonal freezing temperatures and snow storms across the countries.
What is Sudden Stratospheric Warming?
Sudden Stratospheric Warming occurs when the stratosphere – a layer of the upper atmosphere about 30 km from the ground – warms by more than 25°C over several days. Normally the stratospheric temperatures increase gradually over several weeks during spring.
The South Pole points away from the Sun in wintertime. The air above the South Pole doesn’t receive any heat from the sun, so the cold air becomes colder. The circulation becomes more isolated and the winds stronger throughout winter. This strong cold rotating air is called the Polar Vortex.
During winter, the Polar Vortex gets stronger and stronger, rotating quickly, and no longer mixing with the warmer surrounding air. At its coldest and strongest, the atmosphere usually cools to -80° C.
When spring comes around, the sun gradually begins to warm up the air again the vortex slows and warmer air mixes in. In rare cases, Sudden Stratospheric Warming causes a rapid breakdown of the cold polar air circulation changing the shape and location of the Polar Vortex and leading to unusual weather.
Stratospheric Warming brings colder weather
The Polar Vortex is usually centred and stable over Antarctica. The sudden warming in the stratosphere weakens the Polar Vortex. A weaker vortex causes the circulation to move away from over the pole and stretches out like an oval. The vortex might even split into two.
As the Polar Vortex breaks down, the cold stormy weather which is usually locked to Antarctica moves north. Where the vortex splits will affect where the cold conditions will be. Cold stormy weather could affect Australia, New Zealand, South America, or somewhere over the vast ocean.
We don’t know what exactly will happen because it is so hard to predict how and when the vortex will break down. It is also hard to know because of only two other known cases.
What we do know is that this event is unusual and will bring wild weather further north than normal. Depending on how the vortex splits, we might be in for a very wintery spring.