SSW phenomenon is linked to polar vortex, an area of low pressure across the North Pole

There has been keen interest over recent weeks in the much-anticipated sudden stratospheric warming (SSW) event, which only began this week but is now well under way. The SSW phenomenon is linked to the polar vortex, an area of low pressure across the North Pole that forms within the stratosphere during autumn, as temperatures plummet in the absence of solar radiation.

SSW events are very common and occur two in every three winters. It remains unclear how climate change will affect these events in the future. As the vortex develops during autumn and into winter, westerly stratospheric winds increase in strength. But in the event of a SSW episode, stratospheric temperatures rise rapidly in the space of only a few days, leading to the weakening or even reversal of these winds. The zonal mean winds at 10hPa pressure – about 30km high – turned to an easterly direction on 15 Wednesday February, significantly displacing the polar vortex away from the North Pole. The vortex and zonal winds are forecast to stay much weaker than normal for the remainder of February and into the first half of March.