What is a sudden stratospheric warming?

A sudden stratospheric warming (SSW) event could lead to a repeat of the 'Beast from the East'. Photo: Jeff J Mitchell

Caroline O'Doherty

March 02 2023 08:09 AM

Just as the days are lengthening, buds are swelling and birds fly overhead with twig-filled beaks, there’s a warning that winter may be about to reappear.

A ‘sudden stratospheric warming’ (SSW) event could mean freezing conditions and heavy snow.

The word ‘could’ is important here – a similar event in February 2018 brought us the ‘Beast from the East’.

A repeat in 2019 and again 2021 caused much excitement but ultimately resulted in not a single extra sale of thermals and no rush on sliced pans.

SSWs occur extremely high up, the stratosphere being the layer of atmosphere around 15-50km above the Earth.

Up there above the north pole, is the polar vortex, a whirling mass of freezing air.

Every so often an upset occurs, the vortex slows or even reverses direction, and the loss of momentum causes the frigid air to drop. This leaves behind dramatic warming temperatures – the ‘sudden stratospheric warming’.

As the freezing band of air drops, it can knock the lower down jet stream off course.

Jet streams are like currents of air that continuously flow around the globe.

Because they follow a fairly reliable pattern, so does our weather. An interruption – such as the intrusion of frigid air from the polar vortex – can send the weather awry.

Our jet stream is a formidable force and while it wobbles, slows and stalls occasionally, it does not easily take a back seat to other influences.

We can look back to 2018 for an indication of what conditions can be like.

It began snowing on February 27 and kept on snowing all over the country until March 3, with 69cm falling in parts of the east, with drifts deeper again, and temperatures dropping to minus seven.

The cold meant the snow lay for days, occasionally topped up by fresh falls, and when it seemed the last pockets would melt away, there were significant and widespread falls again mid-month.

Similar events happened during the big freeze of December and January 2010-2011 and also in February 2009 but it’s hard to see a pattern.

Although there is some indication that SSWs are increasing, there is not necessarily a corresponding increase in extreme weather events.