The UK could face severe snow and cold in the coming months as this year's record-breaking El Nino may trigger Sudden Stratospheric Warming (SSW), the phenomenon that led to the 2018 'Beast from the East'.
The historic weather system brought 22 inches of snow to some parts of the country in February and March of that year, with temperatures plunging as low as -14C. It was caused by SSW, which, despite its name, can bring bitterly cold conditions to UK shores.
El Nino and the polar vortex
El Nino describes the warming of sea surface temperatures that occurs every few years, declared when temperatures in the tropical eastern Pacific rise 0.5C above the long-term average. It leads to rising global temperatures in the summer, while winters can be stormier, wetter and colder.
This year's El Nino, expected to be the strongest ever, will peak soon. This may lead to SSW again, experts say. The Met Office explains: "Through the latter part of the winter, following a peak in El Nino, the stratospheric Polar Vortex can become weaker, leading to an increased chance of Sudden Stratospheric Warming events unfurling, increasing the potential for cold weather."
What is Sudden Stratospheric Warming?
SSW refers to a rapid warming (up to about 50C in just a couple of days) in the stratosphere, between 10km and 50km above the earth's surface. This means cold air rushes downwards, impacting the jet stream and potentially creating a 'blocking pattern' which can keep snow and cold weather locked in place for an extended period.
The Met Office explains: "The stratospheric sudden warming can sometimes cause the jet stream to 'snake' more, and this tends to create a large area of blocking high pressure. Typically this will form over the North Atlantic and Scandinavia. This means that northern Europe, including the UK, is likely to get a long spell of dry, cold weather, whereas southern Europe will tend to be more mild, wet and windy."
As well as the Beast from the East in 2018, this process also led to extreme cold and snow in 2009-10 as well as 2013. Although SSW doesn't always result in these outcomes, the Met Office says a cold snap "follows more often than not".
