What is sudden stratospheric warming?

The current mild weather in Britain could come to an end in the first week of March as a result of a phenomenon known as sudden stratospheric warming, which is currently brewing above the Arctic, the Met Office has warned.

Theoretically, although there is little to indicate it will happen at present, these conditions could bring a new “Beast from the East” to the UK, reminiscent of that bitter winter storm the British Isles experienced in early March 2018, which saw heavy snow, ice and strong winds cause 17 fatalities and bring the country to a standstill.

Forecaster Aidan McGivern said: “We are seeing a major sudden stratospheric warming taking place above the North Pole and what that means is that the winds in the stratosphere surrounding the North Pole are expected to reverse, instead of going from west to east they are going to go from east to west.

“That can have a drag effect on the jet stream, which can slow the jet stream down, which can, in turn, lead to higher pressure at the surface, a blocking area of high pressure, blocking wind and rain from the Atlantic and sometimes leading to colder conditions. That’s why sudden stratospheric warmings increase the chance of cold weather.

“Not immediately, though, although this is taking place right now, there is a lag effect so we are not expecting an impact, if any, to take place until the first week of March.”

He continued: “Not all sudden stratospheric warmings lead to cold weather at the surface but what we are seeing from the computer models for the start of March is a signal for higher than normal pressure that would be consistent with the slowing down of the jet stream.

“However, we are not seeing a strong signal for colder weather because it would depend for the UK on where that higher than normal pressure ends up, whether it is a strong high to the north of the UK or whether it is centred over the UK, which would lead to more typical temperatures for the time of year.”

As the Met Office explains at greater length on its website, a weakening of the rapid westerly winds roaring above the Arctic, known collectively as the Polar Night Jet, occurs when natural weather patterns or disturbances lower down in the atmosphere disrupt their flow and cause the jet to “break just like waves on the beach”.

When that happens, “cold air then descends very rapidly in the polar vortex and this causes the temperature in the stratosphere to rise very rapidly, as much as 50­C over only a few days”.

This process is what is known as sudden stratospheric warming, which typically takes place “between 10km and 50km above the Earth’s surface”, meaning we do not notice the warming effect on the ground.

The forecaster continues: “As the cold air from high up in the stratosphere disperses, it can affect the shape of the jet stream as the cold air sinks from the stratosphere into the troposphere.

“It is this change in the jet stream that causes our weather to change.”

If the jet stream “snakes”, large areas of blocking high pressure can form over the North Atlantic and Scandinavia, condemning northern Europe to a long period of cold, dry weather while the south of the continent remains mild.

The good news is that the interval of several weeks between the sudden stratospheric warming taking place and its beginning to impact our weather is long enough that it can be reliably tracked with satellites.

The Met Office also stresses that this process does not necessarily take place every year and does not always affect our weather when it does, although the prospect of it occurring next month leaves spring feeling some way off.