New year begins with a sudden stratospheric warming

Meteorologists are already able to note a significant atmospheric observation in their 2021 diaries with the beginning of a Sudden Stratospheric Warming (SSW), which started over the weekend – this was forecast last week.

A SSW sees cold air descending above the Arctic, resulting in warming by as much as 50°C. This is in association with a significant weakening or reversal of westerly winds circulating around the North Pole between 10 and 50 km above the ground – the stratospheric polar vortex.

Professor Adam Scaife – head of long-range prediction at the Met Office – said: “As predicted, atmospheric observations are now showing that the Arctic stratosphere is undergoing a sudden warming event associated with a weakening stratospheric polar vortex.”

During these events the vortex can break down completely, and when this happens the disruption in the stratosphere can trigger a shift from westerly to easterly winds which can be followed by lower-altitude winds shifting in the same direction. On average 70 percent of occasions see this switch to easterly conditions at ground level, with the resulting cold and easterly shift in our weather.

Matthew Lehnert is an Expert Operational Meteorologist with the Met Office. He said: “Although the prolonged cold spell and snow events in February and March of 2018 – dubbed the ‘Beast from the East’ by the UK media – were linked to a sudden stratospheric warming, the record warm spell that occurred in February 2019 also followed such an event.”

Paul Davies is the Met Office’s Chief Meteorologist. He added: “We can’t completely rule out a signal for colder weather following this SSW event later in the month. However, evidence from model data and other drivers of the UK weather support a return to relatively milder and more unsettled conditions next week.”

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