Polar vortex winds reverse in earliest sudden stratospheric warming on record, winter impact uncertain

The spiralling winds of the polar vortex high above the Arctic have reversed direction, marking the earliest 'major sudden stratospheric warming' on record in modern observations.

These polar winds usually blow in a westerly direction, keeping cold air locked high above the North Pole. In recent weeks, however, the polar vortex has weakened and shifted position, and observations now show a switch to easterly winds, which experts say defines a major sudden stratospheric warming, or SSW.

An event this early in autumn is unprecedented in modern observations. The previous earliest SSW on record happened on 28 November, in both 1969 and 2025.

The polar vortex is a normal feature of the climate in the months around winter. As the North Pole tilts away from the sun, temperatures above the Arctic drop and strong westerly winds form in the stratosphere.

A sudden stratospheric warming is a rapid and dramatic rise in temperature high in the stratosphere, roughly 10 to 30 miles (15-50km) above Earth's surface. It can happen when atmospheric disturbances shift the position or shape of the polar vortex and make its winds weaken, eventually reversing the strong westerlies into easterlies.

In the following days and weeks, these changes can slowly filter lower down in the atmosphere, altering the position of the jet stream, which drives the weather experienced on the ground.

Major SSW events have often historically increased the chance of drier and colder weather in the UK, with northerly or easterly winds bringing in Arctic air, and can also boost the possibility of snow, especially across eastern parts of the UK. The 2018 'Beast from the East' followed a major SSW that occurred a few weeks earlier.

However, an SSW does not automatically lead to cold or snowy weather, and any influence would take time to develop. There are very few historical examples of early-season major SSWs to compare with, so it is unclear what impacts may happen a few weeks from now. The report also says this event is definitely not connected to the current cold spell.

Because the stratospheric polar vortex operates at a global scale, disruptions and reversals can have impacts around the world, increasing the possibility of cold weather, ice and snow in many regions, including North America. The possible impacts on the USA and Canada are, as in the UK, very uncertain.

The deadly winter storm that swept as far south as Texas in February 2021 followed a major SSW event the previous month. Widespread snowfall in February 2018 and the record 'snowmageddon' that brought record-breaking snow to Washington DC in February 2010 were also linked to shifts in the polar vortex. Those examples all occurred in late winter, when polar vortex disruptions are most likely.

The difference this time is that the SSW has taken place unprecedentedly early, in autumn, so its expected impacts are even less certain than normal. Even if widespread northerly winds develop, the Arctic has not yet become as bitterly cold as it will be by late winter, so any chill spells may be somewhat muted.

At the same time, the world is heading towards the expected peak of the strongest El Niño on record. In the UK, El Niño tends to bring a higher chance of wetter and stormier conditions in autumn and early winter, and a higher chance of colder wintry weather late in winter. In the US and Canada it can bring a greater chance of a warm and dry winter in northern areas and more risk of flooding rain further south. Any link between a strong El Niño and the early onset of an SSW event is unclear and will need further research.