The polar vortex is an important part of the winter atmosphere that can shape the weather conditions we experience in the UK. We spoke to Dr Matt Patterson, a climate scientist at the National Centre for Atmospheric Science and University of St Andrews, about the weakening polar vortex and the role of sudden stratospheric warming and El Niño.

The polar vortex is a band of strong winds above the Arctic, 15-50km high in the layer of the atmosphere known as the stratosphere. From autumn to early spring, these winds circle the North Pole and help keep very cold air contained in the Arctic region.

When the polar vortex is strong, it tends to support a powerful jet stream. The jet stream is a river of fast flowing air over the North Atlantic, around 10km above sea level, which typically brings the UK its mild, wet, and windy winter weather.

Sometimes the polar vortex weakens or becomes disrupted. “Around once every year or two, the polar vortex can break down during winter in an event known as a sudden stratospheric warming. This increases the chances of the jet stream shifting southwards and becoming more wavy, allowing cold dry Arctic air to move into the UK and Europe. However, this chain of events does not always occur, so a sudden stratospheric warming does not automatically lead to a cold spell,” says Matt. 

Sudden stratospheric warming, also known as SSW by atmospheric scientists, is a dramatic warming of the stratosphere, a layer of the atmosphere around 10 to 50 kilometres above the Earth’s surface. 

Matts explains: “During an SSW event, temperatures high above the Arctic rise rapidly and dramatically over a matter of days, by up to 50°C, weakening the strong winds that form the polar vortex. A major sudden stratospheric warming event is likely to occur in mid November, marking the earliest major SSW on record by a large margin. It is not immediately clear why this has happened so early in the season or what the impact it will have given the rarity of such events. There is debate in the scientific community over the role that the super El Niño may have played, though it is likely that other factors will be at play. SSW events make it more likely that the jet stream will shift south or become more wavy in future weeks, favouring colder, drier conditions for the UK.”

He adds: “There isn’t a clear signal for colder than average weather in long-range forecasts but this could change in the coming days once the SSW is underway. It is quite unlikely that the SSW will bring snow given how early we are in the season, yet drier than average conditions could disrupt the recovery of reservoirs which are at low levels across large parts of southern England, following a dry summer. This SSW raises lots of questions regarding its early timing, underlining how much we still have to discover about our atmosphere, which makes studying it all the more exciting!”

As the polar vortex weakens, it can become distorted, shift away from the pole, or even split into smaller vortices. This can increase the likelihood of changes in the jet stream, causing it to shift southwards and become more wavy. When this happens, cold Arctic air can move further south, making colder conditions in the UK and Europe more likely, including frosts, snow and easterly winds. However, a weakened polar vortex alone does not guarantee a cold spell.

The effects are not immediate either. Changes high up in the stratosphere can take several weeks to influence weather closer to the ground. The atmosphere is complex, so the impacts vary from event to event, and some disruptions have little noticeable effect on UK weather.

“The polar vortex weakening does not guarantee cold weather but changes the odds to make it more likely,” explains Matt.

When a polar vortex weakening occurs earlier than usual, it simply means the atmospheric conditions needed to disrupt it developed sooner in the winter season. Each event is different, and the timing, strength and impacts can vary considerably from year to year.

“This November’s early polar vortex weakening is unprecedented. If sudden stratospheric warming happens, it is usually in January or February. The earliest it has happened on record is 28th December in 1968 and 2025, but this year it looks like it will be 11th November,” adds Matt. 

Large-scale climate patterns, such as El Niño, may influence the atmosphere in ways that affect the polar vortex. Matt explains: “El Niño does this by creating large-scale atmospheric waves, much like how throwing a rock into a pond creates ripples on the surface. These waves can act to disrupt the vortex.”

El Niño is only one of several factors involved, and early weakening events can happen for a range of reasons. “El Niño is a dominant driver of our weather this winter, especially as it is likely to be the largest on record, but there are multiple pathways through which it can affect our weather,” says Matt.