High above the Arctic, strong winds in the stratosphere can play an important role in shaping winter weather patterns across the Northern Hemisphere.
The stratospheric polar vortex, often shortened to SPV, is a large-scale circulation of very strong winds found high above the Arctic during the Northern Hemisphere winter. It sits in the stratosphere, around 10 to 50km above the surface, and circles a pool of very cold air over the North Pole. Although it forms far above the weather we experience day to day, changes in its strength and shape can influence the atmosphere below, including the position and behaviour of the jet stream.
During winter, the SPV is not fixed. It can strengthen, weaken, stretch, shift away from the pole or, in more dramatic cases, split apart. These changes matter because the stratosphere and troposphere, the lower part of the atmosphere where our weather takes place, are connected. When the SPV is strong and stable, it tends to support a strong, relatively straight jet stream across the North Atlantic. For the UK, this often means weather systems are steered in from the west, bringing milder, wetter and windier conditions.
When the SPV weakens, the picture can become more disrupted. A weaker vortex can be associated with a weaker or more meandering jet stream, allowing large north-south bends to develop in the flow. This can increase the chance of colder air moving south out of the Arctic, or of high pressure becoming established in places that interrupt the usual westerly pattern. For the UK, that can raise the likelihood of colder, drier spells, particularly if winds turn easterly or northerly.
What is sudden stratospheric warming?
A sudden stratospheric warming, or SSW, is one of the most significant ways the SPV can be disturbed. During an SSW, temperatures in the polar stratosphere can rise sharply over just a few days and the strong westerly winds that normally make up the vortex are weakened. In major events those winds reverse direction in the mid stratosphere.
Major SSW events can affect the SPV in two main ways. The vortex may be displaced away from the North Pole, or it may split into two smaller vortices. Either outcome can severely weaken the SPV and, at times, temporarily break it down. Once this has happened, it can take several weeks for the vortex to recover, and the effects can often work down through the atmosphere towards the surface.
However, an SSW does not automatically mean cold weather for the UK. The link between the stratosphere and surface weather is influential, but not guaranteed. The details depend on how the lower atmosphere responds after the event, including the position of high and low pressure, the behaviour of the jet stream, and other weather patterns affecting the North Atlantic and Europe.
What can it mean for UK weather?
When an SSW affects conditions near the surface, it can weaken the jet stream and encourage blocking high pressure over the North Atlantic, Greenland or Scandinavia, disrupting Atlantic weather systems and changing the air reaching the UK.
If high pressure becomes established to the north or northeast of the UK, colder easterly winds can impinge on the UK. These tend to bring colder, drier air and can produce sleet or snow showers, especially in eastern areas. In other setups, northerly winds can bring cold Arctic air southwards, increasing the risk of wintry showers, ice or snow, depending on temperatures and moisture.
At the same time, the impacts can vary widely. Some SSW events are followed by notable cold spells, while others have little lasting effect on UK weather. In some cases, the strongest impacts may be felt elsewhere in Europe or the Northern Hemisphere.
Why forecasters monitor the SPV
Monitoring the SPV is an important part of understanding medium-range and extended-range weather risks during winter. Its behaviour can provide useful context on whether the atmosphere is more likely to favour mobile, Atlantic-driven conditions or a greater chance of blocked patterns and colder outbreaks. It is one factor among several, including sea surface temperatures, tropical weather patterns, existing pressure systems and short-term developments in the jet stream.
For the UK, the SPV is therefore best thought of as a background driver that can tilt the odds towards certain weather patterns, rather than a switch that turns cold weather on or off. A strong SPV often supports mild, wet and windy winter weather from the Atlantic, while a weakened or disrupted SPV can increase the chance of colder, more blocked patterns. Sudden stratospheric warming is one of the clearest examples of this connection and it most often leads to cold dry conditions but this depends on how the whole atmosphere responds in the days and weeks that follow.
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