Polar vortex weakens unusually early, UK snow not guaranteed

Scientists say the polar vortex, a ring of powerful winds high above the Arctic, has weakened and shifted in recent days in an event they describe as unprecedented this early in November in modern observations.

The polar vortex develops every autumn as Arctic temperatures fall. Strong westerly winds form in the stratosphere, roughly 10 to 30 miles (15-50km) above Earth's surface, trapping the coldest air near the pole. Atmospheric disturbances can warm the air high up, weakening the vortex winds. This sudden stratospheric warming can eventually reverse the strong westerlies into easterly winds.

In the following days and weeks, those changes can alter the jet stream and increase the chance of a blocked weather pattern. That can bring drier conditions and colder northerly or easterly winds carrying Arctic air, and it can boost the possibility of snow, especially in eastern UK.

Most sudden stratospheric warming events occur from mid-winter, but the current weakening is dramatic. Some forecasts show a reversal of the polar vortex winds by mid-November, and the vortex has stretched and briefly split into two smaller circulations around the Arctic. If the winds continue to weaken and reverse as forecast, it will be classified as a major sudden stratospheric warming event.

Jeff Knight, lead climate scientist at the Met Office, told the BBC that it appeared to be the earliest such event on record in 75 years of observations, with most experts agreeing it was very unusual.

The reasons for such an early event are not clear. It could be part of normal weather variability. In a normal El Niño year, the polar vortex can weaken towards the end of winter. Warmer Pacific waters disrupt tropical weather, and pulses of energy known as planetary waves can disturb the polar regions, pushing the vortex out of position and breaking up the circulation.

With the current record El Niño, Knight suggested it could have contributed to the conditions for this sudden stratospheric warming event. However, he said there was not enough evidence because the event is so rare. Any link between a strong El Niño and an early sudden stratospheric warming is unclear and needs further research.

Met Office long-range forecasts had suggested the UK was more likely to see a stormy start to winter, followed by a colder spell at the end of winter, consistent with El Niño expectations. But this unusually early sudden stratospheric warming was not widely expected.

Professor Andrew Perez-Charlton, a meteorologist at the University of Reading, said the typical response to a sudden stratospheric warming is colder and drier conditions over Europe and the UK. He added that because no similar event has been seen before, it is harder to anticipate a cold spell at the moment.

The Arctic is not yet as cold as it becomes later in winter, and the polar vortex has only recently developed. Any influence on weather patterns could be weaker than during a mid-winter disruption, so cold and snowy weather for the UK is far from guaranteed.

While snow is not likely in the coming weeks, a possible continuation of drier than average conditions would be concerning for reservoir replenishment. Most of England and Wales are still in drought, and a wetter winter is needed to fill extremely low reservoirs.

Perez-Charlton said the concerning thing would be if this event brought the typical drier weather pattern and lack of rain associated with sudden stratospheric warming over the coming months. For now, experts stress it is too early to know whether the recent polar vortex changes will alter the winter outlook.

Source: BBC Science.