A major disruption of the stratospheric polar vortex, the ring of strong westerly winds around 30 km above the Arctic, began in early November, weeks earlier than usual.

A major SSW is defined by the westerly winds at 10 hPa (about 30 km up) and 60°N reversing to easterly (Butler & Polvani, 2011). If that reversal is confirmed, this will be the earliest major SSW in the record that begins in 1958. The previous earliest began in late November 1968: on 27, 28 or 29 November, depending on the dataset (Butler et al., 2017).

An SSW weakens the winds that normally keep the coldest air locked over the Arctic, making it more likely that polar air spills south into the mid-latitudes.

In a 'normal' winter, strong westerly winds circle the pole 10–50 km up. This stratospheric polar vortex holds cold air over the Arctic. A strong vortex tends to be followed by stronger westerlies near the surface, which carry mild Atlantic weather towards the UK (Baldwin & Dunkerton, 2001).

An SSW starts when large atmospheric waves rising from below break against the vortex. Polar stratospheric temperatures can jump by up to about 50°C in a few days, and the westerly winds slow or reverse (Baldwin et al., 2021). With the vortex weakened, displaced or split, cold air outbreaks over North America and Eurasia become more likely.

Over the North Atlantic and Europe, the most common patterns in the weeks after an SSW are high pressure over Greenland and an Atlantic storm track shifted south-east. Greenland blocking brings cold conditions to Europe, while the shifted storm track brings milder weather. Which develops depends partly on the weather pattern in place when the SSW begins (Domeisen et al., 2020).

The disturbance works its way down through the stratosphere, and the altered surface pattern can then last for around 60 days (Baldwin & Dunkerton, 2001). About two-thirds of SSWs are followed by a persistent negative North Atlantic Oscillation, the pattern linked to colder winters in northern Europe; the rest are not (Domeisen, 2019). In observed European data, cold extremes over Scandinavia intensify in the 60 days after an SSW, while southern Europe tends to turn wetter (King et al., 2019).

This winter pairs a record-early SSW with a strong El Niño, two influences that have pushed past UK winters in different directions.

El Niño’s influence on the North Atlantic and Europe shifts during winter. In early winter (November–December) its signal travels through the lower atmosphere. In late winter it also travels via the stratosphere and often projects onto the North Atlantic Oscillation (Ayarzagüena et al., 2018).

Research links El Niño to late-winter cold over northern Europe, working through the stratosphere (Ineson & Scaife, 2009). For the strongest El Niño events, other work suggests the lower-atmosphere route dominates instead (Bell et al., 2009). From 1958 to 2010, major SSWs were about as common in El Niño winters as in La Niña winters, and nearly twice as common as in neutral winters (Butler & Polvani, 2011). In the 1958–2014 record, the two November major SSWs, in 1958 and 1968, both came in El Niño winters (Butler et al., 2017).

Recent winters show how differently these set-ups can play out:

Winter 2018/19 - Major SSW in January 2019; the vortex was displaced, then split (Butler et al., 2020):

Weaker surface impacts than in 2018 (Butler et al., 2020). A record-warm spell followed, with 21.2°C at Kew on 26 February 2019, the first UK winter temperature above 20°C (Kendon et al., 2020)

Winter 2018/19 - Major SSW in January 2019; the vortex was displaced, then split (Butler et al., 2020):

Weaker surface impacts than in 2018 (Butler et al., 2020). A record-warm spell followed, with 21.2°C at Kew on 26 February 2019, the first UK winter temperature above 20°C (Kendon et al., 2020)

Winter 2017/18 - Major SSW in February 2018; the vortex split (Butler et al., 2020):

Persistent coupling to the surface with strong weather impacts (Butler et al., 2020). Cold extremes over north-west Europe were stronger than after any SSW from 1979 to 2016 (King et al., 2019)

Winter 2009/10 - Strong El Niño, with stratospheric conditions that favoured SSWs (Fereday et al., 2012):

Coldest winter since 1978/79 in the Central England Temperature record, with a record-low North Atlantic Oscillation (Osborn, 2011)

No major SSW has been recorded this early, so there is no direct precedent for how this one will play out at ground level.