Colder than average conditions for the start of winter?

In coming weeks you can expect to hear more in the media about the impacts on the UK’s late autumn and winter weather emanating from La Niña: part of a pattern of climate variability in the tropical Pacific.

Seasoned climate watchers will be aware of ENSO (El Niño Southern Oscillation): a pattern of oceanic and atmospheric variability which – depending on the exact phase – can lead to a mild warming or cooling of the planet. Much of the time ENSO is in a so-called neutral state, but sometimes the variation becomes more extreme when we get a warmer El Niño, or cooler La Niña phase.

During La Niña, stronger than normal trade winds blow warm water towards the west Pacific causing an upwelling of cool water from the ocean depths in the east Pacific. This cools the tropical East Pacific and leads to variations in global weather.

La Niña conditions are now present in the tropical Pacific, and forecasters are suggesting these conditions will continue through the next few months.

ENSO 2021

La Niña doesn’t just affect the tropical Pacific as it can even influence the Atlantic jet stream and our weather here in the UK.  In early winter it is associated with Atlantic ridging and northwesterly winds which is consistent with the current Met Office long range outlook. In late winter it is associated with westerly, milder and wetter conditions when compared to normal.

Professor Adam Scaife, Head of long-range forecasting at the Met Office, said: “La Niña has a profound effect on weather across the globe and can have impacts extending as far as the UK.

“In late autumn and early winter La Niña tends to lead to high pressure developing in the mid-Atlantic, which stops Atlantic weather systems from delivering mild air to the UK, and therefore can allow cold conditions to intensify. However, in late winter La Niña tends to drive a strengthening of the jet stream towards the Arctic increasing storminess and heavy rainfall, while bringing milder conditions to our sector.”

However, these conditions are not guaranteed. Last winter there was also a La Niña, but the impacts in the North East Atlantic strayed from usual. It is important to remember that the ENSO cycle is an important driver of global and UK weather, but other drivers, such as the Quasi-biennial Oscillation, can lead to differing conditions. The QBO is a regular variation of the winds that blow – either east or west – high above the equator.

Aidan MgGvern is a Met Office weather presenter. He explained: “This winter the QBO is in an easterly phase and there is an increased chance of a weakened jet stream across the Atlantic, making cold-weather effects on the UK more likely.”

The current 6-30-day outlook from the Met Office highlights the uncertainty around the impacts from global climate drivers on UK weather. Despite the La Niña and QBO impacts there is an emphasis on temperatures being generally near or above average for the rest of October, but there is a suggestion of a more settled and colder spell, with the threat of associated wintry hazards – such as frost and fog – becoming established later in the season.

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