Britain should brace for a stormy and unsettled winter after months of record-breaking heat, as a "Super El Niño" builds in the Pacific Ocean, scientists have said.
The UK sits roughly 5,000 miles from the Pacific, but researchers say warm ocean temperatures there send atmospheric "waves" rippling around the globe, pushing major weather systems such as the Jet Stream off their usual course and altering Britain's winter weather.
Forecasters expect the Jet Stream to steer storms towards the UK on fast-moving currents of air this winter, though it remains too early to say exactly when they will arrive. Will Lang, chief meteorologist at the Met Office, said El Niño conditions increase the chance of storms in late autumn and early winter. He said it was too early to comment on flood risk this winter, "though again some flooding is to be expected and should be planned for in any UK autumn and winter."
What is El Niño?
El Niño was officially declared in June, when sea surface temperatures in the equatorial Pacific climbed 0.5°C (0.9°F) above the long-term seasonal average. The Met Office now suggests the warming of these waters could reach 3C (37F) in the coming months.
El Niño is part of the El Niño Southern Oscillation, a naturally occurring climate cycle that switches between the hot El Niño and cool La Niña phases every two to seven years. During El Niño, the trade winds that normally carry warm water away from South America towards Australia slow down or reverse, stopping cold water welling up and allowing hot water to build near the surface of the equatorial and tropical Pacific.
That build-up drives more evaporation and thunderstorm activity over the central and eastern Pacific, with effects that ripple outward as far as Europe and the UK, including a greater likelihood of storms in autumn and early winter.
Wetter autumn, colder late winter
According to the Met Office, autumn is likely to bring milder, wetter weather, early winter more rain and a higher chance of storms, and late winter very cold, drier conditions.
Dr Chloe Brimicombe, a climate scientist at the University of Oxford, said El Niño affects UK weather through long links in the Earth's weather system known as teleconnections. "If processes line up, it can make the winter wetter and stormier in north-west Europe," she said.
El Niño interacts with the North Atlantic Oscillation and the Jet Stream, the fast-moving body of air that steers weather systems. As hot air rises over the Equator it spreads towards the poles in a pattern known as Hadley circulation, altering circulation patterns worldwide.
Professor Emily Black, a meteorologist at the University of Reading, said the unusually warm tropical Pacific shifts the regions where intense tropical rainfall occurs, with knock-on effects on global circulation including over the UK. "Specifically, the release of heat into the atmosphere during El Niño generates large-scale atmospheric waves that can alter pressure patterns and the Jet Stream over the North Atlantic," she said.
"In early winter, these changes can favour lower pressure to the west or north-west of the UK and a stronger flow of air from the Atlantic. That brings more weather fronts and more moisture-laden maritime air across the country, increasing the likelihood of rain," Professor Black said. She added that the effects are felt most strongly in the parts of the UK most exposed to Atlantic weather systems, though the distribution varies from event to event.
Stormy start, bitter finish
Professor Black said El Niño tends to produce a stormier start to winter rather than a stormier season throughout. "Even a very strong El Niño does not determine what the UK winter and autumn will be like. Instead, it changes the probabilities of particular weather patterns," she said. "For the UK, El Niño tends to increase the likelihood of mild, wet and windy conditions during autumn and the first part of winter, and colder conditions later on in the season."
Bill McGuire, Professor of Geophysical and Climate Hazards at University College London, said: "Powerful El Niños typically bring mild, stormy autumns and early winters to the UK, followed by bitter cold, although this isn't set in stone."
Forecasts remain uncertain
Dr Zachary Labe, a climate scientist at Climate Central, cautioned that the impact on Europe and the UK is harder to predict than in regions such as Southeast Asia and North America. "A bigger El Niño does not necessarily mean bigger impacts everywhere. The relationship is much more complicated, particularly over the UK, where several other factors influence the Jet Stream at the same time," he said.
Drought concerns
Beyond storms, experts said the bigger worry is that El Niño may not bring enough rain to offset the summer's drought. Rainfall in July reached just 26 per cent of the typical average in north-west England and as little as 1 per cent in the south-east, leaving water companies hoping to replenish depleted reservoirs.
Professor Timothy Palmer, a climate physicist at the University of Oxford, said Europe is a long way from the tropical Pacific and that El Niño's impact on UK weather is not consistent from one event to another. "So we don't know what to expect this winter, not least because the El Niño event developing is likely to be unprecedented in magnitude," he said.
Professor Palmer said the first "credible" seasonal forecasts for winter would come in September, though even then uncertainty may remain. "If there is no consistent signal from the various models, then we will have to conclude that we won't know in September how El Niño will impact winter UK rain, and will have to wait another month," he said. "But if there is a consistent signal, the water authorities and the Government need to act on it with urgency, particularly if the prediction is for below average rain."
