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A planet-warming El Niño has just begun in the tropical Pacific, and it’s likely to bring cooler, wetter weather to the southern US and hotter, drier weather to much of South America, Africa and Asia, raising the risk of wildfire and drought.

What will the impact be on western Europe and the UK? That’s notoriously difficult to say, and it will depend a lot on changing weather phenomena around the Atlantic Ocean.

“The direct effects of El Niño on Europe and the UK are really quite unclear because El Niño is so far away and we’re so strongly affected by the north Atlantic,” at the University of Reading, UK, said at a media briefing on 10 June.

“It’s just because of the amount of variability, the amount of chaos in the storm track coming over our region, and how many other influences that are happening at the same time, that it is hard to predict,” says at the Met Office, the UK’s weather service.

El Niño is a natural climate phase that happens when easterly trade winds weaken, allowing the warm water they had piled up in the western Pacific to wash back across the central and eastern Pacific. This swathe of warm water heats the atmosphere and weakens the winds further in a feedback loop.

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Several weather services have that El Niño has started, and the US National Oceanic and Atmospheric Administration says it’s likely to become a very strong or “super” El Niño, which could unleash even more extreme weather. But in western Europe, the impacts could be different at different times of the year, and they could be cancelled out altogether by other climate phases.

Though currently weak, El Niño will gain strength as the Pacific heats up and is expected to peak toward the end of the year. That is far beyond the two-week horizon of typical weather forecasts. By averaging the effects of past El Niños, meteorologists can say that certain weather is more or less likely in different places, but the effects of any individual El Niño can still vary, especially in western Europe.

The clearest El Niño trend in the UK is towards wetter, windier, stormier weather in the late autumn and early winter. That’s what happened during the last super El Niño in December 2015, when record rainfall hit the UK, with one village receiving more than 34 centimetres in a single day. In northern England, rivers burst their banks, hospitals and universities lost power, thousands of homes were flooded and the army had to rescue people with small boats. Flooding caused an estimated £1.6 billion in damages that winter.

This wet weather is likelier because the movement of warm water to the eastern Pacific during El Niño brings rising, warm air with it. The shift of that low-pressure centre of rising air to the east and the north can propagate around the world through “planetary waves” of atmospheric pressure and temperature.

This can intensify an area of low pressure west of the British Isles called the east Atlantic pattern, which causes a kink in the jet stream that directs warm air and storms from the south towards the UK. But it doesn’t always happen.

Dunstone compares these teleconnections between El Niño in the tropical Pacific and faraway places like the UK to “waving a big stick” through the atmosphere.

“The tip of it is hard to control, so it’s hard to know exactly where it’s going to land, but on average it lands [on] giving us a low pressure over the north Atlantic, which then impacts the UK,” says Dunstone.

The other significant impact that El Niño can have on the UK is colder, drier conditions in late winter. The planetary waves from El Niño propagate not only through the lower atmosphere but also upward into the stratosphere, where they can slow down the polar vortex, a ring of winds trapping cold air around the North Pole. If the polar vortex gets weaker and wavier, looping southward in places, it can allow cold Arctic air to penetrate the UK.

A weaker polar vortex can also weaken the North Atlantic Oscillation (NAO), a measure of the difference between the permanent low-pressure system over Iceland and high-pressure system over the Azores. A less pronounced difference between these two systems – known as a negative NAO – slows down the westerly winds that typically buffet Britain, allowing cold air from eastern Europe and Siberia to spill eastward towards the UK.

In 2009-10, El Niño and a very negative NAO “the big freeze”, the coldest winter northern Europe and the UK had seen in 30 years. Much of the UK had a white Christmas, and on 5 January, 40 centimetres of snow in southern England, which usually only sees rain.

But the trend towards a colder late winter during El Niño is even weaker than the trend towards a wet early winter. NAO phases sometimes last only weeks before flipping, and positive NAOs, which are associated with mild, wet winters in the UK, have occurred during El Niños. Moreover, in a strong El Niño like the one expected this year, the warmer, wetter tropospheric teleconnection can essentially cancel out the colder, drier stratospheric teleconnection in late winter.

Climate change is already expected to unleash more frequent extreme winter storms on the UK. With El Niño – which is likely to become stronger and more damaging – potentially exacerbating these impacts, the UK needs to invest in monitoring water supplies and food imports that can be disrupted, according to the scientists at the briefing.

“We need long-term preparedness and planning, really to think about that as we continue with climate change and also continue with El Niño events amplifying that effect,” at the University of Oxford said at the briefing.