This year’s El Niño weather pattern will be the biggest in over a century, warming sea surface temperatures to an “unheard-of” level, Britain’s national weather agency predicted on Friday.

The El Niño weather pattern is declared when sea temperatures in the tropical eastern Pacific rise 0.5°C above the baseline average, triggering worldwide changes in winds, atmospheric pressure and rainfall patterns, and causing warmer global temperatures overall.

Adam Scaife, head of long-range forecasting at the Met Office, told the BBC that “We are expecting the biggest El Niño for over a century, peaking at something over three degrees — that is unheard-of in modern climate records.”

He called it “an unprecedented event”, adding that “I have never seen an El Niño signal this intense in our forecasts”.

The anomalous warmth, which is “up to nearly two degrees already”, will have particularly strong effects throughout the Tropics, Scaife said, “where we expect really severe droughts in places like South America, and the West Pacific”.

Temperatures in the region are currently running about 2.6°C above a rolling 30-year average, according to the Climate Brink dashboard.

“These are not short-term weather forecasts. We’re talking about really long-term, persistent conditions here. There are well-known impacts on other systems including food, food security,” said Scaife, a physicist who specialises in climate modelling.

The weather pattern is also expected to cause “increased rainfall and storminess in the autumn here in the UK around November time”, Scaife said.