Heat is released from the ocean into the atmosphere, lifting global temperatures and shifting rainfall patterns.
Scientists declare an El Niño when sea surface temperatures in a key area of the Pacific are 0.5C or more above average.
Current measurements show that they are well over 2C above normal.
Latest forecasts from the Met Office suggest that these temperature anomalies could exceed 3C later this year as El Niño nears its peak.
This would be unprecedented in records going back to 1950, and probably the strongest event since the 19th century.
Very high temperatures in the deeper ocean – around 8C above normal at a depth of 100m (330ft) in some places – could help to supercharge this El Niño, essentially providing a reservoir of warm water fuel to reinforce the event.
“We have many months to go while this event continues to develop and that’s one of the reasons that we have increased confidence that this will be a record-setting event,” said Dr Zeke Hausfather, a climate scientist at Berkeley Earth, a US research group.
Some other climate groups have projected something even stronger than the Met Office’s forecast, perhaps even 4C above average.
If that came to pass, “It’s not beyond the realm of possibility to say this might be the strongest El Niño event in 500 or even 1,000 years,” Hausfather said, although it is impossible to say for sure that far back in time.
