What is El Nino and how can the Pacific Ocean climate cycle cause extreme weather?

·4-min read
El Nino is the warm phase of the El Nino La Nina Southern Oscillation (ENSO) that occurs across the tropical Pacific Ocean roughly every five years. The ENSO affects weather systems across the world, bringing extreme weather such as floods and droughts. El Nino generally causes drier conditions in Australia and South-East Asia, and wetter and warmer conditions in the Americas.
The El Nino weather pattern has been forecast to return this year. (Getty Images)

Forecasters are predicting the return of the El Nino weather pattern later this year – and it could lead to hotter temperatures around the world.

El Nino is one of the three phases of a Pacific Ocean phenomenon known as the El Nino-Southern Oscillation (ENSO) that is closely tied to global temperatures.

The temperature cycle in the Pacific can affect weather in Britain and around the world with the hottest year on record, 2016, being an El Nino year.

Experts have warned that the coming El Nino could see the crucial 1.5C temperature barrier breached as global temperatures soar. This refers to the global average temperature exceeding 1.5C above pre-industrial levels.

Read more: A 1988 warning about climate change was mostly right

El Nino is the best-known part of El Nino-Southern Oscillation climate cycle, with El Nino events seeing the Pacific Ocean up to 3C warmer than normal.

During La Nina – the other part of the cycle – the ocean is up to 3C colder.

Temperatures around the world increase by about 0.2C during El Nino, and fall about 0.2C during La Nina.

There's also a neutral phase, which the world is entering now, according to the Climate Prediction Service (CPC), run by the US National Weather Service.

While 2022 was the fifth-hottest year on record, that was modulated by the fact that the Northern Hemisphere has seen three La Nina events in a row.

The CPC has suggested that El Nino will develop either during the summer or shortly afterwards.

It said in a release earlier this month: "La Nina has ended and ENSO-neutral conditions are expected to continue through the Northern Hemisphere spring and early summer."

What is El Nino?

El Nino is an event where temperatures in the Pacific Ocean are warmer than normal and is declared when temperatures rise 0.5C above the long-term average.

A landscape of dry, brown and parched grass in Brockwell Park during the UK drought, on 15th August 2022, in London, England. A hosepipe ban remains in place for the Thames Water area that includes London and the south-east. (Photo by Richard Baker / In Pictures via Getty Images)
Forecasters have predicted heatwaves due to the return of El Nino. (Getty Images)

Caused by a recurring cycle, El Nino events can have knock-on effects on weather in the UK, with El Nino years linked to a risk of colder winters in Britain and heatwaves in summer.

The Met Office said: "El Nino is felt strongly in the tropical eastern Pacific with warmer than average weather.

"The effects of El Nino often peak during December. It's name, 'the boy', is thought to have originated as El Nino de Navidad centuries ago when Peruvian fishermen named the weather phenomenon after the newborn Christ."

Professor Adam Scaife, the head of long-range prediction at the Met Office, said it is "very likely" that the next El Nino could take the world past 1.5C above pre-industrial levels.

Earlier this year, Scaife said: "The probability of having the first year at 1.5C in the next five-year period is now about 50:50.

"We know that under climate change, the impacts of El Nino events are going to get stronger, and you have to add that to the effects of climate change itself, which is growing all the time.

"You put those two things together, and we are likely to see unprecedented heatwaves during the next El Nino."

Like El Nino, La Nina affects the Pacific Ocean (Getty)
Like El Nino, La Nina affects the Pacific Ocean (Getty)

What is the difference between El Nino and La Nina?

La Nina refers to the cooling of ocean surface temperatures coupled with winds and rainfall in the Pacific.

It can have knock-on effects on weather around the world, often having the opposite impact on the global climate as the better-known El Nino.

The Met Office said: "La Nina, or 'the girl', is the term adopted for the opposite side of the fluctuation, which sees episodes of cooler-than-average sea surface temperature in the equatorial Pacific.

"The conditions for declaring La Nina differ between different agencies, but during an event sea temperatures can often fall 3C to 5C below average. Cooler, drier than average weather is experienced in the tropical eastern Pacific."