What are El Niño and La Niña, and how do they change the weather?

  • By Esme Stallard & Mark Poynting
  • BBC News Climate & Science

Image source, Getty Images

It has been confirmed that 2023 was the hottest year on record, driven by human-caused warming but also boosted by a natural weather system called El Niño.

The continuing effects of El Niño mean that 2024 could see even higher temperatures.

What is El Niño?

El Niño is part of the natural climate phenomenon called the El Niño Southern Oscillation (ENSO).

It has two opposite states: El Niño and La Niña, both of which significantly alter global weather.

In normal conditions, surface water in the Pacific Ocean is cooler in the east and warmer in the west.

The “trade winds” tend to blow east-to-west, and heat from the Sun progressively warms the waters as they move in this direction.

During El Niño events, these winds weaken or reverse, sending warm surface waters eastwards instead.

In La Niña periods, the normal east-to-west winds become stronger, pushing warmer waters further west.

This causes cold water to rise up – or “upwell” – from the depths of the ocean, meaning sea surface temperatures are cooler than usual in the east Pacific.

The phenomenon was first observed by Peruvian fisherman in the 1600s, who noticed that warm waters seemed to peak near the Americas in December.

They nicknamed it “El Niño de Navidad” – Christ Child in Spanish.

How do El Niño and La Niña change the weather?

Not every ENSO event is the same, and the consequences vary from region to region. However, scientists have observed some common effects:

Global temperatures typically increase during an El Niño episode, and fall during La Niña.

El Niño means warmer water spreads further, and stays closer to the surface. This releases more heat into the atmosphere, creating wetter and warmer air.

But the regional effects are complicated, and some places may be both warmer and cooler than expected at different points in the year.

El Niño could mean that 2024 is even hotter, the UK Met Office says, although this is far from certain.

In early January 2024, El Niño was considered to be close to maximum strength, but it is expected to weaken in the coming months, according to the US science body NOAA.

During El Niño events, the warmer water pushes the Pacific jet stream’s strong air currents further to the south and the east.

This brings wetter weather to southern USA and the Gulf of Mexico.

Tropical regions like southeast Asia, Australia and central Africa typically experience drier conditions.

Under La Niña, the effect is reversed.

El Niño also affects atmospheric circulation patterns, which means there are generally more tropical storms in the tropical Pacific, but fewer in the tropical Atlantic, including the southern US.

During La Niña, the reverse is typically true.

Carbon dioxide (CO2) levels

If plants grow less quickly due to drought, they absorb less CO2, while more wildfires in places like South Asia mean more CO2 is released.

Why do these climate patterns matter?

The extreme weather events worsened by El Niño and La Niña affect infrastructure, food and energy systems around the world.

For example, when less cold water comes to the surface off the west coast of South America during El Niño events, fewer nutrients rise from the bottom of the ocean.

That means there is less food available for marine species like squid and salmon, in turn reducing stocks for South American fishing communities.

Image source, Getty Images

Image caption,

Fishing stocks can be affected by the effects of El Niño

How often do El Niño and La Niña episodes happen?

El Niño and La Niña episodes typically occur every two to seven years, and usually last nine to 12 months.

They don’t necessarily alternate: La Niña events are less common than El Niño episodes.

Is climate change affecting El Niño/La Niña?

In 2021, the UN’s climate scientists, the IPCC, said the ENSO events which have occurred since 1950 are stronger than those observed between 1850 and 1950.

But it also said that tree rings and other historical evidence show there have been variations in the frequency and strength of these episodes since the 1400s.

Some climate models suggest that El Niño events will become more frequent and more intense as a result of global warming – potentially boosting temperatures further – but this is not certain.

Graphics by Visual Journalism team.


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