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

El Niño and La Niña are among the most important natural weather patterns on Earth, and can affect temperatures and rainfall around the world.

La Niña conditions are present at the moment, according to US science agency Noaa, but El Niño is expected to develop later in 2026.

El Niño and La Niña are the two opposite states of a natural climate phenomenon called the El Niño Southern Oscillation (ENSO).

El Niño and La Niña occur in the Pacific but can affect weather systems across the world.

The two states are often identified by sea surface temperatures in the tropical eastern and central Pacific Ocean. During El Niño, these waters are warmer; during La Niña, they are cooler.

The phases can also be distinguished by differences in atmospheric pressure. During El Niño, pressure is above normal at Darwin, Australia (western Pacific) and below normal at Tahiti, French Polynesia (central Pacific). For La Niña, the opposite is true.

In "neutral" conditions - neither El Niño nor La Niña - surface water in the Pacific Ocean is cooler in the east and warmer in the west.

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, 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.

Not every event is the same, and the consequences vary between regions and times of the year.

However, scientists have observed some common effects.

Temperatures

Global temperatures typically increase during El Niño, 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.

The hottest year on record, 2024, was boosted by El Niño conditions, on top of long-term human-caused climate change.

The maps below show some typical effects, but they may only be true for parts of the year.

The way the two systems affect UK temperatures is complicated, and can vary.

But El Niño may increase the chance of a mild start and cold end to UK winter, according to the Met Office, whereas La Niña can make a colder start and mild end to UK winter more likely.

Changes to rainfall

During El Niño, 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 United States and the Gulf of Mexico.

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

La Niña typically brings wetter conditions to parts of Australia, Indonesia and equatorial South America, and drier conditions to the southern United States.

Tropical storms

El Niño tends to bring more tropical storms in the tropical Pacific, but fewer in the tropical Atlantic, including the south-east US.

During La Niña, the reverse is typically true.

Carbon dioxide (CO2) levels

Scientists have also observed that CO2 levels in the atmosphere increase during El Niño events, possibly as a result of warmer and drier conditions in tropical regions.

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.

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.

The droughts and flooding caused by the extreme 2015-16 El Niño event affected the food security of more than 60 million people, according to the UN Food and Agricultural Organisation.

Scientists have also found that El Niño episodes can reduce global economic growth.

El Niño and La Niña episodes typically occur every two to seven years.

They usually last for nine to 12 months, although they can persist for longer, and they don't necessarily alternate.

The current La Niña episode began in mid-to-late 2024.

In 2021, the UN's climate scientists, the IPCC, said the ENSO episodes that 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.

The IPCC concluded there is no clear evidence that climate change has affected these events.

Some climate models suggest that El Niño episodes could become more frequent and more intense as a result of global warming, with greater swings between El Niño and La Niña.

But this is a complex and uncertain area of science and there is no clear consensus.

Additional reporting by the Visual Journalism team.