What is the difference between El Nio and La Nia, and how do they affect the weather

According to the US science agency NOAA, the world has formally entered an El Nio phase.

The typical weather patterns associated with El Nio, according to scientists, are likely to cause global temperatures to soar to record highs in the coming years.

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

Both of its opposing states, El Nio and La Nia, significantly affect the weather on a global scale.

When the tropical eastern Pacific's sea surface temperatures increase by 0°C or more over the long-term average, an El Nio event is declared.

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 the event "El Niño de Navidad", Christ Child in Spanish.

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

Temperatures.

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.

The hottest year on record, 2016, was an El Niño year.

Between 2020 and 2022, the Northern Hemisphere had three La Niña episodes in a row, which stopped global temperatures from increasing as much as they otherwise would have as a result of human-caused climate change.

Despite this very rare "triple dip" La Niña, the EU's climate monitoring service said that 2022 was the fifth warmest year on record.

Scientists warn that a return to El Niño conditions, on top of climate change, makes it almost certain that a new global temperature record will be set in the next five years.   .

How UK and Western European temperatures will respond to El Niño is complicated and uncertain.  It could mean colder than average winters, for example, but that depends on how El Niño unfolds.

Changes to rainfall.

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 opposite is seen.

The record rainfall and flooding which Australia experienced in October 2022 was driven by La Niña.

Tropical storms.

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.

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 caused 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.

A recent study suggested that El Niño events significantly reduce global economic growth, an effect which could intensify in the future.

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.

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.

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

Some climate models do suggest that El Niño events will become more frequent and more intense in future as a result of rising global temperatures - potentially leading to further warming.

But scientists are not certain this will happen.

Graphics by Visual Journalism team.

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