What are El Niño and La Niña?

Strong El Nino event form 1997-1998 Credit: NOAA

The El Niño Southern Oscillation (or ENSO) forms part of a complex relationship between the ocean and the atmosphere and it has the power to cause changes in global temperature and rainfall.

It is the world's biggest climate driver, takes place in the southern tropical Pacific Ocean, and occurs every two to seven years.

What is El Niño? The El Niño Southern Oscillation has three phases

  • Neutral

  • El Niño

  • La Niña

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ENSO in the neutral phase, shows winds moving from east with warmer water piling up on the western side of the Pacifc Credit: Met Office

During the neutral or normal phase, the trade winds blow from east to west across the Pacific Ocean. This pushes warmer water to the western side of the Pacific Ocean towards Asia and Australia.

The warmer waters contribute to increased rainfall in Indonesia, Papua New Guinea and Australia.

As this happens, colder water from deep below the ocean surface rides up the eastern side of the Pacific along the coast of Peru, Ecuador and Chile, a process known as upwelling. This brings nutrient-rich waters to the surface allowing marine life to thrive here fuelling the fishing industry. The air here is cooled, keeping the western side of South America relatively dry.

During an El Niño event, the sea surface temperatures in the central and eastern equatorial Pacific Ocean become substantially warmer than average. These warmer waters lead to a shift in atmospheric circulation. The normal easterly trade winds which blow from South America towards Asia and Australasia gradually weaken and in strong cases can even reverse.

El Nino shows a weakening of the easterly winds allowing the warmer waters to shift centrally across the Pacific ocean Credit: Met Office

This weakening allows the warmer waters from Asia and Australia to drift eastwards. It cuts off the nutrient-rich cold upwelling along the western coast of South America killing off marine life here and reducing fish stocks.

El Niño changes weather patterns over the equatorial Pacific, bringing increased rainfall to South America but droughts to Australia and Indonesia with higher temperatures and greater extreme temperatures too.

There are other impacts of droughts, intense heatwaves and increased rainfall events that happen across the world during El Niño years. This is because vast amounts of heat stored within the oceans are released into the atmosphere. This heat and energy is redistributed around the world impacting global weather patterns and rising global temperatures.

This is why El Niño years often feature within the warmest years on record, however each El Niño event is different and so are the impacts.

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La Nina has stronger easterly winds, pushing the warmer waters further west and a colder upwelling in the east of the Pacific ocean Credit: Met Office

During La Niña events, the opposite happens.

The easterly trade winds strengthen pushing more warmer waters towards Indonesia and Australia, causing colder than average water on the eastern side of the Pacific ocean.

La Niña generally leads to above average rainfall in Australia and Indonesia meanwhile cooler and drier than average weather is experienced in the tropical eastern Pacific.

What does this mean for the UK?

El Niño has the power to increase the risk of colder and drier winters to the UK as well as a less potent North Atlantic storm season. Meanwhile La Niña tends to lead to less cold winters.

Scientists are still unsure how El Niño will react in the future to climate change, however it's thought the impacts will be amplified in different parts around the world.

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