What are El Niño and La Niña, and how do they change the weather?
The world has officially entered an El Nino phase, according to the US science agency NOAA. Scientists believe the weather patterns typically seen during El Nino are likely to send global temperatures soaring to record levels in the next few years. El Nino is part of the natural climate phenomenon called the El Nino Southern Oscillation (ENSO). It has two opposite states - El Nino and La Nina - both of which significantly alter global weather. An El Nino event is typically declared when sea surface temperatures in the tropical eastern Pacific rise to at least 0.5C above the long-term average . 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 Nino events, these winds weaken or reverse, sending warm surface waters eastwards instead. In La Nina 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 Nino 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 Nino episode, and fall during La Nina. El Nino 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, 2016, was an El Nino year. Between 2020 and 2022, the Northern Hemisphere had three La Nina 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 Nina, the EU's climate monitoring service said that 2022 was the fifth warmest year on record. Scientists warn that a return to El Nino 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 Nino is complicated and uncertain . It could mean colder than average winters, for example, but that depends on how El Nino unfolds . Changes to rainfall During El Nino 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 Nina, the opposite is seen. The record rainfall and flooding which Australia experienced in October 2022 was driven by La Nina. Tropical storms El Nino 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 Nina, the reverse is typically true. Carbon dioxide (CO2) levels Scientists have also observed that CO2 levels in the atmosphere increase during El Nino 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 Nino and La Nina 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 Nino 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 Nino 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 Nino events significantly reduce global economic growth , an effect which could intensify in the future. El Nino and La Nina episodes typically occur every two to seven years, and usually last nine to 12 months. They don't necessarily alternate: La Nina events are less common than El Nino 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 Nino 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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