Everything you need to know about La Niña, the climate phenomenon behind this year’s extreme weather
For months the world endured droughts, heat waves, floods and cyclones as one of the strongest El Niño events on record brought chaos to global weather systems.
So severe was the lack of rain in India last summer that the government had to ban the export of some types of rice, while flood waters killed hundreds in East Africa and wildfires tore across Australia.
All those events were linked to a naturally occurring climate pattern caused by fluctuating surface water temperatures in the tropical Pacific Ocean, which – on top of the long-term global warming caused by greenhouse gasses – helped shatter heat records around the world in recent months.
Now though, El Niño has come to an end and the pendulum is swinging, according to meteorological experts, who say its counterpart, La Niña, is about to begin.
What to expect from the swing depends on where you are in the world, but it is likely that more chaos is on the way.
Here’s everything you need to know about the climate phenomenon set to fuel this year’s extreme weather.
What are El Niño and La Niña?
El Niño and La Niña are the two opposing states of a natural climate phenomenon known as the El Niño-Southern Oscillation (ENSO).
The cycle was first discovered by Peruvian fishermen in the 17th century who noticed that the waters off the western coast of the Americas would sometimes become unusually warm in the winter.
They nicknamed the phenomenon “El Niño de Navidad,” or the Christ child, as it usually occurred near Christmas.
Generally, El Niño is defined when the Pacific water temperatures rise by at least 0.5 degrees Celsius above the historical average, for a period of three months or more. La Niña is typically defined when temperatures fall by the same amount.
What causes El Niño/La Niña?
In normal conditions, the trade winds in the Pacific blow westwards along the equator, pushing warm water from South America towards Asia and causing cold water to rise up from the depths in a process known as upwelling.
But during El Niño, the trade winds weaken or reverse and the warm water is pushed back to the east.
In La Niña periods, the trade winds become stronger, pushing the warmer water further to the west and bringing more cold water to the surface in the eastern Pacific.
In both cases, fluctuating ocean surface temperatures – though seemingly small – have a knock-on effect on the atmosphere, with dramatic and far-reaching consequences for global weather systems.
“The devil is in the details,” said Dr Neven Fučkar, climate scientist at the University of Oxford. “Atmosphere responds very quickly – a matter of hours, days, weeks, and even a month or two. But the ocean has slower dynamics.”
While oceans do evolve on time scales of hours or days, they also follow patterns that run for decades and centuries or even longer, he said.
The complex ways in which the tropical Pacific and the atmosphere interact with each other makes the fluctuation between the two opposing states very difficult to forecast.
“One of the biggest problems is that it’s very difficult to predict,” said Dr Simon Boxall, a senior lecturer in oceanography at the University of Southampton.
“We know from satellite data, from history and from very detailed measurements we can make of the oceans that there is no real passion as to when El Niño and La Niña occur,” he said.
It’s a phenomenon that scientists are still working to fully understand.
“I won’t say it’s random,” said Dr Boxall. “There must be something driving it, but I don’t think we’ve got to the bottom of what flips an El Niño on or off.”
How often does El Niño/La Niña occur?
The oscillation between El Niño and La Niña periods occurs at regular intervals of every three to seven years, with “neutral” periods in between the two extremes.
The shift disrupts seasons across the planet and typically lasts between 12 and 18 months, but can sometimes last for years.
Seven El Niño events have been recorded since the turn of the millennium, while it is thought that there have been at least 30 since 1900.
The El Niño periods beginning in 1982, 1997, 2014 and 2023 were among the strongest ever seen.
The 1997-1998 El Niño was blamed for record rainfall in Kenya, and contributed to a devastating extreme weather event in Florida known as the Night of the Tornadoes, when 15 tornadoes touched down, killing 42 people, injuring hundreds of others and doing hundreds of millions of dollars of damage.
There have been 24 La Niña periods since 1900.
Beginning in 2020, the world experienced an unusually long La Niña phase that lasted until the start of 2023.
While the period was credited with helping to keep a lid on rising global temperatures, it also super-charged a record-breaking hurricane season.
The 2020 hurricane season was the most active on record, while 2021 also saw numerous catastrophic tropical cyclones make landfall in the United States and Caribbean, killing hundreds and causing billions of dollars in damage.
Which phase are we in now?
Currently, we are in a neutral phase – neither El Niño or La Nina.
“Right now, the atmosphere is receiving a regular amount of heat from the Tropical Pacific,” said Dr Fučkar.
But La Niña appears to be just around the corner, with the US National Oceanic and Atmospheric Administration (NOAA) estimating that there is a 70 per cent chance of one emerging between now and October.
What effect will La Niña have in 2024?
Normally, air rises over the Amazon and Indonesia because moisture from the tropical forests makes the air less dense and allows it to cascade down over east Africa and the eastern Pacific.
During La Niña, these arcs intensify and generate stormier conditions, often bringing wetter-than-normal weather to Indonesia, northern Australia, and Malaysia.
Dr Boxall said: “La Niña is a stable event, but if it’s too extreme, it can cause problems.”
The impacts can be felt across the globe.
Historically, La Niña-like conditions have caused heavier rainfall and flooding in monsoon season over India and have also been associated with catastrophic floods in northern Australia.
The Queensland town of Gympie is testament to the dangerous impact La Niña can have – no less than four of its worst floods on record came during La Niña periods.
In 1955, a cyclone ravaged the town, bringing a deluge of rainfall that submerged the main street under three metres of water.
And in 1974, some 900,000 million tonnes of rain fell on the surrounding state, making it one of the wettest years on record.
La Niña is also bad news for eastern Africa, where many vulnerable communities are already battling long-term drought.
In May, the NOAA warned that an “extraordinary” Atlantic hurricane season could be looming between June and November, fuelled by exceptionally high sea surface temperatures that allows the storms to pick up more energy, enabling higher wind speeds.
Last month, Hurricane Beryl became the earliest category five Atlantic hurricane on record as it ripped through parts of the Caribbean.
But it’s not all doom and gloom.
“Actually, La Niña is good news for most populations,” said Dr Boxall.
For instance, the switch has a dramatically positive impact on the fishing industry of western South America – the upwelling brings cold, nutrient-rich waters to the surface that allow phytoplankton, fish and other marine life to thrive.
The warmer waters can also draw tropical species into areas that are normally too cold for them.
Does La Niña affect the UK?
Though it originates in the depths of the Pacific, La Niña’s effects can be felt as far away as Britain.
Thankfully, the impact is usually subtle.
El Niño can increase the risk of colder winters in the UK and helped make the winter of 2009 the chilliest since 1979.
In contrast, the effects of La Niña on the summer weather in the UK are “less pronounced”, said Malcolm Mistry, Assistant Professor at the London School of Hygiene & Tropical Medicine.
The effects of La Niña, he said, typically “take a while to emerge” and can lead to wetter and milder summers – much like the one we are currently experiencing.
But he added that the effects in the UK are “not consistent in every El Niño or La Niña phase”.
“This remains an active area of research in climate science.”
Could it help keep a lid on rising temperatures?
El Niño and La Niña are now happening on top of the effects of global warming, causing 2023 to be the warmest year on record.
Since summer 2023, the world has had 13 straight months of record-breaking global temperatures.
This has amplified waves of extreme drought in certain areas.
Food production is extremely sensitive to climate conditions and can therefore compromise food security, with some of the worst food crises events down to Enso.
Some experts are optimistic that a La Niña phase could help cool things down, but while the pendulum swing of Enso might be able to provide some respite from global warming in the short-term, temperatures will likely continue to rise.
“I hope that La Niña will develop quickly and dampen global warming,” said Dr Fučkar. “But that’s not certain, it’s a very noisy system.”
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