The world is full of invisible forces: the planet rotating at a speed of about 1,500 kilometers per hour, plant decay absorbing carbon dioxide, and – every few years – the temperature of the Pacific Ocean changing just enough to tip weather patterns across the globe.
The latter is called El Niño–Southern Oscillation (ENSO): a natural weather phenomenon that, combined with human-made climate change, can drastically alter global temperatures and rainfall patterns.
Right now, the warm phase of the cycle, known as El Niño, has just ended, and temperatures in the Pacific Ocean are steadily declining. Many researchers are now predicting that we’ll see the cycle shift to its cool phase, called La Niña, in the next few months.
But what exactly is La Niña, and how can we expect it to develop and impact different regions of the world in the months ahead?
La Niña is the result of extra-strong easterly trade winds blowing warm ocean surface water west across the Pacific Ocean.
It brings warmer, wetter weather to Asia and Australia and allows deeper colder water to rise along the coast of the Americas, which is often associated with drier, chillier seasons.
Scientists declare a La Niña when the average ocean temperature drops by at least 0.5 degrees Celsius for at least three consecutive months. The greater the temperature difference, the stronger the weather patterns that ensue.
When temperatures rise by 0.5 degrees Celsius for at least three months, scientists declare the opposite phenomenon – El Niño.
These two phases, along with a neutral phase, make up the ENSO cycle, which is always ongoing and regularly shifting between phases every two to seven years.
While these temperature changes may sound tiny, ENSO is actually the second most important driver of global climate variations, runner-up only to the sun.
While the world went into lockdown at the start of the COVID-19 pandemic, it also kickstarted a three-year La Niña that lasted until 2023.
The climate became increasingly warm and wet around Asia and Australia, bringing record-breaking rains and floods to eastern Australia, as well as a record-breaking number of hurricanes in the Atlantic.
These heavy rains across Asia and Australia, however, also enriched agriculture, while heatwaves and droughts wrought havoc across South America.
Then, in 2023, the world experienced a strong El Niño. Ocean temperatures rose to 1.4 degrees Celsius above the long-term average, and the world has seen record high temperatures every month since July 2023.
So, you might be thinking: shouldn’t we be grateful for La Niña if it’ll cool our planet?
Maybe, but it isn’t that simple. Thanks to the climate crisis, it’s harder than ever to predict how things will play out.
Even the Niño 3.4 Index, the system used to measure ocean temperatures and determine the onset of El Niño or La Niña, must now be looked at relative to how the climate crisis has shifted global averages.
Still, a better understanding of the ENSO cycle and general weather patterns can help us prepare for the months to come.
It’s impossible to know for sure. But as ocean temperatures creep lower, here’s what we might expect La Niña to bring.
As the easterly trade winds blow warmer waters toward Asia and Australia, we can expect a wetter winter, stronger monsoon seasons and potential flooding. The Atlantic hurricane season is also predicted to be more intense than usual.
Across North America, the polar jet stream will blow colder air across Canada and the northern U.S., bringing a drier, colder winter. However, milder conditions should prevail in the southern parts of the U.S. and Mexico.
Central America can expect to see more frequent hurricanes and heavier rains due in part to denser air pressure and thicker cloud cover. Meanwhile, South America faces the dire prospect of drought, as was seen in previous La Niñas.
In Africa, La Niña generally brings drier conditions around the equator and wetter winters down south.
Beyond flooding, cyclones, droughts, and other extreme weather conditions driven to new extremes, La Niña also impacts people at our most basic level: our health.
Heavy rains and runoff water from flooding are linked to the increased spread of bacteria and disease. For instance, cases of shigellosis, an intestinal bowel infection, peaked in Southeast Asia during the strong La Niña of 2010–11.
On the flip side, cooler temperatures across South America could hopefully reduce the rampant spread of dengue fever, where cases have soared due in large part to El Niño.
La Niña is also set to greatly impact agriculture and the global economy. During the last La Niña, Australia had its most productive years ever of grain production due to plentiful rains, but in Latin America, those same years brought terrible droughts and reduced crop production.
In Mexico, 76 percent of the population faced some level of drought, while Uruguay experienced its driest summer in over 40 years.
To make matters worse for farmers, many in the Global South lack crop insurance. For example, only 10 percent of Brazil’s crops are insured from extreme weather, compared to a global average of 41 percent.
With 2.6 billion people globally drawing their income primarily from agriculture, huge weather events coupled with a lack of government insurance can cause concern for many during La Niña or El Niño.
With impacts on agriculture comes the question: how will this impact our global food supply? With drier conditions headed for South America, how could this impact cash crops like coffee and corn? In Australia, does a wetter season bring a more positive outlook on grain production?
Everything from weather to insurance, food security and disease is interconnected. We will all feel the impacts of our planet as it warms and cools through the ENSO cycle, but the world’s poorest often bear the brunt of the extreme shifts in weather patterns it unleashes.
The National Oceanic and Atmospheric Administration (NOAA) predicts a 65 percent chance of La Niña unfolding by the end of 2024. If so, it will likely lead to more intense Atlantic hurricanes.
Though nobody is certain, some extreme climate events already underway could be indicative of an upcoming La Niña.
Intense flooding filled the streets of Phuket in Thailand at the end of June and early July. While monsoons are a regular part of Southeast Asia’s weather cycle, La Niña brings heavier rains that can further devastate communities, especially in areas where infrastructure is fragile.
However, as the Thai Office of the National Water Resources correctly predicted, the flooding was nowhere near as devastating as it was during the strong La Niña of 2011, when it left hundreds dead across the country and in neighboring Cambodia.
In the years since, Thai authorities have implemented several projects to reduce flooding as well as capture floodwater for the dry season.
This highlights the importance of research and awareness about the impacts of ENSO to help equip people for greater preparedness.
There’s still debate over whether this year’s La Niña will be particularly strong as it follows a very strong El Niño.
But what we do know is: cooler waters are rising, and with them should come our preparedness for the ways La Niña could impact people and the planet.
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