La Nina is coming. Here’s how it could change the weather.

The planet is officially on alert for La Niña, the counterpart to the El Niño climate pattern, scientists said Thursday. It could have a cooling effect on Earth’s ongoing period of record heat and is likely to spawn a series of intense Atlantic hurricanes this fall.

There is a 70 percent chance that La Niña will develop between August and October, and a nearly 8 in 10 chance that La Niña will occur this winter, scientists with the National Oceanic and Atmospheric Administration wrote in a forecast accompanying their La Niña warning.

The climate pattern linked to the cool Pacific Ocean would have a domino effect of regional weather extremes that would be largely the opposite of what a strong El Niño pattern brought at its peak last winter. In the United States, it could bring droughts in some places and heavy snowfall in others; elsewhere, the most dangerous effects could be droughts in East Africa and floods in Indonesia.

However, there is some uncertainty about how this La Niña episode will develop, as it is occurring at a time when global average temperatures have been at record highs for more than a year and sea surface temperatures are at record highs.

Climate scientists will be watching closely to see whether the typical global cooling of La Niña plays out as usual. And if not, what that might mean for the way humans have transformed Earth’s systems by burning fossil fuels and emitting greenhouse gases that warm the planet.

“It’s going to be interesting to see how this La Niña crosses the generally very warm oceans on Earth,” said Nathan Lenssen, a climate scientist at the University of Colorado. “We’re in really uncharted territory, globally.”

Here you will find answers to frequently asked questions about La Niña and its effect on the planet.

What is La Nina?

La Niña is a global climate pattern in which cold water rises from the depths of the eastern Pacific Ocean to the surface, creating a pool of cooler than normal water along the equator in the central and eastern Pacific Ocean. At the same time, stronger than normal trade winds blow across the Pacific Ocean from east to west, blowing warm surface water toward Asia and allowing that colder water to rise in the east.

The pattern affects conditions around the world as it shifts around the atmospheric forces that drive weather patterns through the mid- and high-latitudes. The contrast between hot and stormy conditions in the western Pacific and cooler-than-normal conditions in the central and eastern Pacific helps drive changes in the normal flow of weather patterns such as heat waves and storm systems.

What does La Niña mean for global weather patterns?

Some La Niña effects may be imminent. The pattern is known for boosting tropical storm activity in the Atlantic Ocean. One of the changes it brings to atmospheric patterns is a reduction in wind shear — a difference in wind speed and direction at different altitudes — over the Atlantic basin. That creates an environment more favorable for tropical systems to organize and strengthen.

The La Niña forecasts prompted meteorologists this week to revise upward a key hurricane season forecast, calling for a record 25 named storm systems, including 12 hurricanes and six “major” hurricanes, which are classified as Category 3 or higher.

In the United States, La Niña is best known for warm and dry conditions in the southern portion of the state during the winter, including Southern California, the Southwest, and the Gulf Coast, and wet and snowy conditions from the Pacific Northwest to the Northern Plains.

Elsewhere in the world, the consequences could include flooding in northern South America and Indonesia, and drought in East Africa. These conditions could exacerbate the hunger crisis during the civil war in Sudan.

What is the difference with El Niño?

El Niño is associated with warmer than normal temperatures in the eastern and central Pacific Ocean. During El Niño, the trade winds are weaker than normal or turn eastward, creating a cycle in which warm surface waters collect and warm dramatically in the eastern Pacific Ocean.

El Niño often heralds La Niña, as large amounts of heat are released from the eastern Pacific Ocean, causing a rapid transition to the cooler conditions of La Niña.

How would this episode of La Niña be different?

Excessive warmth has dominated many corners of the world’s oceans over the past year, including the western Pacific. It’s possible that this could exacerbate the natural contrast between warm water on one side of the ocean and cold water on the other, intensifying what would otherwise have been a relatively modest La Niña episode, said Nathaniel Johnson, a NOAA scientist involved in La Niña forecasting.

“This event could have a huge impact because of the warmth in the western Pacific,” said Johnson, a researcher with NOAA’s Geophysical Fluid Dynamics Laboratory.

Research is underway to determine whether climate change could alter the behavior of La Niña and El Niño, Lenssen said. El Niño, which is known to raise global temperatures, helped push the planet to what scientists say were the warmest conditions in more than 100,000 years last July — and closer than ever to a dangerous threshold of warming, 1.5 degrees Celsius (3.7 degrees Fahrenheit) above pre-industrial temperatures.

Climate scientists are closely monitoring whether and to what extent La Niña can counteract accelerated global warming.

How long will La Niña last?

La Niña typically lasts nine to twelve months, but can sometimes last three years. It is too early to say how long it will last.

For now, long-term climate models suggest a period of so-called “neutral” conditions — the absence of El Niño or La Niña — could arrive, but those predictions are far from set in stone, Lenssen said. A two-year La Niña is “certainly something that is possible,” he said.

The stronger the preceding El Niño, the longer a La Niña can last, Lenssen said. After an El Niño pattern was one of the strongest observed in the winter of 2015-2016, weak La Niña conditions persisted for two years.

But amid a relatively weak and short El Niño in 2018 and 2019, La Niña lasted three years in what climate scientists called a rare “triple-dip” La Niña, from 2020 to 2023.

This time around, the planet is coming off a historically strong El Niño, though not as intense as the strongest episodes ever recorded, including 2015-2016, 1997-1998, and 1982-1983.

Why is it called La Niña?

The pattern’s name comes from a legend associated with El Niño, a name that means baby Jesus in Spanish. Fishermen off the coast of Peru noticed periods of unusually warm water in the eastern Pacific that sometimes developed in the winter, changing fishing conditions around Christmas. La Niña is simply the opposite of El Niño.

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