La Niña is coming. Here’s how it could change the climate.

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

But there is some uncertainty about how this La Niña event might play out, as it comes amid more than a full year of record global average temperatures and unprecedented ocean surface warmth.

Climate scientists will be paying close attention to whether La Niña’s typical global cooling influence plays out as usual — and if not, what that might mean about how humans have transformed Earth’s systems by burning fossil fuels and emitting planet-warming greenhouse gases.

“It’s going to be interesting to see how this La Niña intersects with the world’s oceans, which are typically very warm,” said Nathan Lenssen, a climate scientist at the University of Colorado. “We’re in really uncharted territory, globally.”

Here are answers to some common questions about La Niña and its effects on the planet.

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

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

Some effects of La Niña may be imminent. This phenomenon is known to fuel tropical storm activity in the Atlantic. Among the changes it causes in atmospheric patterns is a reduction in wind shear (a difference in wind speed and direction at different altitudes) over the Atlantic basin. This creates an environment more conducive to the organization and strengthening of tropical systems.

The La Niña forecast prompted meteorologists to upgrade a key hurricane season forecast this week, now predicting a near-record 25 named storm systems, including 12 hurricanes and six “major” hurricanes, rated Category 3 or higher.

In the United States, La Niña is best known for its warm, dry conditions in the southern tier during winter (including Southern California, the Southwest, and the Gulf Coast) and for its wet, snowy conditions from the Pacific Northwest to the northern Plains.

Elsewhere in the world, its effects could include flooding in northern South America and across Indonesia, as well as drought in East Africa – conditions that could worsen a food crisis amid civil war in Sudan.

El Niño is associated with warmer than normal temperatures in the eastern and central Pacific. During El Niño, ocean trade winds are weaker than normal or even reverse to blow eastward, creating a cycle that allows warm surface waters to accumulate and warm significantly in the eastern Pacific.

El Niño often leads to La Niña episodes because it releases large amounts of heat from the eastern Pacific, allowing a rapid transition to cooler La Niña conditions.

Last year, many of the world’s oceans experienced unprecedented warmth, particularly in the western Pacific. That may have accentuated the natural contrast between warm waters on one side of the ocean and cold waters on the other, intensifying what could have been a relatively modest La Niña event, said Nathaniel Johnson, a NOAA scientist involved in La Niña forecasting.

“This event may have a larger magnitude than it deserves because of the warm western Pacific,” said Johnson, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory.

Research is ongoing 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 toward what scientists consider the hottest conditions in more than 100,000 years last July — and closer than ever to a dangerous threshold of warming, 1.5 degrees Celsius above pre-industrial temperatures.

Climatologists will be watching this phenomenon closely to see if and to what extent La Niña could counteract this acceleration of global warming.

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

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

According to Lenssen, the stronger the preceding El Niño, the longer the La Niña can last. After one of the strongest El Niños ever observed in the winter of 2015-2016, weak La Niña conditions persisted for two years.

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

This time, the planet is emerging from a historically strong El Niño — although not as intense as the strongest episodes on record, including those of 2015-2016, 1997-1998 and 1982-1983.

The name of this pattern comes from a legend related to El Niño, a name that means baby Jesus in Spanish. Fishermen off the coast of Peru noticed periods of abnormally high temperatures in the eastern Pacific, which sometimes developed in the winter, changing fishing conditions around Christmas. La Niña is simply the opposite of El Niño.