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

The climate pattern linked to cool Pacific Ocean conditions would have domino effects on regional weather extremes that are largely the opposite of what a strong El Niño pattern brought during its peak last winter. In the United States, it can deliver drought conditions in some places and heavy snow in others; elsewhere, its most dangerous effects can include drought in East Africa and floods in Indonesia.

But there is some uncertainty over how this episode of La Niña could play out because it arrives amid over a full year of record average global 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 could signal about how humans have transformed Earth 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 generally very warm global oceans,” 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 effect on the planet.

La Niña is a global climate pattern in which cool waters from deep in the eastern Pacific Ocean well up to the surface, creating a pool of cooler-than-normal waters along the equator in the central and eastern Pacific. At the same time, stronger-than-normal tradewinds blow across the Pacific from east to west, blowing warm surface waters toward Asia and allowing those colder waters to rise up in the east.

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

Some La Niña impacts may be imminent. The pattern is known for fueling Atlantic tropical storm activity. Among the changes it brings to atmospheric patterns is a reduction in wind shear — a difference in wind speed and direction at varying altitudes — over the Atlantic basin. That creates an environment more conducive for tropical systems to organize and strengthen.

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

In the United States, La Niña is best known for warm and dry conditions across the southern tier through 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 around the world, its effects can include floods in northern South America and across Indonesia, and drought in East Africa — conditions that could exacerbate a hunger crisis amid civil war in Sudan.

El Niño is associated with warmer-than-normal temperatures across the eastern and central Pacific. During El Niño, the ocean tradewinds are weaker than normal, if not reversed to blow toward the east, creating a cycle that allows warm surface waters to pool and heat up dramatically in the eastern Pacific.

El Niño often ushers in episodes of La Niña because it releases vast amounts of heat from the eastern Pacific, making for a quick transition to the cooler conditions of La Niña.

Off-the-charts warmth has dominated many corners of the world’s oceans over the past year — including the western Pacific. It’s possible that could exacerbate the natural contrast between hot waters on one side of the ocean and cool waters on the other side, intensifying what might otherwise have been a relatively modest episode of La Niña, said Nathaniel Johnson, a NOAA scientist involved in La Niña forecasting.

“This event could punch above its weight because of how warm the western Pacific is,” said Johnson, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory.

There is ongoing research to determine whether climate change may be altering the behavior of La Niña and El Niño, Lenssen said. El Niño, which is known to boost planetary temperatures, helped push the planet to what scientists say were its 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 preindustrial temperatures.

Climate scientists will be watching closely to see whether and to what degree La Niña could counteract that acceleration in global warming.

La Niña typically lasts nine to 12 months but can sometimes last three years. It’s too early to say how long this one will go.

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

The stronger the preceding El Niño, the longer a La Niña can last, Lenssen said. After an El Niño pattern during the winter of 2015-2016 became one of the strongest observed, 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 called a rare “triple-dip” La Niña, from 2020 through 2023.

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

The pattern’s name stems from legend tied to El Niño, a name that signifies baby Jesus in Spanish. Fishermen off the coast of Peru noticed periods of unusually warm waters in the eastern Pacific that sometimes developed in the winter, which changed fishing conditions around Christmas. La Niña is simply the opposite of El Niño.