La Niña is approaching, let’s see what changes it will bring to the weather.

The cold Pacific weather and associated weather patterns will have a domino effect of regional weather extremes, roughly the opposite of what a strong El Niño event caused at its peak last winter. In the United States, droughts could occur in some areas and heavy snowfall in others. Elsewhere, the most dangerous effects could include droughts in East Africa and floods in Indonesia.

But how La Niña will play out is unclear, as it comes amid more than a year of record-breaking global temperatures and unprecedented warming of sea levels.

Climate scientists will be watching closely to see whether the typical global-cooling effects of La Niña play out as usual, and if not, what that means about how humanity has altered Earth’s systems by burning fossil fuels and emitting planet-warming greenhouse gases.

“It’ll be interesting to see how this La Niña interacts with the world’s oceans, which are generally very warm,” says Nathan Lensen, a climate scientist at the University of Colorado. “We’re really in uncharted territory on a global scale.”

Here are answers to some frequently asked questions about La Niña and its impact on the planet.

La Niña is a global weather pattern in which cold water from deep in the eastern Pacific Ocean rises to the surface, creating colder-than-usual pools of water along the equator in the central and eastern Pacific. At the same time, stronger-than-usual trade winds blow from east to west across the Pacific, blowing the warm surface waters towards Asia, causing the cold water to rise eastward.

This pattern affects weather conditions around the world because it alters the atmospheric forces that drive weather patterns at mid- and high-latitudes. The contrast between hot, stormy weather in the western Pacific and cooler than normal weather in the central and eastern Pacific contributes to changes in the normal course of weather patterns such as heat waves and storms.

The impacts of La Niña may be imminent. This pattern is known to promote tropical storm activity in the Atlantic Ocean. One of the changes it brings to atmospheric patterns is a reduction in wind shear (wind speed and direction that varies with height) in the Atlantic basin. This creates a more favorable environment for tropical systems to organize and intensify.

In response to the La Niña forecast, meteorologists this week revised their forecasts for the main hurricane season upwards, now predicting a near-record 25 named storms, including 12 hurricanes and six “major” hurricanes rated Category 3 or higher.

In the United States, La Niña is best known for causing warm, dry weather throughout the winter across the South, including Southern California, the Southwest and the Gulf Coast, and rain and snow from the Pacific Northwest to the Northern Plains.

Elsewhere around the world, impacts could include flooding across northern South America and Indonesia, and drought in East Africa, threatening to worsen a hunger crisis amid Sudan’s civil war.

El Niño is a phenomenon that causes warmer than normal temperatures across the eastern and central Pacific Ocean. During an El Niño, ocean trade winds are weaker than normal and may even reverse to blow easterly, creating a cycle of warm surface water pooling in the eastern Pacific Ocean and causing rapid temperature increases.

El Niño often triggers the development of La Niña because it releases a lot of heat from the eastern Pacific Ocean, causing a rapid transition to the cooler conditions of La Niña.

Many parts of the world’s oceans, including the western Pacific, have been unusually warm over the past year, exacerbating the natural contrast between hot water on one side of the ocean and cooler water on the other, which could intensify a normally relatively mild La Niña event, said Nathaniel Johnson, a NOAA scientist who works on La Niña forecasting.

“This event may have a larger impact than expected because of warmer temperatures in the western Pacific,” said Johnson, a researcher at NOAA’s Geophysical Fluid Dynamics Laboratory.

Lensen said research is underway to determine whether climate change is changing the behavior of La Niña and El Niño. El Niño, known for raising global temperatures, pushed the planet to what scientists described as the hottest temperature in more than 100,000 years last July, bringing it ever closer to a dangerous warming threshold of 1.5 degrees Celsius above pre-industrial times.

Climate scientists will be watching closely to see to what extent La Niña can slow the acceleration of global warming.

La Niña typically lasts for nine to 12 months, but can sometimes last up to three years. It is not yet known how long this La Niña will last.

Lensen said that for now, long-term climate models suggest a so-called “neutral” period — one in which neither El Niño nor La Niña events occur — could be next, but that such predictions are not firm. A two-year La Niña event is “definitely possible,” he said.

Lensen said the stronger the preceding El Niño, the longer the La Niña will last. The winter of 2015-2016 was one of the strongest El Niño patterns on record, followed by two years of weak La Niña.

But unlike the relatively weak and short El Niño events of 2018 and 2019, the La Niña lasted for three years from 2020 to 2023, what climate scientists call a rare “triple dip” La Niña event.

This time, the Earth is heading toward the end of a historically strong El Niño event, but it is not as intense as the most powerful El Niño events on record, such as those of 2015-2016, 1997-1998, and 1982-1983.

The pattern gets its name from a legend surrounding El Niño, Spanish for Baby Jesus. Fishermen off the coast of Peru noticed periods of unusually warm waters in the eastern Pacific Ocean during the winter, which caused changes in fishing conditions around Christmas. La Niña is the exact opposite of El Niño.