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How the arrival of La Niña later this year could change the world’s weather

Submerged trees stand in the flooded Los Angeles River during an atmospheric river storm in Los Angeles on Monday.Kyle Grillot/For The Washington Post

Signs of a historically strong El Niño global climate pattern became obvious in recent weeks - including deadly fires in South America and deluges in California. Yet scientists are now predicting that the regime could disappear within months.

Forecasters at the National Weather Service issued a La Niña watch Thursday, projecting that there is about a 55 percent chance that this pattern - which is the opposite of El Niño - will develop by August.

The development of La Niña would have major consequences for weather in the United States and around the world. It could also temporarily slow the rapid global warming that began about nine months ago, when El Niño first took hold.

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La Niña is known for encouraging active and destructive Atlantic hurricane seasons, as well as promoting dry conditions for Southern California and the Southwest.

It also tends to subdue global temperatures. While it won’t turn back a decades-long rise in planetary warmth, it could moderate the extreme levels of warming scientists have observed as of late.

Last month was Earth’s warmest January on record, an eighth consecutive monthly global temperature record. And it marked the end of a 12-month period during which the planet hit a long-feared level of warming: 1.5 degrees Celsius (2.7 degrees Fahrenheit) above a historic average from the 19th century, before the widespread consumption of fossil fuels.

Whether Earth’s climate is under the influence of a La Niña or El Niño depends on conditions along the equator in the Pacific Ocean.

When normal trade winds that flow from east to west become weak or even reverse, warm water pools along the surface of the central and eastern Pacific, heralding El Niño and triggering domino effects on weather around the world. That can include wetter-than-normal conditions along the southern tier of the United States, including in Southern California, diminished Atlantic hurricane seasons and droughts and fires in Indonesia and southern Africa.

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It is named after the Christ child in Spanish because fishermen long noticed its effects off the coast of Peru around Christmas.

During La Niña, Pacific trade winds push warm waters to the west. The result is wetter-than-normal weather on the western side of the Pacific, as well as in southern Alaska, the northwestern United States and the northern Plains. La Niña tends to encourage dry and mild conditions in the southern United States when it typically peaks in the winter months.

Climate forecasters at the Weather Service said Thursday that warmth in eastern and east-central Pacific diminished in January, as did the wind pattern associated with El Niño. Meanwhile, climate prediction models are suggesting that El Niño will continue to weaken through the spring months.

They predict a 79 percent chance that “neutral” conditions - the absence of either El Niño or La Niña - will be in place sometime between April and June.

It can be difficult to predict how El Niño or La Niña will behave during the spring months, the scientists noted, because the patterns tend to be in transition at that time, making it harder to detect trends. But a quick switch to La Niña is common after historically strong episodes of El Niño, like the one that is now winding down.

Regardless of how quickly it weakens, El Niño is likely to continue to drive global weather patterns for months to come.

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“Even as the current El Niño weakens, impacts on the United States could persist through April,” the Weather Service forecasters wrote.

Scientists will be closely watching how a transition from the planet-warming El Niño pattern to the cooling influence of La Niña might affect global average temperatures. It could help show answer a critical question: Is the rate of global warming and climate change accelerating?

It’s possible the switch to La Niña could moderate global temperatures in 2024 and prevent them from surpassing 2023, which marked a surprising new peak in global warmth. That would back up a theory that 2023 was an El Niño-driven “blip” in what has otherwise been a steady trajectory for global temperatures, said Gavin Schmidt, director of the NASA Goddard Institute for Space Studies.

But if 2024 ends up warmer than scientists might otherwise expect, given the presence of a La Niña pattern, that could, in effect, “add to the evidence that something systematic has changed,” Schmidt said in an email.

And the climate transitions also provide more data and opportunity to learn how global warming may be influencing El Niño and La Niña. Climate scientists don’t have a definitive answer yet, but they do suspect that the frequency of strong El Niño and La Niña events is likely to increase throughout the next century.