La Niña is over. Scientists now say it was stronger than it seemed.

Washington Post

10-04-2025

La Niña is over, scientists declared Thursday. And while the climate pattern was notably brief and had been waning, scientists said the episode was not as weak as it once seemed.
When the planet-cooling climate pattern emerged this year, scientists said it developed later and in a weaker state than they had expected. But researchers say that is because unusual warmth that has dominated the Pacific and other global oceans for the past two years masked La Niña, which is defined by a patch of cooler-than-normal Pacific waters.
La Niña's end, which the National Oceanic and Atmospheric Administration announced Thursday, marks the beginning of an uncertain spell in global weather, with no strong driver of seasonal patterns and trends in temperature or precipitation. Forecasters instead will have to look toward factors that influence the climate on a weekly to monthly scale that can be harder to predict, even as some of even as some of La Niña's characteristics, such as a drying influence in the Southwest United States, may increase the likelihood of an active fire season.
But forecasters said the pattern that follows this one will drive more uncertainty than usual.
The traditional measure of La Niña and its better-known inverse, El Niño, looks at differences in temperature across specific zones of the Pacific. And based on that metric, the La Niña episode that officially began this winter was weak and expected to have relatively minor impacts on global weather.
But the surge in baseline ocean temperatures tied to global warming made La Niña's signature zone of cool Pacific waters look warmer than they otherwise would have been. Cooler waters mean a stronger La Niña.
When accounting for that warming, the La Niña was closer to moderate strength, scientists said, fueling dry conditions across the southern United States that have set the stage for a potentially active fire season for the Southwest.
Scientists have been tracking the difference between the typical way La Niña and El Niño are diagnosed, and a new standard that takes the trend of global warming into account.
'It's one of the first times we've seen such a large difference,' said Emily Becker, associate director of the Cooperative Institute for Marine and Atmospheric Studies at the University of Miami.
The index that takes into account nearly 1.5 degrees Celsius (2.7 degrees Fahrenheit) of global warming more accurately captured the weather impacts La Niña caused in the United States, she said.
Had scientists at NOAA and other climate agencies used the new index to track La Niña's development, they also likely would have declared its arrival earlier and more accurately, said Pedro DiNezio, an associate professor at the University of Colorado at Boulder.
But it's not clear that the new measure is better. Scientists are investigating whether they should replace the traditional metrics applied to what is known as the El Niño-Southern Oscillation, or ENSO, with the new global warming-adjusted index. Just because the new index worked better this time doesn't mean it always will, Becker said.
'We're going to need to compare the two across many different situations, with many different global background conditions before we can make a call like that,' Nathaniel Johnson, a climate scientist at NOAA's Geophysical Fluid Dynamics Laboratory, wrote on the agency's ENSO Blog.
It was just three months ago that NOAA declared La Niña had arrived, with requisite signs of the cooler-than-normal waters along the equatorial Pacific and wind patterns that push warmer waters and towering clouds to the west, toward Indonesia.
Typical La Niña events can last anywhere from nine months to two years. The planet's most recent La Niña lasted, unusually, for three years.
But now, the planet is entering what are known as 'ENSO neutral' conditions, without the influence of La Niña or El Niño to suggest what sort of temperature or precipitation patterns that parts of the world can expect.
That also means the outlooks for wildfire season in the West and hurricane season in the East will depend on monthly and weekly variations in weather patterns that cannot be as reliably predicted so far in advance. While La Niña's drying influence has raised the likelihood of an active fire season, recent early predictions of a busy but not hyperactive Atlantic hurricane season carry more uncertainty than usual, forecasters said.
Forecast model updates for April have started to arrive, and the consensus is that neutral conditions will continue across the tropical Pacific Ocean into summer.
However, from late summer into fall, forecasts start to diverge.
Some model outlooks suggest that the Pacific will warm up, while others suggest that a dip back toward La Niña could occur — this uncertainty is at least partially tied to the 'spring predictability barrier,' the time of year when long-range forecasts for El Niño and La Niña have lower skill.
If the ocean begins to lean even slightly toward El Niño or La Niña later this year, it can have an influence on hurricane season. La Niña-like patterns tend to cause winds that are more conducive to hurricanes in the Atlantic.
For now, NOAA scientists estimate about a 43 chance of neutral conditions by late fall, with a 38 percent chance of La Niña and less than 20 percent chance of El Niño. But those odds are likely to shift over the coming months, as the climate patterns are difficult to predict at this time of year, during Northern Hemisphere spring, when the climate is naturally in flux.
'After the spring, I think we'll see a clearer picture,' Becker said.