The mystery of the missing La Niña continues – and we don’t know why

An expected shift to cool La Niña conditions in the Pacific Ocean has been delayed again. Forecasters now project only a weak event to emerge by the end of November, which is likely to limit the cooling influence of the climate pattern on global average temperatures.

“I do not know why it has slowed down,” says Michelle L’Heureux at the US National Oceanic and Atmospheric Administration (NOAA). “If someone did, we might have been able to predict it.”

La Niña is the cool phase of the cycle of sea surface temperatures in the equatorial Pacific Ocean known as the El Niño Southern Oscillation (ENSO). The climate pattern is among the largest influences on the global climate, and irregularly swings between La Niñas, neutral temperatures and warm El Niños every several years.

A rare “triple-dip” La Niña between 2020 and 2022 gave way to a strong El Niño in 2023. On top of warming due to greenhouse gas emissions, the release of stored heat in the oceans from that event boosted global average temperatures, making 2023 the hottest year on record.

As the El Niño faded and neutral temperatures emerged in May of this year, forecasters projected a rapid shift to La Niña conditions would follow. In June, researchers at NOAA gave La Niña a greater than 60 per cent chance of developing between July and September and a greater than 70 per cent chance of developing between August and October. This contributed to forecasts for an extreme Atlantic hurricane season.

While sea surface temperatures in the Pacific have cooled since then, a full-blown La Niña still has not emerged, possibly contributing to a “suspiciously quiet” lull in hurricane activity. NOAA now gives La Niña just a 60 per cent chance of developing by the end of November. The agency projects a weak and short event lasting through January to March.

That bullish early forecasts were far off the mark is not hugely surprising, says Emily Becker at the University of Miami in Florida. Forecasts made that early in the year tend to be less accurate because small fluctuations in wind or water can have big effects, she says. It is also not unheard of for La Niña to arrive so late, even following a strong El Niño event. Four La Niñas have emerged this late in the year since 1950.

But the reasons for the delay are unclear. “The trade winds have been slower than expected in parts of the eastern Pacific, which may be keeping waters on the equator warmer than forecasted,” says L’Heureux. “But this may be more of a symptom than a cause.”

Whether human-caused climate change played a role in the delayed La Niña remains an open question, says Becker. Some research suggests climate change will make ENSO more variable, but this remains a contentious area of climate science. “There have been studies published in the past five years or so saying practically everything,” says Becker.

Despite the delays, a La Niña is slowly emerging, and it is likely that it will have its characteristic influence on global weather, from dry weather in the southern US to rain in Indonesia. But as a weak event, it won’t cool global average temperatures as much as a strong and long-lasting one. “We have seen the global average temperature come down, but it’s still very elevated,” says Becker. “We probably won’t see as much of a decrease.”

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