An invisible barometer hangs over the Pacific Ocean. Its needle swings from the western shores to the central waters, and in December it shows what the next storm will be like: not just whether it will come, but where it will hit hardest. El Niño, an anomalous warming of water in the tropical Pacific, comes in two types: eastern Pacific (EP), when the hot spot hugs South America, and central Pacific (CP), when it spreads across the middle. The consequences depend on the type: floods in Peru and Ecuador for EP, or droughts in India for CP. But until now, operational models could only name the type about a month before the event, which is too late to prepare fields and reservoirs.
The researchers found that the needle of this barometer is a simple temperature difference. They took sea surface temperature data from 1950, subtracted the 30-year average for each month to remove the influence of global warming, and for each December calculated the difference in anomalies between the western equatorial region (120°E–165°E) and the central region (165°E–150°W). If the west is warmer than the center, the difference is positive, and EP is likely in the following year. If it is colder, the difference is negative, and CP is likely. It is like looking at the tilt of a seesaw: whichever way it leans in December, that is the way it will swing a year later.
The three misses occurred in years when CP happened instead of the expected EP: 1962, 2008, and 2017. But even with these slips, the method works far better than operational models, which, with a one-month lead, give only about 50% correct type forecasts. Moreover, the precursor does not require complex calculations: just one number in December. And if you combine it with a climate network that predicts the very occurrence of El Niño, you can name both the onset and the type a year in advance. In such a tandem, all CP forecasts were correct, and EP forecasts were correct in two-thirds of the cases.
Why is the sign of the difference linked to the type? Probably, westerly wind bursts play a role: brief intensifications of wind from the west that interact with the ocean differently depending on where heat has already accumulated. This is reminiscent of how a light push can shift a swing in one direction if it is already tilted. The physical mechanism still needs to be clarified, but the practical benefit is already visible: agriculture, water resources, and humanitarian planning can start preparing a year in advance, not just a month.
The barometer needle does not always point exactly, but now it is visible long before the storm. In October 2022, the temperature difference was +1.6°C, which pointed to a likely EP El Niño in 2023. And that is what happened: the 2023–2024 event was classified as eastern Pacific. The barometer worked. The remaining task is to learn to read it without errors, and then the Pacific will cease to be unpredictable.
In October 2022, the temperature difference was +1.6°C, pointing to a likely EP El Niño if one began in 2023. And indeed, the 2023–2024 event was classified as eastern Pacific.
In Kim Stanley Robinson's novel 'Forty Signs of Rain', climate scientists try to predict abrupt climate changes, and the accuracy of long-term forecasts becomes a matter of life and death. This work on an early precursor for El Niño type resonates with that theme: a small but reliable gain in lead time can change how we prepare for a disaster.