In the Pacific Ocean, there is a warm current that flows eastward, against all the others. It carries warm water from Indonesia toward the coasts of the Americas. Before the strongest El Niño events, especially warm water has been observed in this current. Scientists wondered: could this current be more than just a warning sign, could it actually trigger El Niño?

To find out, they used a computer climate model and ran 52 paired experiments. In each experiment, they raised the ocean surface temperature by 1°C in a narrow band along the current. The band was roughly 4000 by 400 kilometers, about the size of India. In the control experiments, nothing was changed. Then they compared how the atmosphere responded to the warming.

It turned out that the warm water makes the air rise faster. As the air rises, clouds and rain form in that area. At the same time, rainfall decreases over Indonesia and Southeast Asia. Air pressure shifts: it drops in the eastern Pacific and rises over the Indian Ocean. This weakens the trade winds that usually blow along the equator. And when the trade winds weaken, warm water piles up off the coast of South America. That is El Niño.

Previously, it was unclear why El Niño begins. Now we can see that even a small warming of the warm current can set off the whole chain. This knowledge will help us better predict El Niño and prepare for its impacts, such as droughts in India and Australia.