In the tropical Pacific, the ocean sometimes warms more than usual, sometimes cools. Fishermen off the coast of Peru noticed this centuries ago and named the warm phase El Niño, which means 'little boy,' because it arrived around Christmas. Today, these temperature swings affect weather across the planet: from droughts in Australia to downpours in California. But exactly how the ocean switches between warm and cold states remains unclear. Classical theories talk about waves that run along the equator and rock the layer with a sharp temperature gradient with depth. The new work offers a different view: horizontal currents that carry warm or cold water to the right regions play the leading roles.

To test this idea, the author decomposed data on sea surface temperature from 1950 to 2017 and current velocities from 1993 to 2016 into oscillations with different periods. The method, called ESMD, extracts individual modes from a messy signal and shows where and when they change. Then indices were built for four sensitive regions that correspond to pathways of warm water from the north and cold water from the south. Comparing these indices with temperature anomalies revealed which current triggers what.

It turned out that the central equatorial countercurrent (CECC) starts strengthening two to three months before a warm anomaly appears in the central Pacific. The northern equatorial countercurrent (NECC) leads warming in the eastern part by several months, sometimes up to two years. For the eastern type of El Niño, the Niño-B index proved decisive, reflecting the transport of warm water from the coast of Central America southward. For the central type, the Niño-A index, linked to a break in the northern countercurrent, plays the main role. For La Niña, the picture is different: the southern branch of the South Equatorial Current (SSEC) strengthens five to seven months before cooling and drags cold water from the southeastern ocean straight into the central equatorial zone.

This work changes our understanding of how the climate cycle is born. Whereas scientists previously focused mainly on vertical processes, such as how the layer with a sharp temperature gradient rises and falls, the spotlight now is on horizontal currents. Current indices can serve as early indicators of events: a strengthening northern countercurrent warns of El Niño months ahead, and the southern branch of the South Equatorial Current warns of La Niña half a year in advance. For countries that depend on the whims of tropical weather, this is time to prepare: from water reserves to strengthening coastlines. The ocean's conveyor belts, it turns out, can be tracked in advance, and thus we can predict what goods they will deliver next time.