In the tropical Pacific, water sometimes becomes warmer than usual, sometimes colder. The warm phase is called El Niño, the cold one La Niña. These changes affect weather worldwide: droughts in some regions, floods in others. Previously, scientists believed the cause was the ocean oscillating up and down. New work shows that the main factor is horizontal currents that carry water from one place to another.

The researcher decomposed data on ocean temperature and current velocities into simple oscillations to identify which current drives what. It turned out that the central equatorial countercurrent begins to strengthen two to three months before El Niño in the central ocean. The northern equatorial countercurrent leads warming in the east by several months. For La Niña, the strengthening of the southern branch of the South Equatorial Current is key: it brings cold water from the southeastern part of the ocean five to seven months before the cooling. This is why El Niño and La Niña are so different: warm and cold water come from different places.

Now that we know which conveyor belts are switched on, we can monitor them and provide early warnings of upcoming anomalies. If the northern countercurrent accelerates, expect El Niño. If the southern branch of the South Equatorial Current accelerates, La Niña is ahead. This gives people time to prepare for droughts or floods.