The tropical Pacific Ocean sometimes becomes warmer than usual off the coast of Peru, which is called El Niño. Sometimes it cools instead, and La Niña sets in. These states can last for months, then suddenly shift. Why this happens has long been puzzling: some said it was random, others that the system had its own rhythm.
New work shows that the atmosphere over the ocean resembles an uneven surface with two pits. In one pit, air circulation warms the ocean, which is El Niño. In the other, it cools the ocean, which is La Niña. Between the pits lies a ridge that the system finds hard to cross. But water temperature changes the height of that ridge: as water warms, the ridge lowers, making the transition easier. As water cools, the ridge grows, and the system gets stuck.
The scientists built a mathematical model and showed that under real conditions, there are indeed two pits. This explains why El Niño and La Niña occur irregularly, even though the transition itself is predictable. Now we can think about predicting these events by assessing which pit the system is in and how close it is to the ridge.
The name 'El Niño' (Spanish for 'the Christ child') originated with Peruvian fishermen, who noticed that a warm current sometimes appeared off the coast during the Christmas season. The phenomenon has been known since the 19th century, but its global significance was only realized in the late 20th century.
The idea of metastable states and sudden transitions between them echoes the concept of 'climate tipping points' in science fiction, such as in Kim Stanley Robinson's novel 'Forty Signs of Rain', where abrupt climate changes occur when critical thresholds are crossed.