Off the coast of Peru, a warm current sometimes appears. It brings rain to places that are usually dry, and drought to places that are usually wet. For a long time, scientists couldn't understand why this current appears not every year but intermittently.
One researcher proposed a simple model. He imagined the ocean as a bowl of water. The Moon and Earth's rotation rock this bowl, making the water slosh. Sometimes the sloshing intensifies, and then the warm current reaches the shores of Peru.
The model managed to reproduce observations with 80% accuracy. Crucially, it explained the strange behavior of the current in 1981. Back then, the rhythm changed, and the model shows that this was linked to a phase shift in the two-year cycle.
Now scientists can better predict El Niño. This will help farmers prepare for droughts and cities for floods. The bowl of water turned out to be a useful metaphor for the entire Pacific Ocean.
The 4.25-year period characteristic of the thermocline is close to half of the 8.5-year cycle of the lunar perigee, which may point to a tidal origin of the main frequency.