The Pacific Ocean works like a giant seesaw. Sometimes its surface warms up, and then more clouds gather over South America. Sometimes the ocean cools, and the sky stays clear more often. These swings are called El Niño and La Niña. They move slowly, over several years, and they determine how much sunlight reaches the panels.

Scientists took sunlight data from 31 weather stations in Brazil and Chile. They compared three years: one warm, one cold, and one neutral. It turned out that in a neutral year, the weather behaves 'stubbornly': if it is clear today, it will likely be clear tomorrow. In a warm El Niño year, this 'stubbornness' becomes even stronger, especially in northern Brazil. But in a cold La Niña year, in some cities the weather becomes more variable: a clear day is more often followed by an overcast one.

This matters for people who build solar power plants. If they measured sunlight during an El Niño year, their ten-year forecasts could be off. Previously this was unknown, but now adjustments can be made so that plants generate as much energy as promised.