Why do scientists "treat the two as parts of one cycle"?

The Pacific is the largest ocean on the Earth, and what happens to its water can change the weather far away from it.

In normal years, steady winds called trade winds blow westward along the equator. They push warm surface water from the coast of South America across the ocean towards Asia. Cold water rises from the depths near South America to take its place, a process known as upwelling.

Every few years this pattern breaks. In an El Niño, the trade winds weaken, and warm water spreads back east towards the Americas. In a La Niña, the opposite happens: the winds grow stronger, and more warm water is pushed towards Asia. Scientists treat the two as parts of one cycle.

Each episode usually lasts nine to twelve months, and sometimes longer. They come every two to seven years on average, but not on a fixed timetable, and El Niño happens more often than La Niña. Both can affect weather, wildfires, the natural world and the economies of countries around the globe.

The name is old. Fishermen in South America noticed spells of unusually warm water in the Pacific as early as the 1600s. Because the warming tended to peak around December, they called it El Niño de Navidad, the little boy of Christmas.