How can a warm patch of the Pacific change the whole world's weather?

A warm patch of the Pacific can change the world's weather because the ocean and the wind there are locked in a tug-of-war that reshapes where rain falls. Normally, strong trade winds blow west along the equator, piling warm surface water up near Indonesia, where the sea stands about half a metre higher than off Peru. Cold deep water wells up off South America to replace it. The warm west gets the clouds and rain; the cool east stays dry. Every two to seven years, the winds weaken. The warm water sloshes back east, upwelling slows, and the rain follows the heat. That is El Niño.

The shift spreads far beyond the Pacific. Rain fades over Indonesia, India and northern Australia while storms build over the tropical Pacific, and the whole planet runs a little warmer for about a year. Its opposite, La Niña, brings stronger winds and cooler water in the east. The two together are called the El Niño–Southern Oscillation, or ENSO.

The name comes from the sea. In 1892 a Peruvian captain, Camilo Carrillo, reported that local sailors called the warm current El Niño, the Christ Child, because it usually arrived around Christmas. The air-pressure half of the story was found by an English mathematician, Gilbert Walker, sent to India with no meteorology experience to predict the monsoons after earlier forecasts had failed disastrously. His Southern Oscillation, and the Walker circulation named after him, became pieces of one global puzzle.