4.9 El Niño and La Niña

Normally, the trade winds blow from east to west across the tropical Pacific. They push warm surface water west, so it piles up near Indonesia and Australia. That warm water heats the air, which rises and brings heavy rain to that region.

Along the coast of Peru and Ecuador, as warm surface water is pushed away, cold, deep water rises to replace it. This upwelling brings up nutrients from the deep ocean, which feed huge amounts of phytoplankton. That makes the waters off Peru one of the world's richest fisheries, especially for anchovies.

The whole system is called ENSO, the El Niño–Southern Oscillation. The Southern Oscillation refers to a seesaw in air pressure between the eastern and western Pacific that goes along with the ocean changes. ENSO swings irregularly, with an El Niño or La Niña developing every two to seven years.

During El Niño, the trade winds weaken or even reverse. Warm surface water spreads back east across the Pacific toward South America.

With warm water off Peru, upwelling weakens, so fewer nutrients reach the surface. Phytoplankton decline, and so do the fish and seabirds that depend on them. Peru's anchovy catch can drop sharply.

• Peru and Ecuador: heavier rain and flooding on the coast.
• Indonesia and Australia: drier conditions, drought and more wildfires.
• Southern U.S.: often wetter, cooler winters.
• Atlantic hurricanes: usually fewer, because El Niño increases wind shear over the Atlantic.
• Global temperature: tends to be higher in El Niño years, because the Pacific releases heat to the atmosphere.

La Niña is roughly the opposite. The trade winds get stronger than normal, pushing even more warm water west. Upwelling off South America strengthens and the eastern Pacific becomes cooler than normal.

• Indonesia and Australia: wetter than normal, with more flooding.
• Peru: dry coast; the fishery generally does well with strong upwelling.
• Southern U.S.: often drier, warmer winters, raising drought risk.
• Atlantic hurricanes: usually more active.

Scientists monitor sea surface temperatures in the central and eastern tropical Pacific. A common measure, the Oceanic Niño Index, compares the average temperature over three months with the long-term average. If it stays at least 0.5°C above average for several overlapping three-month periods in a row, it's classified as El Niño; at least 0.5°C below average means La Niña. Predicting ENSO months ahead helps farmers, fisheries and emergency planners prepare.