El Niño was named by Peruvian fishermen after the Christ Child
Warm water showing up off Peru around Christmas gave El Niño its name, the Boy. Today the term describes one phase of a Pacific-wide swing in winds and sea temperatures that brings storms to some regions and drought to others, and briefly nudges the whole planet's average temperature up.
The El Niño-Southern Oscillation is one climate pattern with three phases: neutral, El Niño and La Niña. El Niño is the warm phase of Pacific surface waters and La Niña, the Girl, the cool one; Older names for La Niña include anti-El Niño and El Viejo, meaning old man. The Southern Oscillation half refers to a seesaw in air pressure between the eastern and western tropical Pacific. The name appears in a climate sense in 1892, when Captain Camilo Carrillo told a geographical congress in Lima that sailors had named the warm current for the infant Christ.
Normal conditions set the stage. Strong equatorial trade winds push surface water westward, where the Sun warms it, so the sea near Indonesia sits about half a metre higher than off South America. Waters northeast of Australia average around 28 to 30 degrees Celsius, while the eastern tropical Pacific is roughly 8 to 10 degrees cooler. In the east, cold deep water wells up to replace what the wind removes, helped by the Coriolis effect and the cold Humboldt Current. The boundary between warm and cold water lies about 140 metres down in the west but only about 30 metres in the east.
When the trade winds weaken, upwelling slows, warm water slides east and rain clouds follow it. That shift feeds on itself through what is called the Bjerknes feedback, identified by Jacob Bjerknes in 1969: changes in the air alter sea temperatures, which in turn alter the winds.
The disruption is global, upsetting normal weather far from the Pacific. An El Niño lifts global average surface temperature for about a year, while La Niña cools it briefly. The pattern is irregular and hard to forecast, and each monitoring country uses its own threshold to declare events. How climate change will alter ENSO remains uncertain, though warming is making its droughts and floods worse.
Source: El Niño–Southern Oscillation
