Peruvian fishermen named El Nino after the Christ child because the coastal water turned warm and the anchovies vanished around Christmas. The El Nino Southern Oscillation, or ENSO, is a coupled conversation between ocean and atmosphere. In the neutral state, trade winds push the sun-warmed surface of the Pacific west until sea level near Indonesia sits about a foot and a half higher than off Peru, while cold, nutrient-rich water wells up along South America. The western Pacific runs 8 to 10 degrees Celsius warmer, driving the Walker circulation, first pieced together by Gilbert Walker, in which moist air rises over Indonesia and sinks dry over the east. Jacob Bjerknes described the self-reinforcing loop in 1969.

A burst of westerly winds, often from the Madden-Julian Oscillation, can break the loop, sending warm water sloshing east. The warm pool caps the upwelling, rain follows it east, and Australia, Indonesia, and India face drought while the eastern Pacific floods. La Nina, once called El Viejo or anti-El Nino, exaggerates the neutral state, and ENSO Modoki, from a Japanese word for similar but different, stalls warm water mid-Pacific. Released heat expands the troposphere and shoves the jet streams through teleconnections, shifting storms far from the Pacific. NOAA declares El Nino when the Nino 3.4 region holds 0.5 degrees Celsius above average for consecutive overlapping seasons.

• The 2023 El Nino cut maize output by up to 70 percent in parts of Zimbabwe, Zambia, and Malawi.
• A 2014 University of Cambridge study found El Nino lowers GDP in Australia, India, and South Africa while the US, Canada, and Mexico often gain.
• Columbia University researchers linked ENSO to 21 percent of civil conflicts since 1950, with civil war risk doubling in El Nino years.
• The strong events of 1997 to 1998 and 2015 to 2016 killed up to 99 percent of live coral on some equatorial reefs.
• The Southern Oscillation Index compares air pressure at Tahiti and Darwin, Australia.