Summary

The El Niño Southern Oscillation is considered to be the most significant form of “natural” climate variability, although its definition and the scientific understanding of the phenomenon are continually evolving. Since its first recorded usage in 1891, the meaning of “El Niño” has morphed from a regular local current affecting coastal Peru, to an occasional Pacific-wide phenomenon that modifies weather patterns throughout the world, and finally to a diversity of weather patterns that share similarities in Pacific heating and changes in trade-wind intensity, but exhibit considerable variation in other ways. Since the 1960s El Niño has been associated with the Southern Oscillation, originally defined as a statistical relationship in pressure patterns across the Pacific by the British-Indian scientist Gilbert Walker. The first unified model for the El Niño-Southern Oscillation (ENSO) was developed by Jacob Bjerknes in 1969 and it has been updated several times since, but no simple model yet explains apparent diversity in El Niño events. ENSO forecasting is considered a success, but each event still displays surprising characteristics.