El Niño-Southern Oscillation (abbrieviated as ENSO and commonly called simply El Niño), is an intensification of monthly or seasonal fluctuations in the air pressure difference between Tahiti and Darwin, Australia caused by warming of surface waters of the tropical Eastern Pacific Ocean that occurs every three to eight years. The name is from the Spanish for "the little boy", refers to the Christ child, because the phenomenon is usually noticed around Christmas in the Pacific near South America. A period of cooling in the tropical Pacific is the opposite extreme in the natural ENSO cycle and is called La Niña.
The mechanisms that sustain the El Niño - La Nina cycle remain a matter of research, but El Nino is associated with disruption of Pacific trade winds and a stronger than usual so-called Madden-Julian oscillation, which is the frequent and regularly occurring eastward progression of tropical rainfall over the Pacific.
El Niño is associated with floods, droughts and is linked to other weather disturbances in many locations around the world. El Niño's effects in the Atlantic Ocean lag behind those in the Pacific by 12 to 18 months. Developing countries dependent upon agricultural and fishing are especially affected. But El Niño's effects on weather vary with each event, and ENSO's intensity or frequency may change as a result of global warming. Research suggests that treating ocean warming which occurs in the eastern tropical Pacific separately from that of the central tropical Pacific may help explain some of these variations.