El Niño is the term used to describe an extensive warming of the upper ocean in the tropical eastern Pacific lasting three or more months. Peruvian fishermen first used the name El Niño to describe unusually warm waters off their coast. It is now known that other climate variations around the world are associated with El Niño.
El Niño events are linked with a change in atmospheric pressure between the western and central regions of the Pacific Ocean, a shifting of tropical rainfall from the western to the eastern Pacific Ocean, a weakening of Pacific trade winds, and sea level changes. The ocean and atmospheric changes are closely linked, collectively known as El Niño/Southern Oscillation, or ENSO.
During the warm phase of ENSO, the South Pacific trade-wind system undergoes a change of state, or seesaw," in which the westward-blowing trades weaken along the equator as the normally high pressure in the eastern South Pacific decreases and the low pressure over northern Australia and Indonesia rises. The pressure change and diminished trade winds cause warm surface water to move eastward along the equator from the western Pacific, while the warm surface layer in the east becomes thicker.
The warm ocean conditions in the equatorial Pacific induce large-scale anomalies in the atmosphere. Rainfall increases manifold in Ecuador and northern Peru, causing coastal flooding and erosion and consequent hardships in transportation and agriculture. Additionally, strong El Niño events are associated with droughts in Indonesia, Australia, and northeastern South America and with altered patterns of tropical storms in the tropical belt. During the stronger El Niño episodes, the atmospheric "teleconnections" are extensive enough to cause unusually severe winter weather at the higher latitudes of North and South America. Rainfall extremes linked to El Niño can adversely affect human societies by triggering food shortages, floods, and landslides.
The effect of El Niño is so great that the global burden of natural disasters increases in the year after the onset of an El Niño event. The ENSO appears to be a truly disruptive force that wreaks havoc on life. Clearly a better understanding of the El Niño and its associated atmospheric effects is needed, and WMO has concentrated a large part of its resources in the effort to manage to predict El Nino events, in the hope of reducing damage and loss of life.