Effect of global Warming
El Niño-Southern Oscillation is a periodic change in the atmosphere and ocean of the tropical Pacific region. It is defined in the atmosphere by changes in the pressure difference between Tahiti and Darwin, Australia, and in the ocean by warming or cooling of surface waters of the tropical central and eastern Pacific Ocean. El Niño is the name given to its warm phase of the oscillation -- the period when water in that region is warmer than average. La Niña is the name given to the cold phase of the oscillation, or the period when the water in the tropical Eastern Pacific is colder than average. The oscillation has no well-defined period, but instead occurs every three to eight years. Mechanisms that sustain the El Niño-La Niña cycle remain a matter of research.
The El Niño-Southern Oscillation is often is abbreviated in scientific jargon as ENSO and in popular usage is commonly called simply El Niño. The name is Spanish for "the boy" and refers to the Christ child, because periodic warming in the Pacific near South America is usually noticed around Christmas. Conversely the name for the cool phase of the oscillation, "La Niña," is Spanish for "the girl."
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. El Niño's effects on weather vary with each event. Recent research suggests that treating ocean warming in the eastern tropical Pacific separately from that in the central tropical Pacific may help explain some of these variations.
Climate scientists define El Niño and La Niña based on sustained differences in Pacific-Ocean surface temperatures when compared with the average value. The accepted difference is anything greater than 0.5C (or 0.9F) averaged over the east-central tropical Pacific Ocean. When this happens for less than five months, it is classified as El Niño or La Niña conditions; if the anomaly persists for five months or longer, it is called an El Niño or La Niña "episode." Typically, this happens at irregular intervals of 2–7 years and lasts nine months to two years.