The name El Niņo (referring to the Christ child) was originally given by a Peruvian fisherman to a warm current that appeared each year around Christmas. What we now call El Niņo seemed to them like a stronger event of the same type, and the usage of the term changed to refer only to the irregular strong weather related events. It wasn't until the 1960s that it was widely realized that this was not just a local Peruvian occurrence, but was associated with changes over the entire tropical Pacific and beyond.
The name El Niņo now refers to the warm phase of a large variation in which the surface temperature of the central/eastern part of the tropical Pacific varies by up to about 4°C, with related changes in the winds and rain-fall patterns. The complete phenomenon is known as the El Niņo/Southern Oscillation(ENSO). The warm El Niņo phase typically lasts for 8-10 months or so. The entire El Nino cycle lasts usually about 3-7 years, and often includes a cold phase (known as La Niņa) that may be similarly strong, as well as some years that are neither abnormally hot nor cold. However, the cycle is not a regular change like the change of seasons, but can be highly abnornal in strength and timing. Even at this present time we do not fully understand what causes these changes in the El Nino cycle.
EL Nino Conditions:
The Normal Conditions: