What is an El Nino event?
El Niño is a warming of surface ocean waters in the eastern tropical Pacific. La Niña episodes represent the opposite - periods of below-average sea surface temperatures across the east-central Equatorial Pacific. Global climate La Niña impacts tend to be opposite those of El Niño impacts. In the tropics, ocean temperature variations in La Niña also tend to be opposite those of El Niño. These events can have profound effects on weather patterns around the world.
During normal weather conditions, trade winds blow towards the west causing warm water to culminate in the western Pacific Ocean and cold water to culminate in the eastern Pacific. However, during El Niño, these trade winds weaken in the central and western Pacific Ocean causing warmer water to culminate in the eastern Pacific Ocean. The rainfall follows this warmer water eastwards, decreasing the amount of rainfall in the western Pacific. During La Niña years, the opposite occurs.
El Niño events tend to happen every three to seven years. They can last from six months to two years. The last most severe El Niño happened in 1997/98.
El Niño, and its opposite La Niña, are both naturally occurring phenomena that have happened in cycles over history. These events are not caused by climate change but climate change could make their impacts more severe.
Sources - NOAA