The weather phenomenon known as El Niño is caused by weakened trade winds over the Pacific Ocean. Normally, warm water is pushed to the western Pacific Ocean by the trade winds, causing cold water to rise in the eastern Pacific Ocean. Weakened winds inhibit this, causing an El Niño event.
The strong trade winds that would normally cause the deep ocean water to rise off of the coast of South America are weakened during an El Niño event. The exact cause of the weakened trade winds is unknown; however, scientists have noticed an oscillation of water surface temperatures in the Pacific ocean, referred to as the El Niño Southern Oscillation.
The area of the Pacific Ocean around Indonesia is often referred to as the Pacific Warm Pool by scientists. Normally, the warmer waters of the Pacific Warm Pool are pushed westward by the trade winds. El Niño is characterized primarily by the weakening of this phenomenon.
The effects of El Niño can be felt across the world, but primarily affect South America. El Niño often results in increased rainfall, thunderstorms, mass die-offs of fish and, in extreme El Niño events, massive flooding. El Niño also causes temperature changes in regional climate patterns across the globe. El Niño occurs during the months of December, January and February.