An El Nino may begin with a high pressure area in the Indian Ocean. Air pressure falls over the eastern Pacific. The steady Trade Winds of the Pacific die away or move east. Warm air lifts over the Peruvian deserts, and rain begins to fall there. More warm air spreads from the Indian Ocean to the east Pacific, keeping off the storms, the Indian monsoons falter or fail, and drought may begin in Indonesia.
A warm flow of sterile tropical water replaces the nourishing Humboldt Current along the western edge of South America, and fisheries suffer if it persists. Schools of fish shift their range. Throughout the continent, there is rain where it does not usually fall.
In North America, winters are warmer in the north central United States and Canada, while California, the desert southwest, and northwest Mexico have a cooler and wetter winter. There is increased erosion of California beaches, but probably fewer hurricanes in the Atlantic.
Australia and Southeast Asia are dry, often much drier than their norm. East Africa gets more water in the long rains of late winter and spring, but south central Africa is usually drier. This weather shift affects the entire world.
El Nino is Spanish for the Christ Child, and the phenomenon is so named because its effects are first felt in South America around Christmas time. Scientifically, it is called the El Nino Southern Oscillation, or ENSO. The oscillation referred to is a fluctuation in the air pressure difference between the eastern and west tropical Pacific. When the east is high, the west is low, and the reverse. These reversals generally happen in concert with El Nino. It occurs every two to seven years, and may last one to two years. An El Nino is generally followed by a period of La Nina, which, speaking very roughly, reverses the previous weather extremes.
Officially, ENSO is a consistent rise in the average surface temperature of the tropical Pacific of half a degree Celsius or more. If it lasts for five months or less it is a condition, if more than five months it is an El Nino episode. It disrupts the normal patterns of agriculture, fishing, and trade.
Some authorities hold El Nino at least partly responsible for the fall of the Moche, a Pre-Columbian high culture of Peru. Others say that this culture was well dispersed and adapted to its environment and that varied social stresses more likely brought it down. Similarly, the French Revolution, which took place after crop failures in 1888-89, is sometimes attributed to El Nino.
Scientists differ about the causes of ENSO. It may be a consequence of volcanic activity in the tropics which alters the amount of solar radiation retained in the air above the ocean, heating the area.
The prevailing theory has to do with a change in the winds. Normally, winds blow to the west in the Pacific, and pile up warm water in the western Pacific and Indian Ocean. Cold waters then rise to the surface along the eastern edge of the Pacific. If those winds slacken however, the warmer water flows back towards the east, and cold water can not well up so much. This actually affects the wind, weakening it, which makes the eastern Pacific warmer, which makes the winds less strong, and so on, in what is called a positive feedback loop.
Another way of looking at the cause of El Nino is that each episode is part of a complicated cycle, in which conditions progress from El Nino to La Nina to what we call normal according to timing which we do not yet completely understand.
The practical goal of weather research is prediction, to guide preparation. That is what has chiefly driven world efforts to understand El Nino. At present, the effects of ENSO can be predicted to a degree, but not with perfect accuracy. The more thoroughly affected nations can understand El Nino and La Nina, the better they can work in concert to prepare.