The El Nino circulation pattern interrupts the normal circulation of the Pacific Ocean at irregular intervals ranging from 2 to 7 years. During the normal years that occur between El Ninos, surface temperatures along the coasts of Peru and Ecuador and in the eastern equatorial Pacific are near 18 C in winter far cooler than typical tropical temperatures (25 C) and in fact the coolest tropical surface water on Earth.
The lower non-El Nino temperatures result from upwelling driven by strong winds in southern hemisphere winter (August). The winds from the south drive warm surface waters westward away from the coast of South America, and cooler water wells up from below. The winds then turn to the west near the equator and drive warm surface water toward the southwest, causing cool water to well up near the equator. Upwelling water brings nutrients to the surface, supplying food to an ecosystem ranging from plant plankton to fish, sea birds, and marine mammals. Non-El Nino years are also dry along the coast of South America because cool, upwelling waters are a poor source of water vapour for the atmosphere. As a result, the coastal deserts of Peru and Chile are normally among the driest regions on Earth.
El Nino years change all this. During El Nino winters, strong southerly winds fail to blow in the eastern and tropical Pacific, upwelling does not occur, and the surface waters along the South American coast warm by 2 to 5 C. Without upwelling, the plankton populations crash, and most fish die or move away. Without fish, sea birds on tropical islands cannot feed their young, and they abandon their nests to fly elsewhere in search of food. In severe El Nino years, a significant fraction of the year's population of young sea birds and mammals dies. Ocean warming near the coastal South American deserts produces a large source of moisture, and rain falls in cloudbursts that produce flash floods in regions with little or no natural vegetation cover to absorb the water. The warm rains also favour the breeding and spread among humans of tropical diseases such as malaria and cholera.
Even though the El Nino circulation pattern reaches its height during southern hemisphere winter in August, the first hint of unusual warming of the surface ocean is often detected during the previous summer, near Christmas. For this reason, Peruvian fishermen named this phenomenon "El Nino," or "the boy child."
El Nino events are part of a larger-scale circulation spanning the entire tropical Pacific. In the 1920s the atmospheric scientist Gilbert Walker found matching changes in atmospheric pressure between the western Pacific (northern Australia and Indonesia) and the south-central Pacific island of Tahiti. High pressures over Australia correlate with low pressures in the south-central Pacific and vice versa. Low atmospheric pressures are associated with rising air motion and rainfall, while high surface pressure is associated with sinking motion and dry conditions. The opposing pressure trends through time across the tropical Pacific are part of an enormous circulation cell called the Southern Oscillation. Sinking and rising motions occur at opposite times over northern Australia and Indonesia in the west and across the south-central Pacific in the east.
El Nino and the Southern Oscillation are linked. El Nino years, with warm ocean temperatures and heavy rains in Peru, are times of high pressure and drought over northern Australia and of low pressures and high rainfall in the south-central Pacific. Non-El Nino years, with cool ocean temperatures near South America, are times of low pressure and increased rainfall in northern Australia and of higher pressures and reduced rainfall in the south-central Pacific. This linked circulation is known as ENSO (El Nino-Southern Oscillation).
The physical link between these two systems occurs in the lower atmosphere and the upper ocean. Strong trade winds that cause upwelling in the eastern Pacific during non-El Nino years also drive warm surface water westward across the tropical Pacific. Warm water piles up in the western Pacific at a height several tens of centimeters above the level of the eastern Pacific and forms a natural source of moisture for evaporation and precipitation in northern Australia and Indonesia. Some of the rising air flows eastward at high elevations and sinks in the east-central Pacific, contributing to the normally cooler and drier conditions near South America.
During El Nino years, without strong trade winds pushing water westward, some of the pool of warm water in the western Pacific flows back eastward and becomes a source of latent heat and moisture for local rains. When the flow reaches the Americas, it deflects northward and southward along the coast, bringing warmer and wetter conditions north to California and south to Peru, along with reduced upwelling. In the western Pacific, slightly cooler conditions during El Nino years result in the drying in northern Australia and Indonesia. Eventually El Nino conditions subside and the tropical ocean reverts to its normal state. ON occasion it overshoots its normal state and produces abnormally cool sea-surface temperatures in the eastern Pacific. This overshoot is called La Nina, or "the girl child."