The EL NIÑO is a weather phenomenon that affects the entire equatorial Pacific region from as far as Peru in the east to Indonesia in the west and recurs every three to seven years. It causes massive weather disruptions that last from six to eighteen months. It was called El Niño by South American fishermen because it usually occurs at Christmas time, hence, the Boy Child.

Under normal conditions, the trade winds blow strongly from east to west across the tropical Pacific. This east-west flow of the winds forces the warm surface water to move to the west along the equator and produces a big pool of warm water in the western equatorial Pacific. The trade winds along the equator push the water away from the equator, northward in the northern hemisphere and southward in the southern hemisphere. The water pushed away from the equator is replaced by an upwelling; water pulled up from deeper in the ocean. In the west, upwelling brings up warm nutrient-poor water, while in the east, upwelling brings up cold nutrient-rich water which feeds small underwater plants and animals (plankton) which in turn feed larger crustaceans and fish.

When the trade winds in the western Pacific slow down or even reverse their direction, the El Niño episode begins. The weak winds can no longer hold the warm pool in the west, so the warm nutrient-poor water from the west moves back along the equator towards the coast of Peru where the plankton slowly die out. Fish and other creatures that feed on plankton then leave the area or starve to death.

A few months after the water warms up in the eastern Pacific, weather patterns around the world begin to change. Warm water heats the air, which rises and produces heavy rains. During El Niño, the rain follows the warm water eastward into the central Pacific and changes wind patterns which disrupts weather all over the world. The warm water in the ocean causes colder parts of the ocean to warm up and warmer parts of the ocean to cool off.

In one to two years, the trade winds return to normal, warm water is once again pushed to the western Pacific and cold water upwelling returns in the east. This back-and-forth reversal of wind patterns in the equatorial Pacific is called the Southern Oscillation, which, when related to El Niño, is called the El Niño Southern Oscillation or ENSO for short.

Positive MEI values represent the warm ENSO phase (EL NINO) while negative MEI values represent the cold ENSO phase (LA NINA).

El Niños can last from six to eighteen months. In recent years, very strong El Niño conditions occurred in 1983 and 1997. A strong one in 1987 and a long episode occurred from 1990 to 1995 with peaks in 1992 and 1994.

The closely related phenomenon, called La Niña, is an over-compensation as Pacific weather conditions swing the other way in an attempt to return to normal. This "overswing" causes weather conditions opposite that of the El Niño, i.e., heavy rains in the western Pacific and very dry weather in the eastern Pacific.

In July 2002, scientists and meteorologists monitoring weather and climate conditions (e.g. sea level and temperature, wind speed and direction, air temperature and humidity, etc) have noted the start of an El Niño episode which would bring dry conditions (no rain) in the western Pacific (e.g. Philippines) and wet conditions (more rain) in at the eastern Pacific side (e.g. California). These weather patterns will have significant impact on agriculture (poor harvest), fisheries (lesser catch), infrastructure (flood or drought), utilities (low water level) and other sectors of the local economy.