El Niņo is a large scale oceanographic / meteorological phenomenon that develops in the Pacific Ocean, which is associated with extreme climatic variability; i.e., devastating rains, winds, drought, etc. It is the migration, from time to time, of warm surface waters from the western equatorial Pacific Basin to the eastern equatorial Pacific region, along the coasts of Peru and Ecuador. This condition can prevail for more than a year, adversely affecting economies in both local and global scales.

El Niņo translates from Spanish as the "Boy Child" or the "Little One". It used to be considered a local event along the coasts Peru and Ecuador. The term was traditionally used by the Peruvian anchovy fishermen to describe the appearance of a warm ocean current flowing along the south American coast around Christmas time.

Under normal conditions, the prevailing southeasterly trade winds produce a surface current flowing toward the equator along the western South American coast. The waters leaving the coast are replace by colder waters from below (upwelling), which is rich in phytoplankton, the food source of anchovy.

The warm current (El Niņo) temporarily displaces nutrient-rich upwelling cold water resulting in the heavy harvest of anchovies. The abundant catch, however, is shortlived. What follows is a sharp decline in the fish population , resulting in a lesser catch. At times, warming is exceptionally strong and ruins the anchovy harvest.

In the Philippines, drought events are associated with the occurrence of El Niņo episodes. Second and third order impacts of El Niņo related drought events in the Philippines include:

(a) environmental (degradation of soil which could lead to desert-like conditions if persistent, effect on water quality like salt water intrusion, high forest/grass/bus fire risk, domestic water supply shortages, etc.);

(b) social (disruption of normal human activities, migration to urban communities, human and health problems, etc.); and

(c) economic (unemployment, food shortages, significant reduction in the productivity and subsequent revenue of various industries, hydro-electric power generation, etc.).

La Niņa develops over the central and eastern equatorial Pacific and is characterized by unusually cold surface temperatures of the ocean. La Niņa is associated with extreme climatic variability such as devastating rains, winds, drought, etc.

This condition can prevail for two to three seasons (six to nine months) thus affecting the economy on both the local and global scales. The term La Niņa (the Little Girl) was used by many scientists and meteorologists to differentiate it from El Niņo. It is sometimes called El Viejo (Old Man), Anti-El Niņo, or simply "cold event" or "cold episode".

La Niņa events are also linked to a change in atmospheric pressure known as the Southern Oscillation (SO). This is characterized by a seesaw (positive) in the atmospheric pressure between the western and central regions of the tropical Pacific Ocean, with one center of action located in the vicinity of Indonesia and the other center located over the central Pacific Ocean. The index that measures the magnitude of the SO is known as the Southern Oscillation Index (SOI) and it is obtained by calculating the difference in atmospheric surface pressure between Tahiti and Darwin, Australia. The Southern Oscillation results from pressure variations which cause changes in the wind circulation, also known as the Walker circulation. In normal conditions, the prevailing wind comes from the southeast to east. During La Niņa, stronger than normal easterly winds occur throughout much of tropical Pacific. These stronger winds push greater amounts of warm surface waters far into the western tropical Pacific. Below normal sea surface temperatures (SSTs) over eastern tropical Pacific and simultaneously above normal or positive anomaly SOI indicate a global-scale climate variation defined as the La Niņa phenomenon.