El Niņo-Southern Oscillation (ENSO)


El Niņo
(The Warm Phase)

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.

Characteristics of El Niņo


- It occurs in the Pacific basin every 2 to 9 years;

- It usually starts during the Northern winter (December to February);

- Once established, it lasts until the first half of the following year,
although at times, it stays longer;

- It exhibits phase-locking in annual cycles (El Niņo and rainfall

- fluctuations associated with it tend to recur at the same time of the
year; and

- It usually has a biennial cycle (El Niņo events will often be preceded
and/or followed by La Niņa).

Climatic Indicators of El Niņo in the Philippines

Abnormalities such as:

- delayed onset of the rainy season
- early termination of the rainy season
- weak monsoon activity
    *isolated heavy downpours with short duration
- weak tropical cyclone activity
    *far tropical cyclone track
    *less no. of tropical cyclones entering the PAR
    *less intense tropical cyclones

Effects of El Niņo in the Philippines

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
(The Cold Phase)

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".

Southern Oscillation/Walker Circulation

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.

Is La Niņa a New Phenomenon?

La Niņa is not a new phenomenon. Evidence suggest that La Niņa events have existed for thousands of years in the past. However, it is only in the last decade that a satisfactory understanding of how they form and are maintained has been gained. Some of the La Niņa events on record are 1955-56, 1964-65, 1970-71, 1973-74, 1975-76, 1988-1989 and 1995-1996.

How Are La Niņa Events Detected?

La Niņa events in the tropical Pacific Ocean can now be detected by many methods, including satellites, moored buoys, drifting buoys, sea level analysis and expendable bathythermographs. This research observing system is now evolving into an operational climate observing system. Large computer models of the global ocean and atmosphere use data from this observing system as input to predict/monitor La Niņa, as well as El Niņo. Other global models are used for research to further understand the phenomenon.

Are All La Niņa Events The Same?

La Niņa events share many general characteristics although every one is somewhat different in magnitude, duration and resulting global climatic impacts. Magnitude can be determined in different ways, such as variations in the Southern Oscillation Index (SOI). Another measure of the magnitude is the sea surface temperature anomaly (difference between the observed and average values) which could either be positive (hotter than normal) or negative (cooler than normal) over specific region of the Pacific ocean, particularly in central and eastern regions.

How Does La Niņa Affect Our Climate?

Impacts of La Niņa on Philippine climate include anomalies in rainfall, temperature and tropical cyclone activities. During La Niņa conditions, major parts of the country experience near normal to above normal rainfall conditions particularly over the eastern sections of the country. La Niņa conditions also favor tropical cyclone formation over the western Pacific which tend to increase the number of tropical cyclones.



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