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Demystifying the El Niño Southern Oscillation

As El Niño Southern Oscillation (ENSO) is part of our climate system’s natural variability and is a constant threat and disruption to our way of life, here are a couple of basic things that you should know about.


1.    ENSO is a natural climate variability happening every 2-7 years.

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This depicts El Niño (in red) and La Niña episodes (in blue) from January 1951 to May 2016 showing the fluctuations in Southern Oscillation Index (SOI), a standard measure of the difference between the atmospheric pressure at sea level at Tahiti and at Darwin. Sea surface temperatures, outgoing longwave radiation, Oceanic Nino Index and wind parameters are other indexes used to measure ENSO. Source: http://www.cpc.ncep.noaa.gov/data/indices/soi



ENSO is a natural phenomenon that occurs in the central and eastern equatorial Pacific Ocean every 2-7 years. ENSO involves different aspects of the ocean (El Niño) and atmosphere (southern oscillation) over the tropical Pacific, thus the acronym.


While ENSO only occurs in the tropics, its impact is felt in many parts of the world because the location of the huge mass of warmer water causes the trade winds to shift and therefore weather patterns to change. As a consequence, some regions can become wetter or drier, or warmer or colder. However, each ENSO event is unique and therefore has different types and levels of impact on different areas.

In the Philippines, it is one of the important sources of interannual variability in rainfall.


2.    ENSO has three phases: El Niño, La Niña and neutral.

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Weather authorities normally use this gauge to indicate the three distinct phases of ENSO and the stage of advisories.

Icon adopted from Australian Bureau of Meteorology


El Niño:  A warming of the ocean surface in the central and eastern tropical Pacific Ocean. The SSTs are above-average. The easterly winds, which normally blow from east to west direction along the equator, weaken. In some cases, it blows the other direction – from west to east (westerly winds).

Over most of the country, rainfall tends to be reduced resulting in prolonged drought conditions.


La Niña: A cooling of the ocean surface in the central and eastern tropical Pacific Ocean. Tropical Pacific SSTs are below average. In this case, the normal easterly winds along the equator become even stronger.

In the Philippines, rainfall tends to increase, often resulting in floods and landslides.


Neutral:  Neither El Niño or La Niña - basically in the middle of the continuum. Often, tropical Pacific SSTs are generally close to average.  

Sources: CPC-NOAA, DOST-PAGASA, Australian Bureau of Meteorology

Want to know more? We will provide more information about ENSO and our country’s weather and climate in our next advisories, so stay tuned!


Find out more about the rainy season and how to prepare:

Rainy Season Advisories: Here comes the rain again,Rhythm of the falling rain, Through the rain 

Preparedness Advisory: Achieving a level of preparedness for disasters and watch out for more!

Visit our website for more updates: http://www.omlopezcenter.org/