Economic Issue
of the Day
Philippine Institute
for Development Studies
S u r i a n s a m g a Pa g -a a ral
Pangkaunlaran ng Pilipinas
Vo l . V I I N o . 1 ( J a n u a r y 2 0 0 7 )
Knowing when El Niño/La Niña is here
I
n a previous Economic Issue of the Day (Vol. V, No. 1, July
2005), a basic understanding was presented on what the El
Niño southern oscillation (ENSO) phenomenon is all about,
its characteristics and two phases, and its implications.
ENSO is a phenomenon that takes place in the central and
eastern equatorial Pacific largely characterized by an interaction
between the ocean and the atmosphere and their combined effect
on climate. The mutual interaction between the ocean and the
atmosphere is a critical aspect of the ENSO phenomenon.
Major ENSO indicators are the sea surface temperature
anomaly (SSTA) and the southern oscillation index (SOI).
SSTA refers to the departure or difference from the normal
value in the sea or ocean surface temperature. El Niño events
are characterized by positive values (greater than zero) within a
defined warm temperature threshold while La Niña events are
characterized by negative values (less than zero) within a defined
cold temperature threshold.
The SOI, on the other hand, measures the differences or
fluctuations in air or atmospheric pressure that occur between
the western and eastern tropical Pacific during El Niño and La
Niña episodes. It is calculated on the basis of the differences in
air pressure anomaly between Darwin in Australia (western
Pacific) and Tahiti in French Polynesia (eastern Pacific). These
two locations/stations are used in view of their having long data
records.
Albeit the seeming straightforward description of these
ENSO-related events as noted in the above, it is to be emphasized
that through the years, it has not been easy to come up with a
commonly agreed definition and identification of these ENSOrelated events, i.e., El Niño or La Niña. The reason is due to the
use of more than one standard index as basis in monitoring ENSO
phenomena and the employ of different methods in determining
the magnitude or value of such index and threshold as well as
the length of time that such magnitude persists. In line with this,
the Philippines adopted the World Meteorological Organization
(WMO) Regional Association IV Consensus Index and Definitions
of El Niño and La Niña. Region IV includes the North and Central
America member nations of the WMO, whose operational
definitions in use of the two ENSO phases are the following:
El Niño
Niño: A phenomenon in the equatorial Pacific Ocean
characterized by a positive SST departure from normal (for the
1971-2000 base period) in the Niño 3.4 region, greater than or
equal in magnitude to 0.5 degrees C, and averaged over three
consecutive months. Defined when the threshold or value is met
for a minimum of five consecutive overlapping seasons.
La Niña
Niña: A phenomenon in the equatorial Pacific Ocean
characterized by a negative SST departure from normal (for the
1971-2000 base period) in the Niño 3.4 region greater than or
equal in magnitude to 0.5 degrees C, and averaged over three
consecutive months. Defined when the threshold or value is met
for a minimum of five consecutive overlapping seasons.
When is El Niño/La Niña occurring?
Because ENSO-related phenomena have been a major source of
interannual climate variability around the globe, especially in
recent years, it is important to be able to determine or identify
when an El Niño/La Niña is occurring or will take place.
As noted earlier, monitoring the occurrence of an El Niño/
La Niña involves the use of two most common indicators, the
SSTA and the SOI, with the SSTA based on the magnitude of
departures/anomalies in the sea surface temperature in the Niño
regions (see box), and the SOI based on the difference in air
pressure between Tahiti and Darwin.
PAGASA: monitoring El Niño/La Niña events
in the Philippines
In the Philippines, how is El Niño/La Niña identified/monitored?
The country’s national meteorological agency, the Philippine
Atmospheric, Geophysical and Astronomical Services
Administration (PAGASA), defines and identifies these phenomena
on the basis of the abovementioned indicators which are also
being used by the National Oceanic and Atmospheric
Administration-National Centers for Environmental Prediction
(NOAA-NCEP) of the United States.
Economic Issue of the Day
EL NIÑO/LA NIÑA
Vo l . V I I N o . 1 ( J a n u a r y 2 0 0 7 )
Through the years and based on Box: NINO regions
this definition and data from the
El Niño regions: Although El Niño is a generalized event in the equatorial Pacific,
NOAA, PAGASA has monitored the
there are different regions which show different characteristics and different moments
occurrence of El Niño/La Niña by
in the process. Past studies show that the Philippine climate responds more significantly
category, as follows:
to temperature changes in the NIÑO 3.4 region.
a) weak El Niño/La Niña –
magnitude of +0.5 to +1.0
°C (or -0.5 to -1.0 °C)
b) moderate El Niño/La Niña –
magnitude of +1.0 to +1.5
°C (or -1.0 to -1.5 °C)
c) strong El Niño/La Niña –
magnitude of more than
+1.5 °C (or less than -1.5
°C)
Table 1 shows the years when
these events and their categories have
taken place in the last decade. It is to
be noted that no two ENSO events
Source: International Research Institute for Climate and Society
are alike in terms of climate impacts.
Accordingly, PAGASA gives out the
Table 1. El Niño and La Niña episodes during the past decade
appropriate advisories to the various sectors and
Period
Event
Category
decisionmakers concerned on the occurrence/presence of
El Niño/La Niña for their corresponding action. ❋
References
May 1994 – April 1995
October 1995 – April 1996
June 1997 – May 1998
August 1998 – July 2000
November 2000 – March 2001
June 2002 – April 2003
August 2004 – March 2005
El Niño
La Niña
El Niño
La Niña
La Niña
El Niño
El Niño
weak to moderate
weak
strong
moderate to strong
moderate
weak to moderate
weak
Columbia University. 2006. When can we say El Niño will occur
[online]. Available from the World Wide Web:(http://
www.columbia.edu/~za2121/Peru-ENSO/Peru-ENSO/Webpages/El%20Nino/When%20will%20it%20occur.html).
International Research Institute for Climate and Society. 2006.
____________
Defining ENSO [online]. Available from the World Wide
Source of data: Climate Prediction Center – National Oceanic and Atmospheric
Web:(http://iri.columbia.edu/climate/ENSO/background/
Administration (CPC-NOAA), 2006
pastevent.html).
National Oceanic and Atmospheric Administration (NOAA). 2006.
ENSO cycle: recent evolution, current status, and predictions
Philippine Institute for Development Studies/Australian Centre for
[online]. Climate Prediction Center, National Centers for
International Agricultural Research. 2006. SCF Project Updates
Environmental Prediction. Available from the World Wide
Vol. II Nos. 1&2, 2006. Makati City: PIDS.
W e b : ( h t t p : / / w w w. c p c . n c e p . n o a a . g o v / p r o d u c t s /
Trenberth, K.E. 1997. The definition of El Niño. Bulletin of the
analysis_monitoring/lanina/.
American Meteorological Society 78:2771-2777.
The Economic Issue of the Day is one of a series of PIDS efforts to help in enlightening the public and other interested parties on the concepts
behind certain economic issues. This dissemination outlet aims to define and explain, in simple and easy-to-understand terms, basic concepts as they
relate to current and everyday economics-related matters.
This Issue was written by members of the Philippine team of the ACIAR-sponsored project titled Bridging the gap between seasonal climate
forecasts and decisionmakers in agriculture.
The views expressed are those of the authors and do not necessarily reflect those of PIDS and other member agencies and sponsors. ❋
Philippine Institute for Development Studies
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