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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

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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 NEDA sa Makati Building, 106 Amorsolo Street, Legaspi Village, Makati City z Telephone Nos: (63-2) 8924059 and (63-2) 8935705 z Fax Nos: (632) 8939589 and (63-2) 8161091 URL: http://www.pids.gov.ph

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PAGASA: monitoring El Niño/La Niña events in the Philippines When is El Niño/La Niña occurring? Philippine Institute for Development Studies...