El Niño/La Niña
In 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: 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: 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.
In 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: 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: 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.
Typhoons in the Philippines
Describes the most notable tropical cyclones to enter the Philippine Area of Responsibility and affect the Philippines. Bagyo is a term referring to any tropical cyclone in the Philippine Islands. An average of 6 to 7 tropical cyclones hit the Philippines per year. A bagyo is categorized into four types according to its wind speed by the PAGASA. All tropical cyclones, regardless of strength, are named by PAGASA. Tropical depressions have maximum sustained winds of between 55 kilometres per hour (30 kn) and 64 kilometres per hour (35 kn) near its center. Tropical storms have maximum sustained winds of 65 kilometres per hour (35 kn) and 119 kilometres per hour (64 kn). Typhoons achieve maximum sustained winds of 120 kilometres per hour (65 kn) to 185 kilometres per hour (100 kn), with super typhoons having maximum winds exceeding 185 kilometres per hour (100 kn). The most destructive tropical cyclone to impact the Philippines was Tropical Storm Thelma in 1991, which killed thousands of people from its resultant flooding. The wettest known tropical cyclone to impact the archipelago was the July 1911 cyclone which dropped over 1,168 millimetres (46.0 in) of rainfall within a 24 hour period at Baguio City. At least 30 percent of the annual rainfall in the northern Philippines could be traced to tropical cyclones, while the southern islands receive less than 10 percent of their annual rainfall from tropical cyclones
Describes the most notable tropical cyclones to enter the Philippine Area of Responsibility and affect the Philippines. Bagyo is a term referring to any tropical cyclone in the Philippine Islands. An average of 6 to 7 tropical cyclones hit the Philippines per year. A bagyo is categorized into four types according to its wind speed by the PAGASA. All tropical cyclones, regardless of strength, are named by PAGASA. Tropical depressions have maximum sustained winds of between 55 kilometres per hour (30 kn) and 64 kilometres per hour (35 kn) near its center. Tropical storms have maximum sustained winds of 65 kilometres per hour (35 kn) and 119 kilometres per hour (64 kn). Typhoons achieve maximum sustained winds of 120 kilometres per hour (65 kn) to 185 kilometres per hour (100 kn), with super typhoons having maximum winds exceeding 185 kilometres per hour (100 kn). The most destructive tropical cyclone to impact the Philippines was Tropical Storm Thelma in 1991, which killed thousands of people from its resultant flooding. The wettest known tropical cyclone to impact the archipelago was the July 1911 cyclone which dropped over 1,168 millimetres (46.0 in) of rainfall within a 24 hour period at Baguio City. At least 30 percent of the annual rainfall in the northern Philippines could be traced to tropical cyclones, while the southern islands receive less than 10 percent of their annual rainfall from tropical cyclones
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