definisi El Niño

Agustus 3, 2009 at 2:41 am 10 komentar

Definisi El Niño berbeda-beda dan belum ada konsesus sampai sekarang. Hal ini disebabkan perbedaan cara pandang para ahli dalam memahami El Niño.

Terminologi El Niño awalnya digunakan untuk memberi nama arus air hangat tahunan yang mengalir ke arah selatan sepanjang pesisir Peru dan Ekuador pada bulan desember (sehingga disebut “bayi laki-laki” (El Niño) untuk merujuk pada kelahiran Yesus).

Ketika kemudian pengaruhnya terhadap ekologi, ekonomi dan lainnya mulai disadari dan hubungannya dengan atmosfer (iklim) dipelajari, penetapan definisi El Niño menjadi lebih sulit.  Beberapa ahli merujuk pada fenomena pada daerah pesisir sementara yang lain lebih pada ruang lingkup samudera yang lebih besar. Publik malah tidak membedakan dan sering salah kaprah dengan memberi definisi sendiri bahwa El Niño adalah badai???

Glantz(1996) mendefinisikan:

El Niño \ ‘el ne nyo noun [spanish] \ 1: The Christ Child 2: the name given by Peruvian sailors to a seasonal, warm southward-moving current along the Peruvian coast 3: name given to the occasional return of unusually warm water in the normally cold water [upwelling] region along the Peruvian coast, disrupting local fish and bird populations 4: name given to a Pacific basin-wide increase in both sea surface temperatures in the central and/or eastern equatorial Pacific Ocean and in sea level atmospheric pressure in the western Pacific (Southern Oscillation) 5: used interchangeably with ENSO (El Niño-Southern Oscillation) which describes the basin-wide changes in air-sea interaction in the equatorial Pacific region 6: ENSO warm event synonym warm event antonym La Niña \ [Spanish] \ the young girl; cold event; ENSO cold event; non-El Niño year; anti-El Niño or anti-ENSO (pejorative); El Viejo \ ‘el vya ho \ noun [Spanish] \ the old man

Sementara Scientific Committee for Ocean Research working group SCOR WG 55 (SCOR 1983) memberi definisi El Niño sebagai berikut:

“El Niño is the appearance of anomalously warm water along the coast of Ecuador and Peru as far south as Lima (12°S). This means a normalized sea surface temperature (SST) anomaly exceeding one standard deviation for at least four (4) consecutive months. This normalized SST anomaly should occur at least at three (3) of five (5) Peruvian coastal stations.”

Definisi ini ditolak banyak ilmuwan karena hanya memperhatikan kejadian di pesisir Peru.

Para praktisi memiliki cara pandang yang berbeda-beda juga, perhatikan definisi yang dipakai JMA berikut ini:

JMA defines that the El Niño (La Niña) is such that the 5-month running mean SST deviation for NINO.3 continues +0.5ºC (-0.5ºC) or higher (lower) for six consecutive months or longer.

Bandingkan dengan definisi dari NOAA berikut ini:

NORTH AMERICAN COUNTRIES REACH CONSENSUS ON EL NIÑO DEFINITION

NOAA announced that the NOAA National Weather Service, the Meteorological Service of Canada and the National Meteorological Service of Mexico have reached a consensus on an index and definitions for El Niño and La Niña events.

The index is defined as a three-month average of sea surface temperature departures from normal for a critical region of the equatorial Pacific (Niño 3.4 region; 120W-170W, 5N-5S). This region of the tropical Pacific contains what scientists call the “equatorial cold tongue,” a band of cool water that extends along the equator from the coast of South America to the central Pacific Ocean. Departures from average sea surface temperatures in this region are critically important in determining major shifts in the pattern of tropical rainfall, which influence the jet streams and patterns of temperature and precipitation around the world.

North America’s operational definitions for El Niño and La Niña, based on the index, are:
El Niño: A phenomenon in the equatorial Pacific Ocean characterized by a positive sea surface temperature 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 (0.9 degrees Fahrenheit), averaged over three consecutive months.

La Niña: A phenomenon in the equatorial Pacific Ocean characterized by a negative sea surface temperature 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 (0.9 degrees Fahrenheit), averaged over three consecutive months.

(NOAA News Online, Feb. 23, 2005)

Definisi berbeda akan anda temukan pada situs dan buku referensi yang lain.

Perenungan

Apakah benar-benar perlu menyeragamkan definisi El Niño? Untuk para pelajar dan kalangan awam tentu sangat perlu untuk menghindari kebingungan karena pernyataan yang berbeda-beda, juga memudahkan para ilmuwan dan praktisi mengkomunikasikan El Niño.

Tetapi karena sampai sekarang fenomena ini belum sepenuhnya dipahami, maka perlu fleksibilitas dalam menentukan kejadian El Niño. Sehingga ada pembanding patokan yang dipakai untuk menentukan apakah terjadi El Niño atau bukan.

Jadi misalkan NOAA mendefinisikan indeks ENSO sebagai: “three-month averages of sea surface temperature departures from normal for Niño 3.4“, terfokus pada suhu muka laut, maka sebagai pembanding, Curtis (profesor pada  East Carolina University yang telah meneliti ENSO selama bertahun-tahun) dan Bob Adler (peneliti iklim dari NASA-Goddard Space Flight Center) membangun index berdasar presipitasi yang didapatkan dari citra satelit.  Klaus Wolter (peneliti pada  Cooperative Institute for Research in Environmental Studies pada University of Colorado) membuat indeks lain yang menggabungkan suhu muka laut (sea-surface temperature), tekanan muka laut (sea-level pressure), angin dan data lainnya. JMA menggunakan suhu muka laut pada perairan diatas Papua (NINO.WEST) sebagai tambahan pertimbangan.

Beberapa daerah bisa menjadi lebih kering/basah sebelum terjadi pemanasan/pendinginan pada suhu muka laut samudera pasifik. Karena itulah indeks alternatif tersebut patut dipertimbangkan karena juga menampilkan kondisi iklim samudera pasifik.

Daftar referensi

  1. Glantz, M. H., 1996. Currents of change: El Niño’s impact on climate and society. Cambridge University Press. 194 pp.
  2. Scientific Committee on Oceanic research (SCOR) 1983: Prediction of El Niño. Proceedings No. 19 Paris. Annex VI, SCOR WG 55 47-51.
  3. http://ddb.kishou.go.jp/climate/ElNino/elmonout.html
  4. http://www.weather.gov/ost/climate/STIP/ElNinoDef.htm
  5. http://www.geotimes.org/oct03/WebExtra100303.html
  6. http://www.cgd.ucar.edu/cas/papers/clivar97/en.dfn.html
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Tinjauan Kondisi Dinamika Atmosfir Kejadian El Nino (berdasarkan standar NOAA) 1950-2008

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