El Niño

[Note to reader: topic sentences are in green; remaining weakness in red.]

ABSTRACT

El Niño is part of a periodic oceanic weather phenomenon known as the Southern Oscillation. An understanding of El Niño is important for two reasons: 1) it is a useful model for predicting both long and short term climate changes, 2) it impacts global economies and ecologies.

In 1982, North America experienced exceptionally hot and dry summers. The Indian monsoons, crucial to agriculture, did not arrive. In that same year, unusual weather conditions caused severe droughts in eastern Australia, flooding in Columbia, and the virtual collapse of the Peruvian fisher.[1] These events were the result of a climatic phenomenon known as El Niño the Southern Oscillation, a periodic occurrence in the tropical Pacific ocean of global significance.

Although much is known about the history of El Niño, scientists have only begun to realize the importance of this climatic event as a unique model of the coupled ocean-atmosphere system. It is one of the few examples of dramatic short term climate variation, and it is a paradigm of nonlinear systems. Theoretically, the study of El Niño has led scientists to a greater understanding of broad climate changes.

The features of El Niño are best characterized by observing conditions on the boundary of the tropical Pacific ocean. The western ocean is usually characterized by warm (29° C) sea surface temperature, low air pressure, and high precipitation; the eastern Pacific is marked by cool (21° - 26° C) sea surface temperature, high pressure, and low precipitation. In the early 1900's, British meteorologists, motivated by a desire to predict monsoon behavior, began to study the periodic oscillation in these conditions.[2] They described a barometric seesaw in which pressure would increase in the western Pacific and decrease in the eastern Pacific; however, they were unable to make the connection between these events and the occurrence of El Niño.

[Global view of El Nino]
In the 1960's the meteorologists Jacob Bjerknes proposed a model that integrated atmospheric effects and upper ocean dynamics into a single system.[3] He suggested that the usually strong trade winds blowing from the high pressure east to the low pressure west maintain the sea surface temperature gradient in the tropical Pacific ocean by piling up warm equatorial waters on the western boundary. As cool dry air blows westward across the ocean and the sea surface temperature gradient, it collects moisture, rises and then recirculates eastward. According to Bjerknes, the conduction of strong trade winds and a temperature gradient produce a convection pattern, he called the Walker circulation.

[There is no topical sentence in this paragraph. This in unfortunate as it is the penultimate paragraph. However, sometimes warm water on the western boundary begins to drift eastward. This tiny perturbation decreases the temperature gradient which in turn decreases Walker circulation. As trade winds decrease and Walker circulation diminishes, the temperature gradient begins to vanish. A positive feedback loop is created. The eastern and central pacific becomes warm and rainy; the western pacific becomes cooler and drier; warm equatorial waters flow down the west coast of South America; this dramatic global climate change, El Niño, occurs in less than a year.

[Needs to be better organized to be an effective conclusion.] Although programs such as the Tropical Oceans and Global Atmosphere Program have succeeded in predicting the behavior of an evolving El Niño, many fundamental questions remain unanswered.[4] What starts El Niño? Why does it terminate? The ability to predict climate is a compelling goal. The success in explaining many features of El Niño should provide a useful model for further examination of global climate, and the insight gained may eventually allow forecasters to predict natural disasters such as the 1982 El Niño event itself.

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References


[1] Sarachik, E.S. (ed).  Learning to Predict Climate Variations
    Associated with El Ni&ntiled;o and the Southern Oscillation.  
    National Academy Press, 1996. pp. 5-12.

[2] Tomczak, M., Godfrey, J.S. Regional Oceanography:  An Introduction.
    Pergamon, 1994. pp. 361-7.

[3] Philander, S.G.  El Niño, La Niña, and the Southern
    Oscillation.  Academic Press, Inc., 1990. pp. 9-11.

[4] Sarachik, E.S., p. 123.

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Edited by: wilkins@mps.ohio-state.edu [September 1997]