El Nino: A Predictable Climate Fluctuation

Article excerpt

El Nino is so versatile and ubiquitous - he causes torrential rains in Peru and Ecuador, droughts and fires in Indonesia, and abnormal weather globally - that the term is now part of everyone's vocabulary; it designates a mischievous gremlin. Hence, if the stock market in New York is erratic, or the traffic jams in London are exceptionally bad, it must be El Nino. This is consistent with our practice of using meteorological phenomena as metaphors in our daily speech: the president is under a cloud, the examination was a breeze. We know exactly what these statements mean because we have a life-long familiarity with clouds and breezes.

Despite all the publicity it receives, El Nino, Spanish for "Child Jesus," remains a puzzle. Why is the rascal named after Child Jesus? How can scientists claim that they are able to predict it months in advance when they are unable to predict the weather more than a few days ahead?

WHY "CHILD JESUS"?

Originally El Nino was the apposite name given to the warm, southward, seasonal current that appears along the barren coasts of Peru and Ecuador around Christmas when it provides a respite from the very cold, northward current that otherwise prevails. Every few years the southward current is exceptionally warm and intense, penetrates far south, and bears gifts. A visitor to Peru described one such occasion, in the year 1891, as follows: ". . . the sea is full of wonders, the land even more so. First of all the desert becomes a garden.... The soil is soaked by heavy downpour, and within a few weeks the whole country is covered by abundant pasture. The natural seems impossible." The "wonders" in the sea can include long yellow and black water snakes, alligators, bananas, and coconuts (Philander S. G. H. Is the Temperature Rising? The Uncertain Science of Global Warming. Princeton University Press, 1998).

With time, we reserved the use of the term El Nino, not for the annual, coastal current, but for the more spectacular, interannual occurrences that affect much of the globe. Not only our terminology, but also our perceptions changed; we now have a pejorative view of El Nino, not because its character has changed, but because we have changed. Heavy rains still transform the desert into a garden, but they also wash away homes, bridges, and roads, products of economic development and of a huge increase in population. As our economy and numbers continue to grow, the damage inflicted by natural phenomena such as El Nino, hurricanes, and severe storms is likely to continue increasing, even in the absence of global climate changes.

HOW IS EL NINO RELATED TO THE

SOUTHERN OSCILLATION?

Towards the end of the nineteenth century, scientists discovered that when atmospheric pressure is high over the western tropical Pacific, it is low over the eastern tropical Pacific and vice versa (Walker G.T. and E. W.Bliss. "World Weather V" Mem Royal Meterol. Soc. 4, 53-84. 1932). This see-saw in pressure, known as the Southern Oscillation, has a period of approximately four years, and is associated with fluctuations in a number of other variables. For example, during one phase of the Southern Oscillation the trade winds are intense, and rainfall is heavy over the western Pacific but light over the eastern Pacific. During the complementary phase, the trades are weak, while rainfall is light over the western and heavy over the eastern tropical Pacific.

The reason for this interannual climate fluctuation became apparent in 1957 when, for the first time during the occurrence of El Nino, sea-surface temperatures across the entire Pacific were available. Those measurements revealed that the warming along the shores of Peru associated with El Nino is not confined to a narrow coastal zone but extends thousands of kilometers offshore and influences an area so enormous that the global atmospheric circulation is affected. This led Bjerknes (Bjerknes, J. "Atmospheric Teleconnections from the Equatorial Pacific. …