It had been known for centuries that near the end of each calendar year a weak, warm ocean current flowed southwards down the east coast of South America, providing a welcome respite from the brisk cold northward stream that normally prevails. Long ago the local inhabitants gave this warm current a name; they called it Corriente del Nino, the Christ Child Current, because it came along at Christmas.
It was also noticed that every few years the southward current was exceptionally warm, and the effects on the arid landscape were dramatic. In 1891 a visitor to Peru described them thus: "The soil is soaked by heavy downpours, and within a few weeks the whole country is covered in abundant pasture, the natural increase of the flocks is practically doubled, and cotton can be grown in places where in other years any vegetation seems impossible."
But there was a downside too. The heavy rains that transformed the deserts of Peru and Ecuador also flooded local rivers, with houses and bridges often washed away. The anomalously high sea temperatures played havoc with indigenous fish life, so that the normally abundant anchovy were scarce, causing serious difficulties for the region's fishing industry.
In the 1960s it was noticed each intense episode of the Corriente del Nino was merely a local manifestation of a general warming of the surface waters of large areas of the Pacific. The term El Nino was borrowed and applied to this much larger-scale phenomenon.
In normal times, steady easterly trade winds in the equatorial Pacific literally drag the warm surface water with them to the west, causing a rise in sea level in the western Pacific. At the same time, cold water from below wells up in the eastern part of the ocean near the coast of South America. But when the trade winds relax, as they do periodically as a prelude to El Nino, the warm surface waters in the western parts of the ocean slosh back eastwards to raise the temperature of the sea in the central Pacific and in the vicinity of Peru and Ecuador.
This periodic collapse of the north-easterly trades is a consequence of a complex interaction between the ocean and the atmosphere. In any event, the warmer water over the vast expanse of the tropical Pacific makes a great amount of extra energy available to the air in contact with it, so the periodic heartbeat of El Nino causes oscillations in the world's climate. One such palpitation, as we noted yesterday in Weather Eye, is developing at present and is expected to reach its peak in the early months of 2002.