METEOROLOGISTS in recent years have been paying great attention to a phenomenon they call El Nino, a periodic warming of the tropical Pacific Ocean by a few degrees that reverberates around the world in the form of local droughts and floods.
The origins of El Nino can be traced to the centuries old observation that near the end of each calendar year a weak ocean current starts to flow slowly southwards along the east coast of South America.
Long ago, the inhabitants of these coastal regions gave this warm current a name: they call it Corriente del Nino, the Christ Child Current, because it came at Christmas time.
More recently it was noticed that every few years Corriente del Nino was particularly intense, and in the 1960s came the further discovery that each intense episode 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 large scale phenomenon.
El Nino occurs at regular intervals of between four and seven years, and over the past few decades many of its mysteries have been unravelled.
In normal times the Pacific Ocean in these equatorial regions is under the influence of steady easterly trade winds. This persistent westward flow of air literally drags the warm surface water with it 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. It is when the winds relax as they do periodically as a prelude to El Nino - that the warm surface waters in the western Pacific slosh back eastwards to raise the temperature near Peru and Ecuador.
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 heart beat of El Nino causes oscillations in the world's climate.
At its peak, the normally and coasts of Peru and Ecuador are drenched with rain, and Indonesia, where rain is normally abundant, along with parts of Africa, Australia and Brazil, experience droughts.
El Nino comes and goes at irregular intervals, with an intensity which is different every time and for underlying reasons that are still not fully understood. Moreover, the most recent El Nino event, which began in 1990, lasted for a full five years until the summer of 1995, the longest for at least 100 years.
And scientists are wondering if this anomaly may yet be another signal that something very unusual is happening to our global climate.