"What news on the Rialto?" inquires Shylock of Bassanio in Shakespeare's play. He was not, of course, asking about happenings on the southbound Route 19 of Dublin Bus, but referring, rather, to the bridge across the Grand Canal in Venice where the merchants of that city used to carry out their business. But when meteorologists ask for news from one another nowadays, as often as not they have El Nino on their minds.
The most recent World Meteorological Organisation update on El Nino, published in December 1997, confirms that the present event, which reached its peak around November, has been the strongest we have had since 1982. Sea surface temperatures in parts of the eastern tropical Pacific climbed to more than five degrees above their average values. The latest predictions forecast that this El Nino will continue until April 1998, after which conditions are expected to return to normal.
This general warming of the surface waters of large areas of the Pacific Ocean, which occurs at irregular intervals of between two and seven years, makes a great amount of extra energy available to the air in contact with it. The periodic heartbeat of El Nino, therefore, causes oscillations in the world's climate; at its peak normally arid areas of western South America are drenched with rain, other areas in the low latitudes in which rain is normally abundant experience droughts, and there are suspicions that it may sometimes affect the weather in the middle latitudes as well.
Despite the wide publicity in recent years, El Ninos are not by any means a new phenomenon; unlike the enhanced greenhouse effect, they have been around for centuries. Much work has been done, indeed, in deducing rainfall patterns and periods of drought over the years in places like Peru and Ecuador from writings left by the Spanish conquistadores and from other sources.
Historical El Ninos can thus be identified with reasonable confidence, and it seems that particularly strong events occurred in 1541, 1578, 1614, 1624, 1652, 1701 and so on up to 1814 and 1828. In the last 100 years or so, the strongest El Ninos were in 1891, 1925 and 1982. The difference now is that it is only in the last decade or so that we have gained a satisfactory understanding of how an El Nino forms and is maintained. And it is only very recently that computer models have been developed capable of simulating the complex oceanic and atmospheric interactions in which El Nino has its origins, and of processing the vast amounts of data required to predict the phenomenon.