Weather: What we don't know about El Nino

Almost every piece of bad weather anywhere in the world this year is going to be blamed on El Nino. Yet despite the intense study that has been given to the phenomenon, nobody really has much idea what the effect will be, if any, in the northern hemisphere.

There are many things that I do not understand about the weather, but the current El Nino phenomenon illustrates the single most perplexing of them. Why is it that nobody seems to have any idea what effect El Nino is going to have on Europe? No one is even sure whether the recent Spanish storms, or the Polish floods earlier this year, have any connection with the huge climatic changes in the Pacific.

The problem seems to be that our understanding of the mechanisms of weather change is greatest when it comes to very fast and very slow processes. By plotting the movement of weather fronts and clouds, we can predict changes in the weather with great accuracy up to a day or two ahead. Or by studying the long-term changes in the earth's orbit round the sun, we can predict, to within 10,000 years or so, when there is likely to be another ice age. The motions of ocean currents, however, seem to fall awkwardly between these extremes.

El Nino is the name given to the strange behaviour of the Pacific that occurs every few years. The western Pacific has some of the warmest ocean water in the world, while the eastern Pacific is much cooler. Every few years, however, for reasons that are not understood, the westward- blowing trade winds drop and allow a slow drift of warm water eastwards across the Pacific. These warm-water drifts interfere with the normal welling-up of cold water, and influence air temperatures, and therefore wind and rain, over a wide area. Also, since it is the cold water that carries the most important nutrients for certain types of fish, the result is always devastation among certain species of fish in South America.

For an analogy, you might imagine yourself sitting in a cooling bath. After removing your feet from the danger area, you turn on the hot tap. Warm currents begin to drift through the water, taking the chill off certain areas, but in a pattern that never seems to be quite the same as the time before.

That seems to be what El Nino does to the world's weather. A vast mass of warm water in the Pacific must have the capacity to alter weather patterns world-wide. The whole system of cooling and warming, and winds and rain, can, the chaos theoreticians tell us, be thrown into unpredictability by one flap of a butterfly's wing. So what can turning a gigantic warm tap on do to it?

Recently, a research team in Israel discovered what was described as a striking correlation between El Nino and rainfall in central Israel. The country's record rainfall coincided with the particularly severe El Nino of 1991-92, while there were no El Ninos recorded during the Israeli droughts of the Thirties. Dr Dan Yakir of the Weizmann Institute has even suggested that the seven- year famine in the time of the Pharoahs reported in Genesis may have been caused by an absence of El Nino.

Yet, as with the sudden influx of warm water into the bath, we can never be sure where the next warm area will be. When it comes to predictions, El Nino remains El Great Un-Ninown.