Phenomenon El Niño from 1997 to 1998

Natural catastrophes, droughts or floods, or tropical cyclones, they are images associated with climatic phenomenon El Niño, and no country does not seem to escape at various degree. But, in fact,that in is it?

" El Coriente d' El Niño " (the current of the Child-Jesus, such as the Peruvian and ecuadorian fishermen defined it) was a heat current which appeared at the time of Christmas and remain a few months (during the southern summer). Its hot water replaced water usually colder, and the catches were fewer. Sometimes, this reheating lasted during one year longer or more and, in years 50-60, starting from British Sir Walker’s work at the beginning of the century, one realized that, those years, the reheating was not only coastal but interested all equatorial Pacific.

This reheating is the result of the displacement of the immense hot water reservoir of the Western Pacific (temperatures of the ocean higher than 28°C) towards the Pacific Central and Eastern under the action of the western winds at the equator close to Papua New Guinea. It involves an attenuation of the trade winds in the eastern part of the basin and a stop of the cold water increase towards surface. In its displacement towards the Central Pacific, the hot water tank involves with him the maximum area of precipitations (great atmospheric convection) and influences the atmosphere which reacts by an attenuation of the trade winds on the eastern part of this basin. Consequently, the temperatures of surface are heated at the same time on the American coast, because the cold water upwelling from the deeper layers of the ocean are weaker, even non-existent.

In 1997, the event reached an exceptional amplitude comparable with the event which had proceeded in 1982-83. El Niño had its first demonstrations very early, since in March – April measurements of satellite Topex Poséidon showed in a very clear way an unusual front elevation in the sea level along the equator through all Pacific. As of July and August, the hot anomaly of the temperature of surface (gap compared to the seasonal averages) reached from 4to 5°C in the eastern part of the Pacific basin and along the South American coasts.

At the same time, the area of maximum precipitations (area of great atmospheric convection) moved towards the center and eastern part of the Pacific, with the hot water tank. The consequences were quickly felt on the indonesian islands and New Guinea New Guinea which were reached by serious problems of dryness involving of the beginnings of famines in certain places. The southern winter along the coasts of South America was very warm and rainy.

But El Niño had more remote consequences because it disturbed tropical atmospheric circulation. Thus it is partly responsible for the pluviometric deficit in Western Africa during July to September, as that had occurred for the event from 1982-83.

If El Niño 97 was early in its starting, and if it reached its peak of power in at the beginning of December, its effects however remain particularly important, because Southern summer is the period when the seasonal cycle of temperature is maximal, with maximum effects on the atmosphere, in particular for the tropical rains. One thus observed, since the beginning November, several tropical storms and cyclones on the Pacific Islands Central (Cook islands and certain islands of French Polynesia), areas which are not affected by the tropical cyclones in normal period. The rains are abundant (even torrential) on the coasts of Ecuador, Peru and California of the south, and the dryness prevails on Australia.

Thanks to the observations by satellites and of the oceanographical networks, thanks also to the observations carried out by the merchant ships and to the predictions of the numerical models which give reliable indications to more than 6 months, El Niño of this year is well studied. If one cannot avoid the climatic whims of El Niño, search on its forecast and its impacts makes it possible to prevent and thus to mitigate its effects