lesson 3

The Cause of the El Nino Phenomenon

El Nino is the Spanish name for the baby Jesus. The phenomenon is so-called because warm water moving across the Pacific traditionally reaches South America around Christmas. Scientists have now applied the term El Nino to the major warming episodes over large South American coastal areas and westernly along the equator and the Dateline area. Scientists noted the El Nino has a return period of four to five years and lasts between 12—18 months.

In the late 1960s, it became apparent that the year-to-year variations in the sea surface temperature and consequently El Nino events, were closely linked to the Southern Oscillation, a relationship between atmospheric pressure over the southeastern Pacific and Indian Ocean. When pressure is high in the Pacific Ocean, it tends to be low in the Indian Ocean from Africa to Australia. These conditions are associated with low temperatures in both these areas and rainfall varies in the direction opposite the pressure.Thus, the combination of El Nino and Southern Oscillation (ENSO) is the linkage atmospheric and oceanic events and involves changes in circulations of the atmosphere and oceans across the Pacific Basin. The strongest El Nino this century occurred in 1982—1983 and resulted in droughts and disastrous forest fires in Indonesia and Australia, wreaking economic damage of at least US $8 billion.

A major warming of the ocean waters across the eastern and central tropical Pacific Ocean, known as ENSO, has developed since March 1997. The El Nino developed very rapidly during April—May, and reached strong intensity by June. This event is currently comparable in magnitude and extent to the 1982/1983 episode.