Looking back at last winter's unusual weather, meteorologists realize the world experienced an epochal climatic event. Its significance doesn't lie merely in the weather extremes - disastrous Australian-Indonesian drought, devastating Peruvian floods, and one of the warmest North American winters in 52 years. It doesn't lie in the fact that these events all reflect the influence of an unusual warming in the equatorial Pacific. Such things have happened before. But this time, thanks to the capability of modern weather satellites, scientists have at last been able to follow one of the most important types of short-term climatic fluctuations in detail.
Experts have dubbed it the El Nino-Southern Oscillation phenomenon, or simply the ENSO. It's a combination of what were once studied as separate events but are now recognized as one. Normally in the fall and winter, off western South America, an upwelling of cold, nutrient-rich water supports marine life. But from time to time, a season will have warm, nutrient-poor water. Fisheries fail. Catastrophic rains may flood the land. This is El Nino - ''the little one,'' or Christmas child - so named because it often sets in soon after Christmas.
El Nino is linked across the Pacific with what meteorologists at first took to be an upper-air curiosity. Air pressure differences between the Australian-Indonesian region and southeast Pacific seesaw back and forth every few years. Meteorologists called it the Southern Oscillation. They use the pressure difference between Tahiti and Darwin, Australia (Tahiti minus Darwin), to measure it. The index is positive when Tahiti has relatively high pressure and negative when Darwin is relatively higher.