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.
This pressure oscillation has turned out to be part of a larger, very important phenomenon. When it goes negative, the normally easterly trade winds in the tropical Pacific may fail. Warm surface water may spread into east Pacific regions, where it normally isn't found. This, in turn, is linked to El Nino and to weather in temperate latitudes around the world.
Earth's weather machine is a single, integrated ocean-atmospheric system. And what happens in the tropical Pacific can influence weather in Boston, Moscow, and London. Roderick S. Quiroz of the US National Oceanic and Atmospheric Administration (NOAA) Climatic Analysis Center says last winter ''provided to meteorologists a clear and strong example of tropical-extratropical and ocean-atmosphere interaction.''
Summarizing this in the Monthly Weather Review, he notes that this has been an usually strong ENSO event. Strong intrusion of warm water in the eastern Pacific led to powerful atmospheric convection with massive rainfall. As the rain condensed in the atmosphere, it released heat energy. The tropical atmospheric circulation was strongly perturbed.
Quiroz explains that these events appear to have influenced circulation far away, giving it an ''extraordinary overall strength.'' Among other things, the total energy of east-west winds over the Northern Hemisphere last February reached the highest value recorded since NOAA began keeping such records in 1975 . It's not yet fully understood how, or to what extent, the tropics affect mid-latitude weather. But there seems little doubt now that ENSO events do have considerable influence.
The detail of this latest ENSO event that has been provided by weather satellites is beginning to give experts this kind of insight needed to prepare for extreme weather. Winds can be traced by the movement of clouds. Areas of heavy precipitation and of clear skies and drought are shown by patterns of outgoing infrared (heat) radiation. This is high where the satellite looks through cloudless skies to the surface. It is relatively low where the satellite sees the cold, high tops of convective rain clouds.
Quiroz's NOAA colleague Eugene M. Rasmusson, and A. E. Gill of Britain's Cambridge University, have been able to use this detail to test simple computer models of an ENSO event. Reporting their study in Nature, they made no claim to having a definitive explanation. Yet a plausible picture has emerged.
They find a sequence in which the reversal of the Southern Oscillation index - with pressure high at Darwin, low at Tahiti - is coupled with disruption of the normal easterly trade wind circulation. This in turn is linked to a strong, wind-driven eastward migration of the ''warm pool'' - surface water warmer than 29 C. - normally found in the western Pacific.
The warmth of this water stimulates convection in the air and much rain. As it moves east, drought comes to Australia and Indonesia, while normally drier mid-Pacific areas are deluged. The system tends to feed on itself. Air rising in the convective area is replaced by air flowing in as westerly surface winds. This maintains and strengthens the anomalous surface wind field. Heat released by the rainfall encourages more convection. These effects and associated ocean waves and currents eventually stimulate the El Nino.
No one yet knows what starts the process in the first place or how it ends. Gill and Rasmusson note that more progress could be made if observing networks in the tropics were greatly expanded.
With such dense data gathering, it may be possible to use computer models to predict the ENSO. Certainly, since the ENSO has such far-reaching effects, this would be a valuable international project.