The tropical storms spawning off Mexico, the heavy downpours that have drenched Chile and Peru, the worrisome lack of rain in Australia and Indonesia--all, say scientists, are signs that a powerful El Nino has grabbed control of the world's weather machine.

Already there is reason to think that the El Nino brewing in the Pacific may dwarf just about any other seen in this century. The swath of equatorial ocean over which it holds sway extends some 6,000 miles, a quarter of the globe's circumference.

Temperatures at the sea's surface have been rising so rapidly that they seem likely to equal those of the notorious El Nino of 1982-83, which left 2,000 people dead and $13 billion in economic losses. "That was the biggest El Nino we know of," says climate modeler Stephen Zebiak of Columbia University's Lamont-Doherty Earth Observatory, "until maybe now." El Nino generally peaks around December, which is why Peruvian fishermen long ago gave the Christmastime weather visitor a name that in Spanish means "Christ Child." If the warming trend continues, scientists say, the incipient El Nino could pump so much heat into the ocean that average sea-surface temperatures might rise 3.5[degrees]C, or 7[degrees]F--and if this happens, the effects would be felt far into the new year. Among the disasters that would be likely to result are landslides, flash floods, droughts and crop failures. Ecuadorian cocoa producers estimate that the current El Nino could lower crop yields as much as 60%.

But El Nino would also bestow a patchwork of benefits. Off Chile, fishermen could look forward to catching anchovies normally found much farther north. Peruvians have been enjoying balmy beaches in the middle of their winter. And residents of the U.S. could look forward to fewer Atlantic Ocean hurricanes, an earlier spring in the Northeast and a blessed lull in tornadoes throughout the Midwest. All things considered, says Florida State University oceanographer James O'Brien, Americans should think of El Nino as a "good dude."

TROUBLED WATERS Torrential rains that washed away this bridge in Chile's capital city of Santiago in June are linked to the same meteorological mayhem that can turn climate topsy-turvy from California to China SANTIAGO LLANQUIN - AP

Until recently, most weather scientists paid scant attention to the periodic episodes of warm water that for countless centuries have appeared off the coast of Peru. They seemed to be a local event, one that affected mainly fish--in particular, Peru's lucrative anchovy fishery--and seabirds. Not until the early 1970s, when that fishery's collapse was accompanied by drought and crop failures around the world, did the global reach of El Nino become clear. However, it took the disastrous weather of 1982-83 to convince scientists and policymakers that the tropical Pacific merited close watching.

As a result, the El Nino of 1997-98 will be the most closely observed in history. In the tropical Pacific, ships, satellites and stationary buoys are gathering mountains of continuous data--on sea-surface temperatures, wind speeds and directions, and ocean currents--that scientists at universities and government laboratories are feeding into powerful computers in hopes of creating a model of the climate system that evolves.

Scientists note that something funny seems to be going on. Until recently, El Ninos came more or less periodically every two to seven years. But in the early 1990s several El Ninos appeared in a row, one right after another. Now, after dying down in 1995 and '96, El Nino is back. What is going on? scientists wonder. Are frequent El Ninos a signal of global warming caused by human tampering with the atmosphere? Or do they arise from random fluctuations in the natural cycle? There are as yet no good answers to these questions. Observes Michael Glantz of the National Center for Atmospheric Research: "The discrepancy between what we think we know about El Nino and what there is to know may still be quite large."

In terms of the climate machine, El Nino is more than just a sudden warm current off Peru. It refers to a rise in sea-surface temperatures over much of the equatorial Pacific as well as a change in winds and ocean currents. Indeed, there is a kind of climatic flip-flop, with a reversal of conditions across a wide stretch of ocean. Consequently, climate experts no longer refer to El Nino alone but speak of the El Nino Southern Oscillation.

Rather like a pendulum, the ENSO cycle swings between an El Nino state and its opposite, a cold-water state known as La Nina (the girl) or El Viejo (the old man). Taken as a whole, ENSO is a powerful driver of global weather patterns. In fact, say scientists, it is, besides seasonal variations caused by the earth's travels around the sun, the major cause of month-to-month variation in climate.

To visualize how the ENSO cycle works, think of the Pacific Ocean as a big bathtub, with a fan stirring up air representing the trade winds. In the ENSO cycle's neutral or cold phase, these winds blow from east to west, pushing water away from the South American coast, so that the ocean's surface is a couple of feet lower off Peru than it is off Indonesia. The difference, although seemingly small, has important consequences: to replace the water that the winds have swept away, cold, nutrient-rich water from the depths wells up, and so Peru's waters are loaded with fish. But when an El Nino gets started, the pattern shifts. The trade winds dwindle, and may even start blowing from the west. The upwelling off Peru stops, and anchovies and other fish move to different feeding grounds.

Coastal Peru is hardly the only area affected. El Nino alters winds and currents throughout the tropics, producing what climatologist Nicholas Graham of the Scripps Institution of Oceanography calls "a weird kind of sloshing." As the trade winds slacken, he explains, they give rise to slow-moving waves that surge from west to east and exert downward pressure on the thermocline. This is the boundary layer of chilly water that separates the much colder water in the ocean depths from the sun-warmed water near the surface. Normally, the eastern Pacific's thermocline lies at a shallow depth and thus mixes with water near the surface, cooling it. But in El Nino years, the thermocline sinks too deep, and water temperatures at the surface rise noticeably.

Still, why should a regional phenomenon affect weather around the world? The reason, say scientists, is the extra heat. Like fresh coal tossed on a fire, it creates more and larger storms. And as the warm water spreads into the central and eastern Pacific, these storms inevitably follow in its path, moving the tropical storm belt from one part of the Pacific to another. The rearrangement has reverberations throughout the atmosphere, causing droughts in places as far-flung as northeastern Brazil, southern Africa and Australia, while other regions, from California to Cuba, can be hit by torrential rains. These effects are variable. El Nino may weaken the Indian monsoon--or barely affect it at all.

Scientists are only now beginning to trace this web of long-distance climatological connections--or teleconnections, as they are called--that links the ENSO cycle to major weather changes around the globe, such as shifts in the position of the jet streams. These are the high-altitude winds that serve as weather "tracks" in both the northern and southern hemispheres.

A dip in the northern hemisphere's jet stream, for instance, can be expected to direct moisture-laden storms on a more southerly route over the U.S., while a nudge in the opposite direction will result in snow and rain farther north. But figuring out exactly how a particular El Nino will affect North America is no easy matter because particular effects can be amplified or reduced by other phenomena that are less well understood.

To improve the current state of ENSO forecasting, Scripps and Lamont-Doherty have set up an international research institute dedicated to predicting medium-range swings in climate. Already, says climatologist Antonio Moura, the director, he and other scientists have begun to produce experimental forecasts of the probable impact of the ENSO cycle on selected regions. Thus rice, corn and bean farmers in northeast Brazil, say, could, if adequately forewarned, mitigate the effect of El Nino-associated droughts by planting rapidly maturing varieties of seed. The only hitch is that if they switch and a drought does not occur, their crop yields would be lower than normal.

This puts a lot of pressure on climate modelers, who, even as their forecasts improve, will surely couch them in caveats. As TV viewers know from nightly weather forecasts, an 80% chance of rain doesn't necessarily mean that they'll be unfurling their umbrellas. At an El Nino symposium held at headquarters in Boulder, Colo., University of Washington atmospheric physicist Edward Sarachik suggested that people most affected by the ENSO cycle--not just farmers and fishermen but also commodities traders, water-resource managers and insurance underwriters--should look at a prediction the way a savvy gambler might look at a set of dice that turns up snake eyes more frequently than expected. "You wouldn't want to bet $100 on the first throw," he notes. "But if you bet $1 on a hundred throws, you'll come out ahead."

Because of the high economic stakes and greater public awareness, observes Michael Hall, director of the National Oceanic and Atmospheric Administration's Office of Global Programs, the 1997-98 El Nino is shaping up as something more significant than another mighty misfire of the weather machine. It is also a social experiment that will reveal how people around the world react to climate change that is predictable in its broad outlines but unknowable in its details.

For this reason Hall and others like to think of the current ENSO cycle as a kind of dress rehearsal for the sort of decision-making we could face in the coming century if, as many scientists anticipate, the planet heats up from the accumulation of greenhouse gases in the atmosphere. As Moura puts it, "If we don't know how to react to El Nino, how can we react intelligently to something that is far less tangible?"