IS IT EL NINO OF THE CENTURY?

For six months now a splotch of tropical warmth has been spreading across the Pacific Ocean, from the international dateline to the South American coast. With understandable concern, climatologists have been tracking its progress, for it signifies that El Nino--that mischievous gremlin of the atmosphere and oceans--is once again gathering strength, preparing to unleash meteorological havoc in the months to come. 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."

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.