Coral helps explain El Nino oddities.

The radiation released by early, aboveground nuclear tests harmed people living in the shadow of the mushroom clouds but provided oceanographers with a research opportunity Radioactive carbon-14 atoms that fell into the ocean from the tests, which ended in 1964, provide a way to track water currents. A new study of this radiocarbon may help explain recent quirks in El Nino behavior.

Thomas P. Guilderson of Harvard University and his colleagues measured the amount of carbon-14 taken up by coral living along the Galapagos Islands from 1956 to 1983. During part of each year, the coral was bathed in waters brought up from deeper layers by surface winds--a process called upwelling. The deeper layers have less carbon-14 than surface waters.

Not surprisingly, the coral shows that the concentration of radiocarbon in the upwelling waters increased slowly over most of the period, In 1976, though, the carbon-14 concentration in the coral jumped dramatically in the upwelling season, the researchers report this week at a meeting of the American Geophysical Union in Boston.

The timing is potentially significant because some meteorologists have suggested that 1976 marked a change toward more intense and frequent El Nino episodes, related perhaps to greenhouse warming. Since then, the central equatorial Pacific has produced six El Nino warmings, two of them reaching record intensities.

Guilderson and his coworkers see a link between El Nino and the radiocarbon data. They propose that the warm upper layer of water, enriched with carbon-14, suddenly extended deeper in the eastern Pacific in 1976; thus, during the upwelling season, winds pulled up smaller amounts of the water containing less carbon-14. The thicker warm layer, they say, also spurred El Nino activity.

"It suggests that this is a very sensitive part of the ocean that changed in 1976. We need to understand why," says Harvard's Daniel P. Schrag.

Nicholas E. Graham of the Scripps Institution of Oceanography in La Jolla, Calif., wonders whether the hypothesis is backward: Perhaps the frequent El Nino episodes thickened the warm surface layer. He also notes that some researchers don't agree that El Nino behavior has changed recently.

Oceanographers do agree that something unusual happened to the central equatorial Pacific in the mid-1970s, Sea surface temperatures shot up, trade winds slackened, and rainfall increased, says Graham. The coral data now deepen the mystery by showing that the changes extended below the surface of the ocean as well.