Feb. 2, 1998 COLUMBUS, Ohio -- Astronomers in the Southern Hemisphere need to be extra cautious when doing precise observations during an El Niño event, according to a new study by an Ohio State University researcher.
The report warns astronomers that changes in the water content of the atmosphere during an El Niño/Southern Oscillation event can alter scientists' data by as much as 2 percent. The study was published in the current issue of the journal Publications of the Astronomical Society of the Pacific.
"This doesn't sound like much," explained Jay Frogel, professor of astronomy, "but if you're trying to make measurements accurate to 1 percent, a 2-percent error is enormous." Frogel's research on water-bearing cool stars in galaxies requires that level of accuracy.
Frogel became curious about the possible effect of El Niño as he reviewed data he built from 200 nights of observing. It grew from his use of two telescopes at the Cerro Tololo International Observatory in Chile during a 15-year period from 1978 to 1992. The observatory, one of the world's finest, stands on a mountaintop in the Andes Mountains some 300 miles north of Santiago.
Frogel's conclusions, surprisingly, grow from one of the more mundane tasks astronomers perform to calibrate their observations.
When ground-based telescopes like those at Cerro Tololo are pointed at a star directly overhead, they peer through a specific thickness of atmosphere. But if the star is closer to the horizon, the telescope peers through much more atmosphere and the image is dimmed. How much the atmosphere affects incoming radiation is known as the atmospheric extinction.
To account for this atmospheric extinction, most astronomers take measurements on certain standard stars during their observations. But sometimes, astronomers may rely on standard tables of extinction values instead of making their own measurements. Frogel wondered if relying on the tables might be problematic in years when El Niño events occurred.
His research focuses on observations in the near-infrared part of the spectrum. Scientists have known that atmospheric extinction in the near-infrared is caused mainly by water vapor in the atmosphere. Carbon dioxide also plays a secondary role in that extinction. Frogel proposed that the changes in the amount of atmospheric water vapor brought on by El Niño might be hidden in his 15-year record of observations.
An El Niño event occurs when a dying of the trade winds allows the warm waters of the western Pacific Ocean to flow back east and prevent the upwelling of the very cold waters of the Humboldt Current that flows north from the Antarctic along the west coast of South America. This causes a shift in the pattern of tropical rainfall. These large-scale changes can lead to drier conditions (drought) in Australia, Indonesia, India and Africa, as well as northeast Brazil and Bolivia. It can also cause wetter conditions (flooding) in coastal areas of Ecuador, Peru and Chile, as well as along the west coast and southern United States in winter.
The El Niño event that occurred during 1982-83 was the strongest ever recorded before the current El Niño event which is believed to be even stronger.
Frogel's records included his measurements of atmospheric extinction during that 15-year period. He began analyzing them, looking for identifiable patterns from month-to-month, season-to-season and year-to-year.
He quickly found increased atmospheric extinction in January, February and March -- during summer in the Southern Hemisphere -- and decreased extinction during the winter.
"It's higher in the summer because, with the higher temperatures, the atmosphere holds a lot more water," Frogel said "It was less in the winter when the atmosphere is colder and won't hold much moisture."
Frogel compared the patterns he saw in his data with two sets of climate data that serve as independent measures of El Niño -- the Southern Oscillation Index and Niño 1 and Niño 2. His observation records correlated with the climate indexes almost perfectly.
"During an El Niño year, the water content of the earth's atmosphere is increasing. It's not just that it's raining a lot. The strong correlation between the extinction values and the El Niño measures shows that there really is a lot more water in the atmosphere during these events," he said.
Frogel says his findings aren't earth-shaking but they are interesting, especially for astronomers in South America, Australia and the west coast of the United States. During an El Niño year, he reported, the extinction values can be as much as 2 percent greater than the average in a non-El Niño year.
"I think it's important for people to be aware of this so that they don't take normal mean values (for extinction) and always use them, independent of what is going on in the atmosphere."
Frogel's work was supported in part by the National Science Foundation.
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