By Sid Perkins
An analysis of nearly 2 decades of weather patterns suggests that there’s a link between an abundance of precipitation in the eastern Indian Ocean and a lack of rain in portions of southwestern Asia.
A persistent drought recently afflicted more than 60 million people who populate the swath of land stretching from Iran to western Pakistan, says Heidi M. Cullen, a climatologist at the National Center for Atmospheric Research in Boulder, Colo. Smack in the middle of this area sits Afghanistan, which from 1998 to 2001 experienced its longest and most severe drought in the past 50 years. The dual plagues of drought and political unrest struck the country hard. Only 12 percent of Afghanistan’s land is arable, and 80 percent of its residents are subsistence farmers, says Cullen.
The recent Afghan drought began soon after the appearance of La Nia, a climate trend in which the sea-surface temperatures in the central Pacific remain cooler than normal for at least several months. To investigate a possible connection between La Nia and Asian droughts, Cullen and her colleagues studied the weather patterns for the years 1979 through 1996, a period that excludes the most recent drought.
Not all La Nia years during that period resulted in droughts in southwestern Asia, the scientists found. Typically, precipitation in that region dwindled only when La Nia accompanied warmer-than-average surface temperatures in the western Pacific.
With these conditions in mind, the researchers suggest that the drought-forming patterns develop this way: The combination of abnormally cool waters in the central Pacific and relatively warm conditions in the western Pacific boosts winter rainfall in Malaysia and other island nations that rim the eastern Indian Ocean. Changes in the location of the jet stream that accompany the persistent rainfall in the Malaysian region also produce high atmospheric pressure over Afghanistan. That persistent system blocks moisture-bearing storm systems from large portions of southwestern Asia, Cullen notes. She described the research in Denver this week at the annual meeting of the American Association for the Advancement of Science.
The return of El Nio’s warmer-than-normal surface temperatures to the central Pacific last summer may lead to normal or above-average amounts of precipitation in southwestern Asia, says Cullen. This year’s rainy season, which began last October, is on target to bring above-normal amounts of precipitation.
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