By Sid Perkins
A new study of persistent droughts that occurred in the United States during the past 3 centuries suggests that those dry spells may be associated with prolonged instances of the climate phenomenon known as La Nia. That occurs when sea-surface temperatures in the central Pacific are cooler than average.
La Nia events typically bring drier-than-normal conditions to the southwestern United States, says Edward R. Cook of the Lamont-Doherty Earth Observatory in Palisades, N.Y. In the 20th century, each La Nia typically didn’t last more than 2 years. However, new analyses of coral taken from the central Pacific indicate that the sea-surface temperatures there were significantly lower than normal from 1855 to 1863.
That period lines up with the driest decade in Texas since 1700, as recorded in tree rings, says Cook. Trees farther north, along the edges of the Great Plains, also chronicle a drought beginning in 1855 that was worse than the one that afflicted the region during the Dust Bowl of the 1930s.
Other widespread droughts that coincided with extended La Nias stretched from 1703 to 1709 and from 1818 to 1824. Several additional dry spells–including ones in the 1730s, 1750s, 1890s, and 1950s–overlap but slightly lag corresponding La Nia periods in the central Pacific, Cook notes. He and his colleagues report their findings in an upcoming issue of Geophysical Research Letters.
The team’s analyses suggest that La Nia is merely one of the factors that influence precipitation in the Southwest and Great Plains. The so-called Pacific Decadal Oscillation, an approximately 20-year-long cycle in ocean temperatures in the northern Pacific, seems to play a significant role as well, says Cook. A mild drought in the Southwest, which began in 1879, ended in 1884–the same time that ocean temperatures in the northern Pacific swung from slightly below normal to much above normal–even though La Nia conditions persisted until 1888.
Climate data recorded in living organisms such as trees and coral are particularly important because they enable researchers to examine conditions prevalent in a region before scientific measurements were available, says Gregory J. McCabe, a climatologist at the U.S. Geological Survey in Denver. The new study should help fill in more pieces of Earth’s climate puzzle, he notes.
