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 Nina. That occurs when sea-surface temperatures in the central Pacific are cooler than average.
La Nina 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 Nina 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 Ninas 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 Nina periods in the central Pacific, Cook notes. …