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.