The eight-year drought that plagued the central U.S. in the 1930s, immortalized in The Grapes of Wrath, wracked the Great Plains with devastating dust storms and affected two thirds of this country as well as parts of Mexico and Canada. Paramount to determining why the conditions were so severe and long-lasting is discerning what caused the drought to begin with. Findings published today in the journal Science suggest that unusual sea surface temperatures could be to blame.

"The 1930s drought was the major climatic event in the nations history," explains lead study author Siegfried Schubert of the NASA Goddard Space Flight Center. "Just beginning to understand what occurred is really critical to understanding future droughts and the links to global climate change issues were experiencing today."

Previous research had demonstrated a link between some droughts and peculiar patterns of sea surface temperatures in different parts of the world. Schubert and his colleagues used a computer climate model to analyze conditions during the past 100 years. They found that ocean temperatures, particularly in the tropics, heavily influenced the dry conditions experienced in North America. In the early 1930s, the waters of the tropical Pacific were cooler than normal, and those of the tropical Atlantic were warmer than normal. Those conditions, the scientists say, weakened the jet stream, a strip of fast-moving air that typically flows westward over the Gulf of Mexico before turning northward and depositing rain onto the Great Plains. In its altered state, the jet stream traveled farther south than usual, lessening precipitation over the central U.S. and compounding the drought conditions.

Other droughts that struck the U.S. also correspond to cooler tropical Pacific temperatures, the researchers report, but only the so-called Dust Bowl drought combined these condition with a warmer Atlantic Ocean. A better understanding of the conditions that caused severe climatic events of the past should help scientists better recognize and forecast potentially dangerous conditions in the future.