Eight years of choking dust storms and crippled crops marked the Dust Bowl drought, which devastated Great Plains agriculture in the 1930s. Although severe, the drought wasn't a fluke. According to new climate models, the drought was caused by fluctuations in ocean temperature in the tropics, and these conditions recur--the 1970s narrowly missed seeing a second Dust Bowl.
The central United States is blessed with an unusual climate. In summer, a jet of warm, moist air from the Gulf of Mexico wafts over the Mississippi River Valley and the Great Plains. The moist air works with a high-pressure system sitting over Mexico and the southwest U.S. to produce the summer rains. However, the rains won't fall if the jet doesn't come off the Gulf of Mexico, or the high-pressure system moves or is too intense.
What throws things off course? In the 19 March issue of Science, a team of researchers argues that the sea-surface temperatures of the Atlantic and Pacific tropics are the ultimate cause of droughts in the American Midwest. Their model shows that when a particular part of the tropical Pacific cools while another warms in the tropical Atlantic, a cascade of trouble begins. First, rainfall over the ocean follows the warmer water. The shifting rainfall changes the tropical jet streams, massive air currents in the upper atmosphere, which in turn alter the mid-latitude jet streams. The results are dramatic. If the jet over the Gulf of Mexico shifts just a bit, it flows mostly over Texas and Mexico instead of the Midwest, making it far drier. And if the high-pressure system is too strong, it can prevent the jet from rising and cooling and releasing rain.
"I'm often skeptical of model results," says John Wallace of the University of Washington, Seattle, "but this is one case in which a model really allows us to see these subtle ways in which the atmosphere organizes itself." Surprisingly, the model predicted another Dust Bowl during the 1970s, which had similar ocean temperatures and dry conditions, suggesting that the difference between a dry year and a drought is not large. "Having the Pacific sea-surface temperature cold is like loading the dice," says Wallace.