What Causes Drought?

Image courtesy US National Park Service. Lake Mead, an important water source in the West created in the 1930s by the construction of Hoover Dam, is approaching record low levels - hence the "bathtub ring" around the lake, shown here.

For example, a strong "El Niño" pattern of warm Pacific surface waters near the equator will drive storms into California, while the opposite "La Niña" pattern steers moisture-bearing storms further north to Washington state and Canada. One causes drought, the other alleviates it. But there must be more to the story: While a mild La Niña lurked in the Pacific during the onset of the current drought - as would be expected - a shift to a weak El Niño in 2003 did not reverse the drought.

"It's a really active area of research right now as people are trying to decipher exactly what's causing what," Oglesby says. He and his colleagues at the Global Hydrology and Climate Center are among those working to understand what's going on. In particular, Oglesby is investigating how the land and atmosphere interact with each other during a drought, focusing on the roles that snow cover and soil moisture play.

Part of the difficultly in understanding drought lies in the fact that weather involves many feedback loops that complicate its behavior and defeat simple cause-and-effect explanations. Soil moisture creates such a feedback loop during dry weather. Oglesby explains:

Image courtesy National Drought Mitigation Center. Most of the western U.S. is suffering from some degree of drought. The darkest color on this map represents the most extreme category of drought in NOAA's classification scheme.

"Once you get into a dry pattern and you start to dry the ground out, that reduction in soil moisture can help to intensify and perpetuate the drought." Normally, the evaporation of soil moisture consumes much of the energy contained in the summer sunshine; without this moisture, that energy heats the ground instead and raises temperatures even further. Warmer temperatures create a high pressure system which, in turn, blocks storms from coming into the area. Drought begets drought.