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What Causes Droughts?

The western U.S. is facing yet another summer of severe drought. Science provides some answers as to what causes droughts - and some baffling questions.

by Patrick L Barry and Dr Tony Phillips

People often greet the first warm days of summer with eager anticipation for the sunny weather to come. But for many people in the western U.S., the arrival of warm weather this year is an harbinger of hard times ahead.

Drought has gripped some parts of the West for as many as seven consecutive years, causing one of the worst dry spells in decades. Soils are dry; reservoirs are low. Farmers and golf course managers are vying for irrigation water, residents face water rationing measures, and the politics of water "seniority" rights is heating up between cities and between states.

Hope for a reprieve fades with the departing winter, because little precipitation typically falls in the West during summer months. These regions depend on winter storms to stock the mountains with snow, which melts in summer and replenishes water supplies. The snow pack in April 2004, though, was only 40% to 75% of normal

The ongoing drought, which has affected 20% to 50% of the land area of the contiguous United States, isn't as bad as, say, the Dust Bowl drought of the 1930s. Then 70% of the U.S. was dry. In the Great Plains, precious topsoil blew away, agriculture collapsed, farmers literally lost their land. "The dispossessed were drawn west," wrote John Steinbeck in The Grapes of Wrath. "Families, tribes, dusted out, tractored out. Car-loads, caravans, homeless and hungry. They streamed over the mountains - restless as ants, scurrying to find work to do - to lift, to push, to pull, to cut - anything, any burden to bear, for food."

What causes such severe droughts? Are they predictable? Scientists aren't sure, but they're learning by studying the current dry spell. Earth-orbiting satellites, which didn't exist during the Dust Bowl years, now provide crucial data about winds, rain, soil moisture, and the state of the oceans. Somewhere among those numbers lies the answer.

A key factor is the temperature of water in the Pacific Ocean, says Bob Oglesby, a climate dynamicist at NASA's Marshall Space Flight Center. Sea surface temperatures (SSTs) in the Pacific alter the course of the jet stream as it flows eastward over North America. This high-altitude "river" of fast-moving air is like a conveyor belt for storms, so the path it takes across the continent has a strong effect on where rain and snow will fall. By steering the jet stream, the Pacific Ocean acts like a baton-wielding orchestra conductor directing the symphony of weather patterns across North America.

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:

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.

Drought is a natural part of cyclical weather patterns in North America, notes Oglesby. Physical clues about ancient weather, such as tree rings and lake sediment cores, show that dry spells such as the Dust Bowl and a similar drought in the 1950s typically occur a few times per century. The historical record also reveals a "mega-drought," longer and more severe than any recent episodes, 500 or so years ago.


Sea surface temperature patterns corresponding to El Niño and La Niña. Red/white denotes warmer water; purple denotes cooler. The heat content of the water was calculated from TOPEX/Poseidon satellite measurements of the water's height.

In modern times "there's more to drought than simple lack of precipitation," adds Roger Pielke Sr., a state climatologist for Colorado and a professor of atmospheric sciences at Colorado State University. "You have to consider human factors like the amount of water being drained from rivers for crop irrigation and drinking water. In absolute terms, the ongoing dry spell is not yet as severe as the Dust Bowl of the 1930s, but the impacts have been relatively severe because the demands that people place on the water supply are so much greater now than they were back then."

This makes a complicated situation even more complicated. Land-use and water-use by humans; large-scale atmospheric circulation changes caused by ocean temperatures; feedbacks between the land and atmosphere: they all play a role. Climatologists can't yet put these factors together to predict what will happen many years in advance. Next winter is mystery enough. Will it bring much snow ... and relief? No one knows.

One thing seems sure, though: With levels of moisture in the soil and snow on the mountains both below average, people in the western U.S. are facing at least one more long, dry summer.


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First Science 2014