Physical Processes that Cause Drought

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In the spring and summer of 1988, the skies dried over a region covering about 25 percent of the total U.S. area, centered mostly on the Northern Great Plains. It was North America's worst dry spell since the 1930s, impacting the nation's most productive agricultural lands and causing an estimated $40 billion in crop damages (Kogan 1997). However, researchers were watching closely-with satellite and surface-based instruments-and the event gave them an opportunity to study in detail the physical processes that contributed to the drought.
 

   
 

Generally speaking, there are three main contributors to drought: (1) land and sea surface temperatures, (2) atmospheric circulation patterns, and (3) soil moisture content (Trenberth and Guillemot 1996; Mo et al. 1997). Each of these physical parameters is linked to the others intricately; changing any one of them significantly will typically set up a chain of events that causes the other parameters to change. Sometimes, this chain of events becomes a vicious cycle in which the changing parameters, feeding off one another, are amplified to produce extreme climate conditions-such as flood or drought.

Researchers using global climate models find that as average surface temperatures rise there is an increase in water evaporation leading to more extreme weather events (Dai et al. 1998). In summer, land surface temperatures are linked directly to the availability of moisture (Trenberth and Guillemot 1996). If the soils are wet, then much of the heat from incoming sunlight is used to evaporate water, so temperatures are kept cooler and there is generally more precipitation. But if the soil is dry, then there is little or no water available to evaporate. Consequently, the incoming sunlight can only continue to warm the surface, thereby making conditions hotter and drier, thus beginning the chain of events leading toward drought.

Atmospheric circulation patterns can make or break a vicious drying cycle. Scientists observe that atmospheric circulation is closely connected to the surface temperature of the sea. Heat released from the ocean creates temperature gradients in the atmosphere that cause air currents. And because warm water evaporates more readily than cold water, warmer sea surface temperatures contribute to more cloud formation and more rainfall downwind of the general flow of air currents.

Using satellite remote sensing data, scientists have confirmed there is a direct relationship between sea surface temperature variations in the Atlantic and Pacific Oceans and large-scale atmospheric circulation patterns that bring rain or dry spells (Trenberth and Guillemot 1996). Scientists have used satellites to demonstrate that variations in sea surface temperature can determine where there is high plant growth on land and where there is drought (Los et al. 2000). Researchers refer to the two seemingly unrelated parameters (sea surface temperature and land plant growth) as a "teleconnection" in the Earth's climate system.
 

 

Soil Moisture Animation
Soil moisture plays an important role in preventing or prolonging summer droughts. When the ground is wet, water evaporates as the day warms up. The warm, moist air rises until it encounters colder air high above the Earth's surface, leading to afternoon rainshowers. The water remains in the ground through the cool night, and the cycle repeats the next day. [view animation (1.2MB)]

Dry Soil Animation
Dry soil has the opposite effect on rainfall. As the temperature rises during the day, the air near the Earth's surface heats up and rises, but does not contain enough moisture to form rainclouds. As each day passes more moisture is removed from the ground, enhancing the effect. [view animation (710kb)] (Animations courtesy Susan Byrne, NASA GSFC.)

 

Here's how the teleconnection works. Warm air is less dense than cold air and tends to rise, resulting in an upward transport of heat (called convection). Unusually high or low sea surface temperatures (referred to as “anomalies”) affect the intensity and location of areas of convection. Large-scale anomalies like El Niño and La Niña influence convection on such a large scale that they cause the location of the Intertropical Convergence Zone (ITCZ) to shift southward or northward, respectively. The ITCZ in the Pacific helps determine the course of Pacific air masses flowing eastward toward North America. Thus, changing the ITCZ's position influences weather patterns all over the continent. (The ITCZ is the region of convection that circles the Earth, near the equator, where the trade winds of the Northern and Southern Hemispheres come together. The intense exposure to sunlight in the equatorial region warms the surface water causing increased evaporation and warming of the air near the surface. Thus the air is both warmer and has increased humidity. The warm moist air rises and as it rises it cools, releasing the accumulated moisture in an almost perpetual series of thunderstorms.) When precipitation patterns change across landscapes, so too do plants' patterns of growth.

Certain regions seem particularly susceptible to influence by sea surface temperature and air current variations. In the Great Plains region, for example, researchers find that about 75 percent of a year's worth of precipitation falls from April to September (Laird et al. 1996). Examining recent rainfall data, researchers found that over a recent 17-year period (1979-95) there were 21 “wet” and 19 “dry” events, both of which lasted an average of 17 days (Mo et al. 1997). Both types of events are fairly distributed throughout the summer months (Mo et al. 1997). In North American wet years, the heaviest rains fell over the Great Plains region, while in dry years most rain fell in Florida, along the Gulf Coast, Arizona, and New Mexico (Mo et al. 1997). [Editor's note: because there are many teleconnections between sea surface temperature, rainfall patterns, and plant growth, the Earth Observatory presents each teleconnection as an individual sidebar to this article.]

next Remembering the Drought of 1988
back Dry Times in North America

 

SST vs. NDVI Animation
Sea surface temperatures in the equatorial Pacific affect precipitation (and therefore plant growth) over much of the North American continent. When there is warmer than normal water (dark red) in the Pacific, more rain than normal often falls over western and central North America (green indicates above average plant growth, generally the result of extra rain). Conversely, cold water temperatures in the Pacific (dark blue) lead to decreased rain over western North America (brown indicates a relative lack of plant growth). [view animation (3.9MB)] (Animation courtesy Sietse Los and Marit Jentoft-Nilsen, NASA GSFC)

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