Physical Processes that Cause Drought
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.
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.]