Top U.S. weather story of 2007: the Southeast U.S. drought

The year 2007 is in the record books as the driest or second driest year on record for much of the Southeastern U.S. A mere 31.85 inches of rain fell in Atlanta, Georgia, during the year, 62% of the average of 48 inches. This year's rainfall total just missed breaking the record of 31.80 inches set in 1954. Rainfall records in Atlanta go back to 1930. The drought was worse in Alabama, where Birmingham had its driest year on record--just 28.86", a full 25 inches below average, smashing the record low of 36.14" set in 1931. Huntsville was even drier--a mere 28.65"--29 inches below average. Surrounding areas of Tennessee, North Carolina, South Carolina, Kentucky and Virginia also experienced extraordinarily dry years (Figure 1).


Figure 1. Drought conditions in the Southeastern U.S. were at the highest level possible--exceptional drought--at the end of 2007. Image credit: NOAA.

Although the current drought is exceptional, Atlanta has had much drier 2-year periods. During the drought of 1954-1955, Atlanta received 68.23 inches of precipitation--a 28 inch deficit from the normal 2-year rainfall. The deficit for the past two years is only 16 inches, since an average amount of rain fell in 2006 (48.46 inches). Based on the climatological record, Georgia can expect a two-year drought about once in 25 years, and a drought lasting three or more years about once every 40 years. Drought is part of the natural cycle in Georgia, and it would not be a surprise to see the drought of 2007 continue until the winter of 2008. Although December 2007 saw Atlanta's first above average month of rainfall since November 2006, the coming two weeks look very dry, and the current La Nina atmospheric pattern usually brings below average rainfall.

What is causing the Georgia drought?
Two main factors are responsible for the Southeast U.S. drought. Most importantly, a persistent jet stream pattern has set up that steers storms away from the region, and into Texas instead. Texas, which suffered extreme drought in 2006, found itself awash in floods in 2007, as the jet stream pattern brought storm after storm to the state. Another contributing factor to the current Southeast drought is the absence of tropical storms and hurricanes during 2006 and 2007. These storms are an important part of the annual rainfall budget for the Southeast. For example, in 2005, 29% of Atlanta's yearly rainfall came from five tropical storms:

1.36" Arlene June 11-12, 2005
1.40" Katrina, August 29-30, 2005
5.48" Cindy, July 6-8, 2005
5.41" Dennis, July 9-12, 2005
2.94" Tammy, October 6-8, 2005


Figure 2. Over ten inches of rain fell on Atlanta in 2005 from two tropical cyclones, Dennis and Cindy. Tropical cyclones accounted for 29% of Atlanta's rainfall that year. Image credit: NOAA/HPC.

There is little evidence that global warming contributed to this drought. The Southeastern U.S. has cooled by about 0.1° F over the past 100 years, even as the rest of the globe has warmed 1° F. The reasons for this lack of warming in the Southeast are not fully understood. One theory is that the cooling is due to air pollution blocking sunlight. Another theory, proposed by Georgia State Climatologist David Stooksbury, attributes the cooling to the shift from 80 percent row crops (like corn and cotton) to 60 percent forest. Land use changes like this can have a significant impact on how solar energy heats the air once it is absorbed by the soil.

Why Atlanta is vulnerable to drought
Atlanta, unlike most major cities, grew up around rail lines rather than a major body of water. Although the Chattahoochee River runs through Atlanta, the city lies on top of a watershed, with no large bodies of water upstream. The rivers and streams in the region are small, and the bedrock limits how much ground water is available. The Lake Lanier reservoir that supplies Atlanta with most of its water was constructed in the 1950s. Since that time, the population of Atlanta has quadrupled, increasing the pressure on the reservoir's limited water. Lake Lanier is at its record lowest level--more than 19 feet below average. If the drought continues into the summer of 2008, and no tropical storms arrive to break the drought, Atlanta may run out of water.

By 2030, the population of the Atlanta metropolitan area is expected to increase from 5 million to 9 million. Current water resources in the regions will be unable to support this population increase, unless planners make a concerted and expensive effort to plan ahead. The expected increase in temperatures over the coming decades due to global warming will make future droughts in the region even more severe, so Atlanta has a very tough road ahead of it.

Jeff Masters