After analyzing historical records and climate model data for two major U.S. droughts in the 1930s and 1950s, NOAA scientists found two very different causes, shedding new light on our understanding of what triggers drought. Studies such as this one that expand our insights into drought are essential for improving forecasts and can aid in the creation of an early warning system to help communities take precautions and prepare.
"In the case of the severe 1950s drought of the Southern Plains states, it appears global sea surface temperatures were the principal cause. However, the 1930s 'Dust Bowl' drought over the central and northern Plains states was not caused by ocean conditions, but rather the evidence points to random changes in the atmosphere as the instigator in that event," said Martin Hoerling, lead author and a meteorologist at NOAA's Earth System Research Laboratory in Boulder, Colo. The work, "Distinct Causes for Two Principal U.S. Droughts of the 20th Century (abstract only if not subscriber)," is currently available online and will be published in the October 16 edition of Geophysical Research Letters.
Scientists studied two national events - one over the Southern Plains during 1946-1956 and the other in the central and northern Plains during 1932-1939, commonly known as the Dust Bowl period. The authors note that the two events are considered the most severe and prolonged droughts over the Great Plains since 1895. In the United States, the Plains encompass roughly the area west of the Mississippi River and east of the Rocky Mountains. It has been long held that droughts are influenced by sea surface temperatures. But the NOAA scientists saw differences in the two droughts they studied and delved deeper into the causes.
"Our finding that the 1930s Dust Bowl drought was likely caused by a random change in the atmosphere does conflict with earlier scientific studies," said Hoerling. "But in our analysis, using a more extensive set of model experiments and diagnostic tests, the prior speculation that the Dust Bowl drought had early warning indicators in the ocean temperatures could not be supported." However, the evidence surrounding the Southern Plains drought does point to sea surface temperatures as the driver for that decade-long event that began in 1946. The authors note that the Southern Plains region is more sensitive to sea surface temperatures than the Northern Plains due to its proximity to tropical Pacific Ocean-influenced weather patterns.
"Both the observations, as well as the simulations show strong and frequent La Niñas during Southern Plains drought," said Hoerling. "Droughts are common in that region during La Niña years." La Niña is characterized by unusually cold water temperatures in the equatorial Pacific Ocean as opposed to El Niño which reflects unusually warm temperatures. These events can affect global weather patterns; in the United States, La Niña often signals drier-than-normal conditions in the southwest and central Plains regions, and wetter fall and winter seasons in the Pacific Northwest.
The authors note that while an ocean observing system - a network of instruments including stationary and free-floating buoys as well as satellites - is vital to any drought early warning system, it may not adequately warn of a drought caused by other factors, and may not provide early warning for a drought over the northern Plains states, such as what occurred in the 1930s.
Xiao-Wei Quan and Jon Eischeid from the Physical Sciences Division of NOAA's Earth System Research Laboratory in Boulder, Colo., also are authors on the paper.