| Drought Over the Great Plains | |||
Without moisture from the Gulf of Mexico, the center of North America would be a desert. The prevailing winds that blow from west to east across the continent are relatively dry, as they originate over the cold water of the north Pacific. Much of the moisture these winds do carry is dumped at the foot of the mountain ranges that run up the West Coast. As a result, very little moisture is transported from the Pacific Ocean to the Great Plains. However, the air over the Gulf of Mexico to the south of the Great Plains is saturated with water evaporated from the warm surface waters. In normal summer conditions, storm systems periodically transport moisture northward. |
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During droughts (above), the flow of moisture from the Gulf to the Great Plains is cut off. When the Intertropical Convergence Zone (ITCZ) shifts north, often during La Niña, it pushes the trade winds that blow from east to west in the northern tropics further northward. These winds push moisture from the Gulf over Mexico and Central America, leading to above average rainfall in those regions. The large high-pressure system usually centered near Bermuda moves additional water from the Gulf up the Atlantic Coast of the United States, eventually depositing it in the Canadian provinces of New Brunswick and Nova Scotia. Also, the Jet Stream moves north, and a high-pressure system (dry air that rotates clockwise in the northern hemisphere) develops over the northern Plains. These conditions persisted throughout the summer of 1988, leading to one of the worst droughts in U.S. history. |
An interruption in the flow of moisture from the Gulf of Mexico into the interior of North America causes droughts in the Great Plains. In the above diagram thick white arrows represent high level winds, thin arrows represent surface winds, and extremely thick aquamarine arrows indicate moisture transport. Low-pressure systems are colored red and rotate counterclockwise, while high-pressure systems are colored blue and rotate clockwise. (Image by Robert Simmon, NASA GSFC) | ||
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During wet years, more moisture than normal is transported from the Gulf of Mexico to the Great Plains. The Jet Stream shifts south from its normal position, and a branch extends to southern California. The Jet Stream is deflected by the Rocky Mountains creating a low-pressure zone. This “low” draws moist air from the Gulf into the Plains. At the same time, less moisture than normal moves northwards along the Atlantic Coast, often resulting in drought there. In 1993, these atmospheric conditions caused record floods along the Missouri and upper Mississippi Rivers. During that year, the Ohio River, which receives most of its moisture from the Atlantic, was at exceptionally low levels, and the flooding on the Mississippi subsided below the intersection of the two rivers in St. Cairo, Illinois. |
Increased moisture transport from the Gulf of Mexico into the Great Plains leads to above average rainfall and floods. In the above diagram thick white arrows represent high level winds, thin arrows represent surface winds, and extremely thick aquamarine arrows indicate moisture transport. Low-pressure systems are colored red and rotate counterclockwise, while high-pressure systems are colored blue and rotate clockwise. (Image by Robert Simmon, NASA GSFC) | ||
