Science Briefs

African Wave Disturbances and African Drought

Diagram of three layers of atmosphere (near surface, 3km and 14km) with African wave disturbances on middle layer
Schematic U-shaped AWD is shown at the middle of three levels in the atmosphere. Arrows indicate winds at different altitudes.
TEJ = Tropical Easterly Jet;
AEJ = African Easterly Jet;
ITD = Intertropical Discontinuity, where monsoon winds from the ocean meet dry Saharan harmattan winds.

The people in Africa's Sahel region, a swath of semi-arid territory on the southern flank of the Sahara Desert, live off the land from simple agriculture and cattle herding and are thus extremely vulnerable to drought. A shortfall or irregularity in the June-September monsoon rainfall puts hundreds of thousands of people in Chad, Niger, Mali, Burkina Faso and Senegal at risk of economic and social disruption and possibly famine.

Storms of low pressure which migrate from west to east across the Sahel during the rainy season, called African wave disturbances (AWD), are known to trigger much of the region's precipitation. These AWD can also generate tropical storms over the North Atlantic, some of which develop into hurricanes that threaten North America.

Motivated by this dual importance of AWD, Druyan and Hall (1994, 1996) have studied computer simulations of AWD using a global climate model developed by the NASA/Goddard Institute for Space Studies. They have found evidence that the contrasting worldwide patterns of ocean temperatures between the summers of 1987 and 1988 were a factor in causing the drought in 1987 and near-normal conditions in 1988. Their study suggests that patterns of warmer than normal and colder than normal ocean temperatures in the world's oceans during those summers affected the speed of the Tropical Easterly Jet Stream that overlies northern Africa. When 1988 ocean temperatures were used in the model simulations, a stronger jet stream, stronger AWD activity and more Sahel rainfall all resulted. The research, supported by NASA and the National Science Foundation, highlights the important role that the Tropical Easterly Jet Stream can play in linking the influence of distant oceans with the summer weather in Africa's Sahel.

Reference

Druyan, L., and T. Hall. 1994. Studies of African wave disturbances with the GISS GCM. J. Climate 7, 1316-1323.

Druyan, L., and T. Hall. 1996. The sensitivity of African wave disturbances to remote forcing. J. Appl. Meteorol. 35, 1100-1110.

Contact

Please address all inquiries about this research to Dr. Leonard Druyan.