June 10, 2004

Gretchen Cook-Anderson 
Headquarters, Washington 
(Phone: 202/358-0836)

Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 607/273-2561) 

RELEASE: 04-187

RESEARCHERS SEEING DOUBLE ON AFRICAN MONSOONS

NASA and University of Maryland scientists have found the African 
monsoon consists of two distinct seasons. 

The first season is in the late spring and early summer. The rain is 
concentrated on the West African Coast near the Gulf of Guinea, five 
degrees north of the equator. This season appears strongly influenced 
by sea surface temperatures off the coast of West Africa. 

The second season arrives later in summer in July at around 10 degrees 
north of the equator. Atmospheric waves moving westward over the 
Atlantic Ocean appear to play a major role in this summer season 
monsoon rainfall, according to the research using NASA's Tropical 
Rainfall Measuring Mission (TRMM) satellite. These waves, called 
African Easterly Waves, are evident in the rain patterns. 

The research results appeared in a recent issue of Journal of 
Geophysical Research and in a paper in the Journal of Climate. 
Additional research will explore connections between African Easterly 
Waves and hurricane activity. 

"Most people agree the later season is determined by the atmosphere, 
but we found the early spring monsoon is determined by being close to 
the ocean," said Guojun Gu, lead author of the study. Gu is a 
researcher at NASA's Goddard Space Flight Center (GSFC), Greenbelt, 
Md., and the Goddard Earth Sciences and Technology Center at the 
University of Maryland, Baltimore County. "Also, if the spring rain 
is weak, then the summer monsoon is strong, and vice versa," Gu said. 


A study from 1998 to 2002 of daily rainfall from the TRMM satellite, 
combined with information from other satellites, defined the 
evolution of the African monsoon. This very high-quality dataset of 
rainfall rates helped determine the distinct northward "jump" in the 
monsoon from the early summer season to the later summer episode.

In the May to June season, large differences in sea surface 
temperatures between coastal waters and water further out to sea lead 
to increased rainfall. When coastal waters are warm and equatorial 
waters are colder, rainfall is stronger along the coastline. The July 
monsoon is related to the easterly waves. These waves carry a great 
deal of moisture and provide the seasonal monsoon rain before moving 
off the west coast of Africa.

Previous research suggests the African Easterly Waves may kick off 
small circulations that develop into hurricanes over the North 
Atlantic Ocean. Up to 80 percent of the major hurricanes that impact 
the eastern United States may originate with these waves. "You can 
track a hurricane backward in time to waves coming off the African 
coast and all the way to the mainland of Africa," said Robert Adler, 
a co-author and researcher at GSFC. 

The African Easterly Waves occur only during summer months, from May 
through October. While the waves vary greatly in strength from year 
to year, their numbers stay consistently at about 60 to 70 per year. 
These waves consist of low-pressure systems that pass every three to 
four days, with a wavelength of about 2000 to 2500 kilometers. More 
study is needed to better understand how variations in the waves 
impact hurricanes.

The researchers also found the nature of these waves shifts at about 
the 15-degree north line. The waves that pass south of this line 
carry moisture and create convection that lead to rain clouds. These 
southern waves play a role in both the later summer monsoons and the 
genesis of hurricanes. But waves that pass north of the 15-degree 
line carry hot dry air from the Sahara. These waves may carry tons of 
dust as they move west with trade winds. In about a week they can 
carry dust all the way to the Gulf of Mexico and Florida. 

The mission of NASA's Earth Science Enterprise is to develop a 
scientific understanding of the Earth System and its response to 
natural or human-induced changes to enable improved prediction 
capability for climate, weather, and natural hazards.

For more information about this research on the Internet, visit: 

http://www.gsfc.nasa.gov/topstory/2004/0510africanwaves.html 

For more information about TRMM on the Internet, visit: 

http://trmm.gsfc.nasa.gov/  

	
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