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Where has all the rain gone ?

Thanks to the AMMA program, from 2005 to 2010, twenty countries will join forces with
French research organizations such as CNRS, CNES, IRD, Ifremer and Météo France in order to better understand the African monsoon.1

Weakened in strength, suffering strong changes from year to year, and poorly understood, the West African monsoon is not at its best. Jean-Luc Redelsperger offers some background on the issue: “Yes, monsoons exist in Africa, not just in India. What is striking is that they provide the only rain Sahelian countries get over the entire year.”2 Since the 1970s, however, researchers have observed persistent drought conditions in comparison with 1950s and 1960s levels—a call for concern. “The African monsoon has not disappeared, but rainfall volume has dropped and its northward reach has receded.” The rain begins in February or March in coastal countries (Ivory Coast, Ghana, Togo, Benin, and Nigeria), moving progressively toward the northern Sahel countries (Senegal, Burkina Faso, Mali, Niger, Chad). The monsoon season, which peaks in August, very rarely reaches the Sahara. On the whole, an area of more than 5,000,000 km2 is regulated by monsoons. Redelsperger, a CNRS researcher with the Meteorological Atmosphere Study Group, is in charge of coordinating a national and international scientific program called AMMA (African Monsoon Multidisciplinary Analysis) to deal with this wet weather issue.3 The initiative will proceed by studying monsoon variations. From one year to another, the number of rainy episodes varies significantly, alternating with completely dry periods. Geographical changes are also important. Scientists are trying to understand these variations in order to develop models and help prevent dramatic consequences on local health, agriculture, and water sources.

Initiated five years ago by French researchers, AMMA is also funded by the European Union under the 6th Framework Program.4 Today, this multidisciplinary program has an international scope, spanning Africa, Europe, and America. A total of twenty countries—twelve of which are in Africa—are involved, bringing together sixty research laboratories.5
What exactly is a monsoon? Simply put, it is a giant sea breeze. When the continent gets hot enough, airflow carries moisture from the ocean towards land, from south to north. The moisture helps form clouds, of which only a small proportion (10 to 15%) will generate significant rainfall. Scientists track the appearance of thunderstorm lines, which move very quickly and can cross the entire African continent in a matter of days. For most of the year, the sun does not heat the continent enough to trigger the flow of moist air.

What is the interest in studying African monsoons over those occurring in Asia, Australia, and South America? Over the past thirty years, observed rainfall levels in the Sahel have dropped by 20 to 50%. This persistent diminishing is probably linked to the warming of the Atlantic in the Gulf of Guinea and to changes in land surface due to massive deforestation over the past 50 years, and the use of land for agriculture. In addition, “it is 'easier' to study the monsoon in West Africa,” explains Redelsperger. Scientists must unravel all the factors that have an impact on the water cycle and the radiation budget,6 with respect to the ocean (temperature, salinity, sea currents, etc.), the land-surface (vegetation, hydrology) and the atmosphere (clouds, chemistry and aerosols).
Observation began in 2001, focusing on three sites in Mali, Niger, and Benin. Surveillance of West Africa was reinforced in 2005 to enable careful analysis of change in parameters on a year-to-year basis for three years. It will be intensified during 2006 through the use of airplanes, ships, balloons, sounding devices, lidars, and ground stations.7

“Seven hundred people will work for the program, both directly and indirectly. Our community, already international in nature, has a growth outlook for the future,” enthuses Redelsperger. Recently the program has been rounded off by the introduction of new subjects such as the impacts on agriculture and health. AMMA provides the ideal framework for modeling recurrent epidemics such as meningococcus meningitis that affect many thousands of people living in Sub-Saharan Africa's “cerebrospinal meningitis belt.” Another targeted epidemic is malaria: in a hot atmosphere, heavy precipitation and water pooling encourage proliferation of mosquito larvae, carriers of the disease.

The first international AMMA conference took place in Dakar last November. More than 250 scientists participated to overview the 2005 campaign and finalize programming for 2006.

Magali Sarazin


Notes :

1. CNES: Centre national d'études spatiales, the French space agency; IRD: Institut de recherche pour le développement: Research institute for development; Ifremer: French research institute for sea exploration.
2. Western Africa, countries located between the Sahara and the Atlantic Ocean.
3. Joint lab: CNRS / Météo France, Toulouse. View web site
4. Contract 4089-2.
5. Thirty-five research bodies outside of France are involved.
6. Radiation budget: difference between the solar energy reaching the Earth and the energy emitted by the Earth.
7. Lidars operate in the same way as radars, but with light waves emitted by a laser (ultraviolet, visible, infrared) instead of radio waves.

Contacts :

Jean-Luc Redelsperger
View web site


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