Knowing is half the battle
AMMA - the African Monsoon Multidisciplinary Analyses
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Significant inter-annual variations in rainfall in West Africa in recent decades have resulted in extremely dry years. The environmental and socio-economic impacts have been devastating. This rainfall variability has raised important issues related to sustainability, land degradation and food and water security in the region. |
Implementing relevant monitoring and prediction strategies is of vital importance. In order to reach the desired goals, the multidisciplinary research carried out in AMMA also needs to be effectively integrated with the local prediction and decision-making activity.
The West African Monsoon
The Western part of the tropical African continent gets most of its annual rainfall during the summer months from June to September. This rainy season is associated with the seasonal reversal of the winds in the lowest level of the atmosphere, which is called the monsoon.
In winter, the wind blows from the cool continent to the warm ocean. Following the Sun's movement during the course of the year, the continent warms faster than the ocean. This thermal contrast drives the surface pressure contrast between the ocean (high pressure) and the continent (low pressure) and the set-up of the monsoon circulation in northern summer.
Similar to a giant sea-breeze, at the beginning of the summer, the wind changes and eventually blows from the ocean to the continent. Over the ocean, the air parcels are moist and transported over the continent where this moisture is released, through deep vertical movements within convective storms, in the form of precipitation. This is only a simplified picture of the West African Monsoon, and other factors also contribute to it.
Noticeable anomalies in recent years
It is expected that the science from AMMA will contribute to a better understanding of the strong decennial variability that characterizes the West African Monsoon: an overall decrease in precipitation has been recorded since 1968, and there have been some extremely dry years between 1982-1984. In the past ten years, precipitation has again approached long-term averages.
"I would suggest two possible explanations for this phenomenon: firstly, there is the long-term evolution of global oceans whose sea surface temperatures impact the tropical atmospheric circulation. The warming of oceans in the southern hemisphere and cooling in the northern hemisphere explain most of the long-term Sahelian rainfall decrease. Secondly, inter-annual anomalies could be explained by the occurrence of phenomena such as El Niño. Also, the reduction in vegetation, partly due to anthropogenic pressure seems to have reduced rainfall," explains Serge Janicot, Research Director at the Institut de Recherche pour le Développement, Laboratoire d'Oceanographie et du Climat (LOCEAN).
Mr. Janicot is a meteorologist and a specialist in tropical issues, particularly the African climate. His role in AMMA is to study climate variability, the effects of ocean variability on the African climate, and the seasonal monsoon cycle. He is also actively involved in the AMMA radiosounding program, and coordinates various AMMA activities.
The West African Monsoon also contributes to many global weather phenomena. For example, it plays a role in the formation of US hurricanes, and transfers Saharan dust and aerosols in the atmosphere.
The AMMA observations
Dramatic rainfall changes are likely to occur in West Africa in the future, too, due to natural fluctuations or as a result of the global climate change. An essential step is to improve the ability to forecast the weather and climate in the West African region.
Building AMMA was possible thanks to the participation of the international scientific community, funds from the participating countries as well as the European Commission (EC), and the existence of observation networks in Western Africa. AMMA has reinforced, optimized and completed these networks in order to meet its objectives. The scientists involved wrote a proposal to the EC and received the green light for partial financing of the project. European countries, such as France and the UK, also agreed to share the estimated costs of 50 million euros. Vaisala is a member of the AMMA consortium funded by the European Commission.
“AMMA was launched in 2000. It was preceded by a project called the West African Monsoon Project, funded by the EC. During the West African Monsoon Project meteorologists, climatologists and hydrologists joined forces and realized the need for a field campaign,” Mr. Janicot says.
"Particularly important is the involvement and participation of the African community. Over 200 Africans are now involved in the AMMA activities. All ASECNA (Agence pour la Sécurité de la Navigation Aérienne en Afrique et à Madagascar) radiosounding stations have been upgraded, and new stations and equipment have been bought."
In the summer of 2006, 20 radiosounding stations were in operation in West Africa, performing one, two or four soundings per day with Vaisala equipment. In addition to this, two Intensive Observing Periods were carried out with eight soundings per day.
Good equipment is absolutely necessary
"The great challenge and question is what is going to happen to the observation frequency and the equipment in the long run, after the AMMA activities are finished. This is not only a question of money. Understanding the importance and benefits of good forecasts is vital here. For these, efficient networks and good equipment are absolutely necessary," Mr. Janicot says.
"Even if obtaining and maintaining a radiosounding network is expensive, it cannot be replaced by, for example, satellite observations. It is a very important investment, as in-situ measurements cannot be replaced by anything else at the moment."
"The best support we can offer in the future, apart from money, is enabling the local organizations to carry out the work themselves. We can provide education, and build and maintain strong links between the scientific communities, for example in the form of exchange scholarships. The number of motivated, better educated people willing to improve their countries' situation is growing. We need to support these efforts."
Impacts on people and lives
The impacts of the West African Monsoon are many. Agriculture and food security suffer along with an abrupt decrease in water resources. Cattle populations are diminished, and hydropower electricity plants are shut down. Malaria epidemics occur during the rainy season when the mosquito density increases in response to precipitation, temperature and humidity. During the dry season, meningitis affects 25,000 to 200,000 people annually. One of the AMMA goals is to understand the relationship between intra-seasonal, seasonal and inter-annual climate variability and these diseases.
Airborne observations
Mr. André Gribkoff is the Deputy Director of a French group called Service des Avions Français Instrumentés pour la Recherche en Environnement (SAFIRE). SAFIRE brings together the staff and three research aircraft of the L'Institut National des Sciences de l'Univers (INSU), Centre National de la Recherche Scientifique (CNRS), Météo-France and the Centre National d’Études Spatiales (CNES). SAFIRE’s main task is to operate the aircraft during experimental research campaigns. Mr. Gribkoff's role is technical: he coordinates the research aircraft operations and the necessary modifications to the planes. He also participates in the research flights themselves - for AMMA, Mr. Gribkoff has over 70 hours of airtime behind him.
"For the AMMA scientific measurements, I was in the Falcon 20 aircraft. We had one pilot, one co-pilot and three operators on each flight. Three to four other aircraft were also involved in the measurements. The Falcon 20 aircraft is multi-use, so the equipment varies depending on the campaign. We fly over both land and sea, and measure meteorological parameters around convective clouds and sometimes even inside the clouds, at an altitude of 10-12 km. The objective is to get as close as possible to the nucleus of the cloud, without too much risk," Mr. Gribkoff explains.
"The dropsondes we use for the measurements are all produced by Vaisala. During the last Enhanced Observing Period we also used a lidar and radar. The longest flight-times were around three-four hours, during which we dropped a maximum of sixteen sondes. We are able to launch one dropsonde every six minutes. The Vaisala dropsondes became a real favorite during the campaigns, as they are very easy to operate."
According to Mr. Gribkoff, it takes several years to prepare a plane for flights like these. "There are less than ten companies in the world who are able to provide planes like this. First we launch a tender to find a suitable carrier. The required modifications to the plane are included in the tender. With 35 years of experience in the field, I would say that realistically the purchase takes about one year, and the modifications in theory one year - but in reality up to three years. So it is a long and costly project that takes quite a bit of patience." Mr. Gribkoff was the Project Manager of the Falcon 20 modifications project.
Research aircraft are no small investments, either. For example, the Falcon 20 cost about 7.5 million euros. The purchase costs were covered by INSU and CNRS, and the operating costs are paid by the different participating organizations and the EC.
Further information:
http://www.amma-international.org/
Author: Marikka Metso, Vaisala, Helsinki, Finland