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Title   Waiting for the rainy season

The African Monsoon Multidisciplinary Analyses (AMMA) international programme presents a scientific challenge with enormous repercussions for the black continent. Reports and analysis.

Village South of Niamey, Niger
Village South of Niamey, Niger. The local inhabitants make their living growing millet and rare vegetables and raising livestock.
© Yves Sciama
Bazimbouzou, some 60km from Niamey, the capital of Niger. It is barely ten o’clock in the morning, but already it is blazing hot. The air shimmers above the golden sand, on which a few sparse bushes are scattered here and there. Some soft green shoots, just a few centimetres high, seem reluctant to develop in the overheated atmosphere. The plants in this millet field, the only cereal able to survive in the extreme conditions of the Sahel, should be nearly 40 centimetres high by this time of year. “The rainy season is late this year,” explains Thierry Lebel, hydrologist at the Institut de recherche pour le développement (French Research Institute for Development – IRD). “People here are certainly aware of the hazards posed by the climate and have strategies to try and combat them. When they plant their fields after the first rains they keep some seeds back in case dry weather ruins this initial attempt. They then try a second time. But now the third sowing has failed. Farmers have planted for the last time this season, because soon it will be too late to try again. There is no longer time for the seeds to reach maturity before the dry season returns, and the harvest will be lost.”

Thierry Lebel is not only a specialist in the hydrology of the region, he is also one of the chief officials and founders of AMMA, an international scientific project which operates on an unprecedented scale, at least in Africa. AMMA brings together a large number of teams already working in the region on similar problems. “We have benefited from the strengths of each group,” comments Jan Polcher of the Laboratoire de Météorologie Dynamique (Dynamic Meteorology Laboratory – LMD) in Paris, coordinator of AMMA-Europe, the European part of the project. “The French, for example, have particular expertise in the areas of surface and hydrology, the British in chemistry and atmospheric dynamics, while the Danes, Germans and Italians specialise in impacts.” A spectacular scientific armada has been deployed, complete with fully equipped planes, oceanographic ships, radars, lasers, photometers and other radiosondes, not to mention the five satellites supporting the project. AMMA will, eventually, involve nearly 500 researchers from around thirty countries at an overall cost of over 50 million euro.

Why is there such a concentration of minds in a region, which, on so many other levels, has a shortage of everything? Here in the Sahel, and more generally in the whole of West Africa, the phenomena of climatic changes are a matter of life and, sadly, death. The rainy season (specialists prefer to talk about the monsoon season) governs everyday life. The intensity of the rains, the date they start and end, and their regularity are decisive factors, influencing agriculture, health, water resources, and even interethnic tensions. “Here, when the rains are good, every thing is fine.” This is how Moussa Aksar, Head of the Niger weekly news paper L’Evénement, sums up the situation while sadly contemplating the ongoing drought.

Inexplicable and unforeseeable change
The surprising climatic variation in this region remains an enigma to scientists. To this day, we still cannot explain why and under what conditions one year might see twice as much water as the previous year. In the same way, we do not understand the reasons for the dramatic shortage of rain since the 1970s. On a map of the world showing the evolution of worldwide precipitation over recent decades, Thierry Lebel points to the main red zone, covering almost the entire western part of the African continent like an alarm. “The weakening of the African monsoon is the most important global climatic signal of the last half century,” the researcher warns. “The overall drop in rainfall has reached 30% - and it doesn't help that there is such a large fluctuation between the good and bad years. Bad years can cause major tragedies, mirroring the terrible famines of the 1970s.” If the phenomenon is continental, and therefore a source of tension in several countries, the most severe impact will be in the Sahel, which has the most vulnerable populations. There has, for example, been a 60% reduction in the water level of the River Niger at Niamey since then.

The objective of AMMA, therefore, is to decipher the mystery of the African monsoon's temperamental evolution and discover the root causes. This is not that easy. The phenomenon is, in effect, an atmospheric process of continental magnitude and of enormous complexity. The monsoon is controlled both by the global climate, its evolutions and periodic cycles, and by human activities, which have been throwing things out of balance for decades.

The French Space Agency's (CNES) boundary layer pressurised balloons (BLPB)

The French Space Agency's (CNES) boundary layer pressurised balloons (BLPB) are designed to drift with air masses at a constant altitude of around 1,500 m. They are tracked using GPS. Measuring 2.5 m in diameter and equipped with 3.5 kg of instruments, they gather information about the direction of such air masses, along with various other data (temperature, humidity, pressure, etc.). These balloons, like the sensor balloons, were launched from Cotonou Airport, Benin.
© AMMA/Photo P. Collot (CNES)

Serving the population
We are not just dealing with ethereal research in the upper reaches of pure science. Even if data from AMMA makes the advancement of worldwide climatology possible, it should not be forgotten that the main objective is to assist the people of Africa. Jan Polcher states this clearly: “We have made a commitment to our financers to produce useful science which will improve decision making by those in charge in these countries.” By understanding the monsoon, its interaction with the ocean and the great global climatic cycles, we are also advancing along the path toward forecasting it.

The first steps have already been taken along this path. André Kamga, a young official in charge of the African Centre of Meteorological Application for Development (ACMAD), one of AMMA's African partners, points out that there are already some seasonal forecasts about the intensity of the monsoon, based on various climatic models, whose performances are satisfactory. “The problem,” he says, “is that farmers tell us the information we are giving them is not what they need. They need to be provided with forecasts on late starts to the monsoon, breaks in rainfall, and the risk of any early end to the rainy season. We must learn to provide them with this.”

In a country like Niger, where agriculture represents more than a third of GDP and employs 90% of the workforce, farmers would like to know, for example, when the first rains will fall, and whether the time has come for sowing or if the monsoon has not yet set in. Knowing the probability of long dry spells makes it possible to better manage seed stocks. Finally, the choice of the date of harvest also depends on weather conditions at the end of the monsoon.

How can we predict epidemics?
AMMA includes, moreover, an Impact of the Monsoon section, which goes far beyond merely agronomic considerations. The quantity of rain and its distribution are important in different ways, especially with regard to health. In Niger, several researchers are endeavouring to find out more about the links between the monsoon and various diseases, including malaria, the principal scourge in this region. “In the rainy season, when conditions are most favourable for mosquito reproduction, we observe a peak in the transmission of the disease,” explains Dr Jean- Bernard Duchemin, the Head of the Parasitology Unit at the Centre de recherches médicales et sanitaires (Centre for Medical and Health Research – CERMES), which comes under the authority of the Niger Ministry of Public Health. “However, the situation is not that simple. The Sahel strip displays strongly heterogeneous transmission – both in terms of space and time – and again the logic of this escapes us. Temperature, hygrometry, and possibly other environmental factors should also play a role. In Guyana, for example, we see fairly constant rainfall, while transmission of malaria follows periodical cycles.”

The goal of the CERMES scientists, which explains their participation in AMMA, is to define predictive models which, depending on the characteristics of each monsoon, make it possible for risks to be quantified and geographically localised. “This could allow for timely interventions to deal with the stages at which contamination is at its most acute,” Jean-Bernard Duchemin hopes. “For example, at one of our study sites we have located the temporary ponds that appeared in the rainy season. They are little used and we suspect them to be an important location for contaminated mosquitoes. If this is confirmed, we could suggest that villagers fill them in. We could also use preventative spraying to deal with inhabited areas that are identified as being at risk, or even distribute impregnated mosquito nets in such zones.”

But malaria is far from being the only threat linked to environmental conditions. Most viral diseases transmitted by insects, like dengue fever, yellow fever and Rift Valley fever, for example, are dependent on the surroundings, as are their carriers. There are also water-borne diseases, like dysentery and cholera, which may suddenly emerge after floods or because of drought, as the latter often forces people to drink dirty water. Here too, forecasting might be the key to the optimal use of medical resources that are always insufficient.

A water management tool
The usefulness of accurate forecasting of the monsoon does not stop there. Combined with improved management of the region’s hydrology, which should be another result of the work carried out by AMMA, it would make more optimal use of water resources possible. The sloping basin of Ouémé (Benin), one of the project's intensive observation sites, has, for example, been bristling with hydrological instruments. As Arnaud Zinnou, an engineer at the directorate general for hydraulics in Benin, explains: “There are at least two potential dam sites on this river. A good awareness of the regional water cycle and, if possible, the future evolution of the monsoon, would be valuable in measuring the size of such installations in the most judicious manner.” Investment in dams affects irrigation management, the country’s energy supplies and ecological balance, and it will therefore be up to the scientists to indicate the quantity of water that can be withdrawn and stored, and also to specify the timing of such actions, in order to minimise any environmental impact of these works.

Protecting the future
It goes without saying that this huge deployment of forces mobilised for the AMMA project must have some role in the future. As the Ivory Coast meteorologist Abdoulaye Kignaman Soro, CEO of ACMAD, points out: “We have already come across important scientific issues which we have not been able to follow up in any way. In the end the instruments and researchers were withdrawn and we did not even have access to data. The AMMA certainly seems to be setting out on a different path. In time, we wish to develop our own capabilities in matters of meteorology and to convince our governments that this is an important step.” Africa today has some failings in this area: there are fewer weather stations per square kilometre on the continent than anywhere else on the planet (eight times less than the minimum recommended by the World Meteorological Organisation) and local institutions suffer from a severe lack of resources. It is this situation, which is prejudicial to worldwide climatology and especially to Africa’s development, that those in charge at AMMA would like to see changed. But do they hold all the right cards?

Version imprimable
Dossier 1 2 3 3
  Waiting for the rainy season
  European Involvement
  Extreme climatic tensions
  Deciphering the timing of the monsoon

  The West African monsoon

“A monsoon is any seasonal rainfall phenomenon resulting from the difference in temperature between a continental expanse and an ocean,” explains Jean-Luc Redelsperger...
  The Harmattan and meningitis

>CERMES, the African body for medical research in Niger, is particularly involved in the fight against meningitis, an illness which is fatal ten percent of the time in this country, and for which a vaccine exists. “This disease appears when there is wind, dry conditions and dust all at the same time, and stops with the first rains,” states Jean-Bernard Duchemin...

Dossier 1 2 3 3
  The West African monsoon

"A monsoon is any seasonal rainfall phenomenon resulting from the difference in temperature between a continental expanse and an ocean," explains Jean-Luc Redelsperger, who is part of a joint research unit run by CNRS and Météo France, as well as AMMA scientific management. “There is an Indian monsoon – the best known – but also monsoons in Australia, South America, in the South East of the United States, and even in East Africa, to mention but a few, each with its own particular characteristics.”

The African version is triggered by a thermal disparity between the colder Atlantic Ocean and the heart of the continent, where the overheated air is less dense. A flow of humid ocean air then rolls over the continent from the Gulf of Guinea in the South. This is accompanied by strong rains, which reach several metres per year, providing water for coastal countries and causing the formation of meteorological phenomena, known as squall lines, further inland. Precipitation results from hot, humid air rising towards cold, dry zones in the high troposphere. This rising air causes heavy condensation, and leads to a huge number of rainstorms. Given that it is accompanied by a release of heat, this phenomenon expands both outwards and upwards, reaching heights of around 17 km - the limits of the stratosphere. A European storm rarely exceeds an altitude of 8,000 m.

La mousson de l'ouest africain
© AMMA/Photo P.Taburet (Météo-France)

As it rises, this hot air chimney is struck by an altitude current at around 4,000 m, and then by a second at around 12,000 m, both flowing east to west. These collisions stabilise the rising air chimney (usually by tilting it), thereby preventing it from finally being suffocated under the precipitation it has generated and ensuring its longevity and rapid spread to the west. This series of phenomena results in a kind of giant storm forming a strip of between 200 km and over 1,000 km in length, accompanied by torrential rains and a huge release of energy in the form of wind and lightning. Such a system can remain active for several days, sweeping the entire continent until it emerges over the Atlantic Ocean. Most of the rain collected in the most arid zones in West Africa is generated by such squall lines. Some of the instruments used by the AMMA will record phenomena such as the speed and direction of winds at their centre, the size, shape and location of raindrops and ice particles, and the concentration of aerosols and trace gases.
Ballon d'héliumBallon d'héliumBallon d'hélium
This helium balloon, which is equipped with a number of sensors (temperature, pressure, ozone concentration, etc.), allows radiosondings to be taken from a vertical atmospheric profile. Climbing to an altitude of 30km, it sends data via radio at intervals of a few seconds. The AMMA project will extend the African radiosonde network to 17 stations.
© AMMA/Photo P. Taburet (Météo-France)

  The Harmattan and meningitis

CERMES, the African body for medical research in Niger, is particularly involved in the fight against meningitis, an illness which is fatal ten percent of the time in this country, and for which a vaccine exists. “This disease appears when there is wind, dry conditions and dust all at the same time, and stops with the first rains,” states Jean-Bernard Duchemin. It is therefore prevalent in the dry season and researchers have long recognised the existence of a “meningitis belt”. This shifts with the seasons and, driven by the Harmattan (a dry and dusty northerly wind), can reach cities on the South coast of the Gulf of Guinea, like Cotonou.

Doctors long believed that it was the dust that carried the germs, however recent studies suggest that many subjects are in reality healthy carriers of meningococcus, which occurs in the throat. The germ proliferates due to micro-lesions pro - duced by sharp, irritating dust particles which enter the body and activate the illness. “If we had a clearer understanding of these mechanisms, especially on the geographic level, we would be able to identify the zones at risk,” the doctor points out. “For the moment, vaccination campaigns are triggered when the number of patients exceeds a certain threshold. With improved knowledge of the phenomenon, we would be able to establish different thresholds and vaccinate earlier if there is a suspected risk of epidemic.”