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  CLIMATE  -  Deciphering the timing of the monsoons

Pour percer les secrets des caprices de la mousson, pas d’autre solution que d'ausculter ce phénomène en détail. C’est pourquoi les scientifiques sont allés placer leurs capteurs dernier cri au coeur même du terrain. Plusieurs stations réparties sur le sol africain sont ainsi dédiées aux échanges continent/atmosphère.

The German Falcon 2
Two types of aircraft are used in the framework of the AMMA project. The French ATR 42 (here, at touchdown) flies at between 300 and 7,000 metres. The German Falcon 2 (the inside of which can be seen opposite) is capable of flying at 14,000 metres and is primarily dedicated to studying winds at altitude. These are measured with the aid of a powerful laser beam. These two pieces of equipment complement each other in measuring dozens of parameters, from solar radiation to the concentration of chemical components and the diameter of aerosol particles.

© AMMA/Photo P. Taburet (Météo-France)
In an isolated part of the countryside near Djougou, in Benin, there is an impressive battery of equipment for storing massive amounts of data. Quantities of sulphur oxides, nitrogen oxides, ozone, carbon monoxide, carbon dioxide, particles, etc., are measured here.

Sitting opposite a machine called the Cloud Condensation Nuclei counter, Laurent Gomes, researcher at the CNRS in France, explains that this apparatus makes it possible for us to distinguish which of the particles suspended in the air are liable to form condensation nuclei. “Some particles such as soot are termed hydrophiles. In other words, they are good at condensing water vapour. Conversely, mineral particles are more often hydrophobes. To complicate the situation, these properties may develop during their journey in the atmosphere. If a mineral particle is in contact with salt molecules emitted from sprays, it will gradually become a hydrophile. This appliance takes dust particles present in the air and sends them into a chamber where we can expect the formation of water vapour. All we have to do then is count the drops that form on the sample. At present, of 10,000 particles in suspension, 1,700 are hydrophiles.”

On Earth as it is in Heaven
At the same time, one of the aircraft in the AMMA fleet is equipped with the same apparatus, measuring the condensation capacity of aerosols at a defined altitude. This allows for the properties of dust particles circulating on the ground to be differentiated from those in the upper layers of the atmosphere. The presence of five fully equipped planes, probing the centres of tropical storms as well as the heights of the troposphere, is one of the project’s strong points.

500 km away, near the village of Komakoukou in Niger, we find Jean-Louis Rajot’s laboratory, which is entirely dedicated to the study of aerosols, or all solid and liquid particles suspended in the air. These particles are collected, sorted by diameter, counted and chemically analysed. One of the objectives is to understand the proportion of dust of local origin compared to that coming from elsewhere. Researchers have placed instruments in cultivated fields and on fallow land in order to carry out differential measures, with the experiment showing that fallow land suffers hardly any erosion, while cultivated land areas concentrate this phenomenon. Another piece of apparatus collects raindrops in a sequential manner – the last drops justify special analysis, as they are the ones most likely to contain materials that make condensation possible.

Understanding the water cycle
the Ronsard radar

The role of the Ronsard radar, at the Centre for the Study of Terrestrial and Planetary Environments, is to measure the speed, intensity and size of precipitation systems across the region. It performs genuine “cross sections” of these huge tropical storms. It is a valuable tool for scientists trying to decipher the dynamic.

© AMMA/Photo P.Collot (CNES)

Different place, different concerns. On the Benin side of the River Ouémé basin, a plan of action has been set up in order for detailed analysis of the water cycle to be conducted on a local scale. Rainfall, river discharges, the height of water tables and the amount of water present in the soil, are continually examined by scientists, supported by African collaborators from the Benin hydrology service.

Several slopes, some of them intensely cultivated and others that are not, are being monitored together and compared so as to be able to deduce the impacts of agricultural activities. The nature and density of the vegetation cover form part of the measured parameters. Estimates must be made of the amount of infiltrated water, the amount transpired by plants, the amount that evaporates naturally and the amount reaching rivers.

The initial results (obtained by a chemical comparison of water from rivers and groundwater) seem to indicate that while the water table in this region is situated at a depth of around fifteen metres, a much shallower water table of less than two metres is established during the rainy season. This temporary layer feeds flows of water, while the deep and permanent layer is accessible to trees – but not to cultivated land. If this hypothesis is confirmed, the replacement of the forest by cultivated land, which draws on the superficial layer, could result in a reduction in the portion that feeds into rivers. This type of information is particularly useful for modelling the future of the water cycle in the region. Analogue stations have been set up in Mali and Niger in order to make similar assessments based on different climate and soil conditions.

A human adventure
Conducting a programme on the scale of those carried out by the AMMA is no mean achievement in poor countries with a hostile climate. But the adventure has a human focus, as well as a scientific one. Luc Redelsperger is happy to explain: “When atmospheric specialists understood that their data could be used to combat meningitis, and not only to improve understanding of the subtleties of the climate, their enthusiasm was palpable”.

Even though AMMA scientists launch balloons in torrential rain, burn under the sun while measuring dust, interview hundreds of patients in clinics, or fly for hours in planes transformed into furnaces, they all seem eager – whatever their temperament, nationality or discipline – to contribute toward developing a continent that is too often abandoned and forgotten.

  An important ocean section  
  The ocean/atmosphere interaction is subject to particular attention within the AMMA project. It is the thermal gap between the ocean and the continent that causes humid air to come onto the land from the Gulf of Guinea - a gulf that is therefore the source of practically all energy and water released by the monsoon. This part of the tropical Atlantic is the seat of an annual phenomenon that oceanographers define as an upwelling: a rise to the surface of colder water, coming from the depths, which quickly reduces the temperature on the surface of the ocean by around 3°C.

The links between such an upwelling (its timing, intensity and methods) and the monsoon seem to be important, to the point where it might be a key element in forecasting rains. To this end, within the framework of the AMMA, three boats will supply data from the Atlantic Ocean (a French boat, Atalante, the German Meteor, which will cross from Recife in Brazil to the islands of the Cape Verde, and the American Ron Brown, which will move between America and Africa). Moreover, a network of fixed buoys has been set up, providing information on the entire water column, while floats that drift with water masses periodically send data.

  Covering an entire continent  
  The unprecedented instrumentation of AMMA is deployed with a strategic concern worthy of a military campaign. A series of relatively simple parameters are monitored over a long period (2001-2010), while another, less important, dataset is only subject to monitoring for three years (2005-2007). Finally, several so-called “intensive observation” periods took place in the course of 2006, during which the scientific arsenal deployed (planes, boats, etc.), reached its peak.

Observation sites fixed in space respond to these observation phases over time. This means hydrometeorological measures are very dense on restricted sites, such as certain basin slopes, which have the lowest levels of instrumentation, and are judiciously distributed over the continent. The programme thus functions through “transects” (large North/South or East/ West lines) along which the measuring sites are concentrated. These directions are the same as the flow of the monsoon for the first, and those followed by squall lines for the second, putting scientists in an ideal position to track the evolution of the monsoon.

  And the cyclones of the Caribbean…  
  When the huge storm systems or squall lines set off over the Atlantic Ocean at the end of summer, they first encounter a cold water zone (an upwelling) off Senegal, and therefore generally tend to regress. But if they have sufficient energy and speed, they can even withstand the 500 km or so of relatively cold ocean and reach zones closer to the Caribbean, where the Atlantic reaches its maximum heat. Then, in the Gulf of Mexico, water can exceed 30C, which represents a huge release of latent energy in the form of vapour. These systems of African origin may then be set off again, fed by the abovementioned energy, to form particularly violent cyclones, which then come crashing into the coasts of the United States. The participation of several important American scientific institutions in the AMMA is justified, not only by scientific interest in the project, but also by the desire to know more about this cyclogenesis which, according to some authors, could be enhanced by climate warming in years to come.