Since the end of the 1960s West Africa has continuously been suffering hard drought. The rainfall deficit for the 1970s and 1980s, calculated to compare with the 1950s and 1960s, thus reached as high as 50% over the northern part of the Sahel. The hydrological cycle as a whole is affected by this drought, which results in serious consequences for agriculture and food security.
IRD researchers, aiming to understand the mechanisms behind this situation, examined rainfall data from 1950 to 1990. Two sub-periods emerge : a wet one (1951-1969), followed by a dry one (1970-1990). This finding led them to modify drastically the classic model which presents the monsoon as a process that unfurls in a continuous sweep from South to North. Two rainfall dynamics regimes are in evidence, distinct in time and in space, separated by a sharp transition. The mean date for this, a jump heralding the monsoon onset, is around 22 June, which proved to be highly constant for both the wet and the dry sub-periods.
The proposed model predicts that the first monsoon phase starts off from the Atlantic coast in February and propagates steadily northwards. By May it has reached the central Sahel (13°N longitude, Niamey). After a period of stabilization, rainfall abruptly becomes much heavier. It touches the whole Sahel zone simultaneously. A short dry season then appears on the coast before a second rainy season arises there from September, linked to the retreat of the monsoon towards the South.
Marie-Lise Sabrie | alfa
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