Monsoon in West Africa: Classic continuity hides a dual-cycle rainfall regime

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.

Rainfall variations within the first phase of the monsoon, which corresponds to a stable oceanic regime, are only small. However, the second phase, regulated by the prevailing conditions in the continental regime, shows wide inter-annual differences. So much of the Sahel’s rainfall is the fruit of a continental-derived cycle – 90% in fact – that any loss of monsoon intensity during this second phase can favour severe drought. The duration of the rainy season did not, however, vary much over the two periods, the research team found. The drought is therefore expressed by a lengthening of the average time between two rainfall events, which increases the risk of a dry spell intervening at the core of what should be the rainy season and rendering crop plants such as millet more vulnerable to stress.

Since a severe drought that hit the Sahel between 1971 and 1974, several investigations have been devoted to determining the different factors (relief, oceanic parameters and so on) that could explain this climatic event. These latest studies have prompted IRD researchers to postulate a discontinuous, non-linear, model for monsoon development. Such a model broadens the field of investigation in terms of potentially relevant parameters in drought development in West Africa. The approach the team is adopting favours advances in the identification of mechanisms behind the decline in rainfall-event frequency after the monsoon jump. It also provides the foundation for the international programme AMMA* devoted to improving systems for predicting the monsoon.

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* AMMA = Analyse multidisciplinaire de la mousson africaine (Multidisciplinary Analysis of the African Monsoon), a programme launched at the beginning of 2002 on the initiative of the IRD, the CNES, the CNRS, Météo-France and the African organizations ACMAD (African Center for Meteorological Applications to Development) and AGRHYMET (Centre d’Agro-hydro-meteorologie, Niamey, Niger)