In West Africa this rainfall gradient extends over a remarkable short distance, including the Upper Niger Basin and Inner Niger Delta. The air circulation around the equator is the main driver of the annual rainfall sequence in Africa. Ascending warm and moist air immediately north and south of the equator is sucked into the itcz and transported at altitudes of 10-15 km further north and south of the equator. To compensate for the rising air in the convergence zone, the northern flow descends in the desert zone, normally centred between 20° and 30°N. Descending air heats up as the pressure increases, becoming under saturated with water vapour and leading to the typical clear skies and general aridity of the Sahara.
Annual rainfall in West Africa (left) in two different years (2010 and 2012) based on daily satellite measurements given on the Famine Early Warning System (FEWS) website. The average annual rainfall in Africa in 1960-1990 (right); source: Hijmans et al. (2005). The Upper Niger Basin is indicated. The satellite-derived rainfall estimates are correct for the Sahel zone and also for the Upper Niger Basin, but systematically too low along the Atlantic coast between Senegal and Nigeria.
This air circulation system, known as the Hadley cell, ensures that the prevailing wind over the Sahara, the Harmattan, always blows from the northeast and not from the north. The Harmattan, a well-known phenomenon in West Africa, brings dry, dusty air to the Sahel and further south. When, during the northern summer, the sun is overhead in the Sahara, a low pressure belt forms over the Sahel, bringing clouds, rain, frequent thunderstorms and a monsoon from the southeast.
Schematic cross section of the air circulation system of the Intertropical Convergence Zone (itcz). B Low pressure zone with ascending warm and humid air, H high pressure zone with relative dry descending air, causing prevailing winds over het Sahel (Harmattan). Taken from Beintema et al. (2009).
The weather in West Africa shows a distinct seasonal differentiation with, in general, a hot and dry season from November to April, and a humid to rainy season from May to October. This seasonal pattern is the result of the seasonal shifting of the itcz, north and south of the equator. The itcz moves according to the position of the sun, between the Tropic of Cancer (21 June) and the Tropic of Capricorn (21 December).
The seasonal pattern varies accordingly from north to south. In the southern part of the Upper Niger Basin, in the mountainous region of the upper reaches of the Niger, the rainy season extends from March to October. In Tombouctou and the Gourma region in Mali rainfall, if any, falls from June to September. In all regions in West Africa, the maximum amount of rainfall falls in August.
The rainy period in the Sudan and Sahel zone is limited to a few months, typically punctuated by local downpours and tropical thunderstorms causing a huge local variation in daily rainfall. As a consequence, adjacent rainfall stations can show remarkable differences over the entire rainy season. The existence of such large variations requires many rainfall stations to record adequately the annual variation of rainfall in West Africa. For historical data on rainfall see Long term rainfall data.
In order to obtain a reliable picture of the annual variation of rainfall over time, an analysis of rainfall data is done in several studies. To remain valid, such analyses must be derived from data from a large number of rainfall stations, in order to take into account the substantial local variation. Missing data in series of average annual rainfall tallies, based on a set of different stations, corrupt the results in analyses. This problem is usually resolved by standardising the data for all rainfall stations. First, the average yearly rainfall is calculated. Subsequently, the rainfall for each year is converted into the difference relative to the long-term average, divided by the standard deviation – this is known as the ‘anomaly’ or ‘rain index’.
The Sahelian rainfall index is shown here for the years 1900-2005 as annual rainfall anomaly (%). The distinctive pattern of wet and dry years, with sometimes very large differences between consecutive years, is characteristic for West Africa, and the Sahel-Sudan region in particular.
Annual rainfall in the Sahel (1900-2005), expressed as percentage departure from the average calculated for the 20th century. The smooth curve gives the 9-year running mean. Source Zwarts et al. (2009).
The Sahel and Sudan zones in West Africa not only experience unparalleled rapid transitions from dry to wet, but also has suffered a historically unprecedented decline in rainfall since about 1960, with distinct periods of long drought. During the 20th century, three periods of drought can be discerned: the first two, in 1900-15 and 1940-49 respectively, were followed by periods of improved rainfall. Again 30 years later, another drought occurred, but instead of the expected recovery in rainfall, there was a further decline in rainfall until 1984. This last period is known as the Great Drought in Africa – La Grande Sécheresse (1972-1993). Since then, rainfall has gradually improved.
Sources and more information:
- CILSS 2016. Les Paysages de l'Afrique de l'Ouest : Une Fenêtre sur un Monde en Pleine Évolution / Landscapes of West Africa. U.S. Geological Survey EROS, 47914 252nd St, Garretson, SD 57030, UNITED STATES.
- Beintema, A.J., J. van der Kamp & B. Kone (éds.). 2007. Les forêts inondées: trésors du Delta Intérieur du Niger au Mali. A&W-report 964. Altenburg & Wymenga conseillers écologiques, Veenwouden. Wetlands International, Sévaré. Pays-Bas / Mali.
- Hijmans, R.J., S.E. Cameron, J.L. Parra, P.G. Jones and A. Jarvis, 2005. Very high resolution interpolated climate surfaces for global land areas. International Journal of Climatology 25: 1965-1978.
- Zwarts, L, Bijlsma RJ, van der Kamp J, Wymenga, E 2009. Living on the Edge. Wetlands and birds in a changing Sahel. KNNV Publishing, Zeist. p. 1-564.