Overview of the West African climatology
Etymologically, the climate means the inclination of the sun's rays (coming on the earth's surface). In a simplified way, there is here the idea that the sun is the external source to the earth system engendering variations in heat flow, mainly at the atmospheric level. The statistical values of atmospheric parameters such as temperature or precipitation are used to define climate zones. Based on precipitation and temperatures, West Africa can be divided into two broad categories: dry and tropical plains of the Sahara through the Sahel (the "shore" of the Sahara), and down to coastal countries. The «desert and semi-arid» subdivisions compose the first category, while the «equatorial, monsoon and savannah» types are part of the second (Koopen classification).
In West Africa, for CILSS / ECOWAS member countries, we can oppose coastal regions, enjoying a warm and humid climate to Sahelian regions, with average drier and warmer climate (Figures 1 and 3).
Figure 1: Diagrams showing mean temperatures of Tombouctou (Mali, solid line) and Banjul (Gambia, dotted line); (source: the CRA climate database).
-West African Monsoon and rainy seasons
Photograph 1: Sandstorm ("Haboob") before a stormy downpour, typical during the rainy season in the Sahel. Photography: L.Labbé, ACMAD, Niamey, June 2012).
Summarizing, we can say that the rainfall is due to the seasonal movement of the Inter-tropical Convergence Zone (ITCZ) which performs a seasonal flapping between the Gulf of Guinea and Sahel.
Figure 2: Extracted from the CSAO / CILSS / FAO / ECOWAS Atlas and showing the average seasonal flapping of the ITCZ.
Thus, we can simplify by saying that there are countries with unimodal rainfall patterns (eg Niger, one rainy season per year, centered in the month of August) and countries having two rainy seasons (Bimodal pattern type); one rainy season in the month of June and a short rainy season around the month of October (see Figure 2).
Figure 3: Comparison of mean monthly rainfall in Niamey (Niger) vs. Cotonou (Benin) (source CRA and WMO)
Although West Africa has 11 river basins (Figure 4), we are interested in this part to the Niger, a major river which flows through four countries and over more than 4200 km from its source to its entrance into the Atlantic in Nigeria; although it receives water from 8 countries.
Figure 4: The main river basins in West Africa (source CRA 2012)
The rainfall in the countries of the lower basin primordially affects the great flood of Niger, although local rainfall also plays a role (Figure 4).
It is noted that the local rainfall results in a double river flood: a local flood in August appears at Niamey when most of the river water is concentrated at the beginning of the following year, and when the water resulting from the monsoon rain from the middle and lower basin countries reaches Niamey. Additional information can be found in following pages: Water Management and Adaptation of irrigated agriculture
Figure 5: Evolution average flow rates over three time periods in Niamey (source CRA, Niger)
In a context of Climate Change:
The chemical composition of the atmosphere is very important because through complex process of reflection / refraction in particular, the different types of heat (latent and sensible, in particular) are trapped in the lower layers of the atmosphere, creating a ”thermal surplus” called Greenhouse Effect. Without this effect, the Earth's mean temperature which is 15 ° C would be -18 ° C.
The water vapor is the main component of the atmosphere and thus the main gas contributing to the greenhouse effect. However, since the industrial era and especially the beginning of the extraction and consumption of fossil fuels and related combustion phenomena, there is an increase in the concentration of greenhouse gases such as Carbon dioxide (CO2) or methane (CH4). Thus, this increase has a corolaire, the global temperature increase that is also called global warming. This is proved, for recent evidence, the month of April 2012 is the warmest on record at the global scale (+0.89 ° C compared to the mean of the April months of past century (source of NDCC NOAA).
In the Sahel, there is also a significant increase in minimum temperatures compare to the maximum but above all an increase of high temperatures greater than the global level increase (IPCC, 2002).
In terms of precipitation, the graph below clearly shows the recent alternation of three contrasted rainfall periods: a wet period before 1970, and a drier period until the early 2000s and finally a return to average wetter conditions but still contrasted in the first decade of the 21st century.
Figure 6: Climate Data showing Climate Change (A. Ali, special bulletin CC CILSS 2010)
The river flow has also undergone temporal variations, particularly due to the complex phenomena of climate variability listed above, which may be associated with anthropogenic pressure (population growth has resulted according to zones to phenomena of sand encroachment vs. over digging, construction of hydroelectric dams and irrigation facilities) and changes in the use of natural resources (water, soil, vegetation). This entails the need for adaptation to climate change.