Climate information

The climate of West Africa is dominated by the seasonal movement of the West Africa monsoon and it is a general consensus that differential changes in SSTs (Sea surface Temperature) of the Atlantic (North and South) and the Indian Ocean represent the primary driver of climatic conditions in the region.

In the three TroFCCA target countries, annual distribution of rainfall, like other parameters, is zonal. Precipitation ranges from 2200 mm (In South West Ghana) to less than 50 mm in the desert of Northern Mali. A monomodal rainy regime peaking in August, varies from less than 3 months north of the 14th parallel to over 9 months along a bimodal coastal Ghana. The Sahel, receives around 200 mm of rain a year but unevenly distributed, mostly between June and September. While climatic conditions in Southern Ghana are relatively stable, variability increases as one move North-wards. Hence, in the Sahel the standard deviation of mean annual precipitation could be over 50%. Mali has a climate that is very similar to that of Burkina Faso. The potential evapotranspiration depends on temperature, radiation, wind speed and relative humidity.

Generally speaking, the temperature in both Burkina Faso and Mali is relatively high most part of the year, and maximum temperature ranges between 45º C to 48º C, with heat peaks in May to October, when humidity and cloud are greatest. During the low-sun period from November to March, drier air is brought by the North-East Harmattan winds laden with dust, with a clear decline in temperatures in January from the South to the North and low night temperatures (average 100 C) in the desert due to high out-going radiation and relatively low day temperatures during the period. In Ghana, temperatures ranging from 18º C to 40º C or more are common in the Southern and Northern parts respectively.

Extreme events

The Sahelian droughts

The 20^th century droughts across the Sahelian countries had devastating effect on local populations and livelihoods with estimated deaths of over 500,000 people. The reduction in precipitation across the Sahel between the 1960s and 1980s preceded a period of sustained precipitation increase during the 1920s to 1950s. This period is considered the largest multi-decadal regional climate perturbation ever observed (Hulme 2001) and provides a good example of a directional change in regional climate.

Through the analyses of rainfall regimes of the sub-region found that the drought years were related to the number of rainfall events than their relative size (Le Barbe 2002).

Protracted dry spells are not unprecedented in West Africa even in recent times.

Dust

Dust is considered to be a suppressant of rainfall but of lesser extent than land surface. Once drought conditions are established, they may be reinforced by high atmospheric dust loadings. The relationship between atmospheric dust-loading, vegetation cover and climate goes back to the Holocene period whereby warmer, wetter conditions could decrease dust-loading to offset any increases in dust generation caused by changes in land-use and anthropogenic devegetation. Lau et al.  postulated that dust layer over the ocean could intercept sunlight reaching the ocean, thereby causing the SST beneath the dust layer to be cooled. The Saharan dust may also provide a positive feedback mechanism, further sustaining the Sahel drought. It appears that air-borne dust causes droplet size in clouds to drop below the 14 micron radius needed for the onset of rainfall (Foley 2003). Large inter-annual changes in dust carried by wind across the Atlantic from Africa are negatively correlated with rainfall in the Sahel.

Following are summary tables of climate conditions in the TroFCCA countries: