Rainfall Variability

Most of Africa is subhumid to semi-arid and experiences one or more prolonged dry seasons Figure 3.4 Illustration showing the mean position of the mid-level African Easterly Jet (AEJ) during August. Colours correspond to wind velocity in m/s shown in legend. Data from NOAA-CIRES Climate Diagnostics Centre.

quantity and duration towards the equator where the northward and southward passages of the ITCZ induce two rainy seasons each year (figures 3.5 and 3.6). The majority of continental Africa lies in tropical or subtropical latitudes with only the poleward extremes of the continent experiencing a true cold season as part of a Mediterranean-type climate (Hastenrath, 1991). Although many areas of the continent are arid or semi-arid, rainfall regimes are highly variable and locally complex, creating areas of desertification within regions that are ordinarily subhumid (Nicholson, 2001). Anthropogenic activity has further intensified the impact of such extreme climatic conditions (Balling, 2005).

Figure 3.5 Map showing mean annual rainfall distribution over the African continent. Seasonal positions of the ITCZ are shown by dashed grey (January) and dotted grey (July) lines. Modified from Nicholson (2000).

Environmental change in Africa is most directly related to rainfall and the variability that arises in the intensity and duration of the rainy seasons (Goddard and Graham, 1999). In contrast to the high latitudes, where temperature is the dominant climatic variable, low latitudes exhibit extreme variations in precipitation associated with the seasonal migration of the ITCZ and monsoonal circulation (figure 3.5) (deMenocal and Bloemendal, 1995; Nicholson, 2001). Rainfall regimes over Africa are particularly complex and are controlled by a number of dynamic factors such as the interactions of different air-masses, sea-surface temperatures (SSTs), monsoonal circulation and El Niño Southern Oscillation (ENSO) (Nicholson, 2001). It is important to distinguish that the factors which exert a control over mean climate in Africa, which is regionally diverse and highly variable, are different to the large-scale atmospheric and oceanic aspects of circulation which control the temporal variability of precipitation (Nicholson, 2000).

3.4.2.1 Continental rainfall variability

Climate in northern Africa is characterised by a wet-and-dry season regime (Buckle, 1996). Following a three-month dry period during boreal summer, the onset of the rainy season is controlled by the West African monsoon which results in the land to the north of the Sahara becoming progressively drier as precipitation is inhibited by a layer of dry air which sits above moist air at lower levels (Nicholson and Flohn, 1980; Nicholson, 1986).

In West Africa dry stable Saharan air dominates during boreal winter imposing a dry season, whilst an increased influence of humid, maritime air from the Atlantic Ocean between April and October creates a rainy season with monsoonal conditions that are most intense during July and August (Buckle, 1996).

In Central Africa, rainfall is highly seasonal with one primary wet season lasting throughout the boreal winter with the majority of precipitation falling during November, December and January. Precipitation in this region is largely associated with the migration of the ITCZ such that during some years when the ITCZ does not advance any further than 12 °S, countries such as Zimbabwe and southern Zambia receive very little moisture (Buckle, 1996; Nicholson, 1993).

Rainfall variability in southern Africa is controlled by subtropical anticyclones which induce seasonal drought in the region from April to September as a result of the subsidence

replaced by a shallow surface low pressure cell which draws in moist maritime air permitting summer rainfall (Buckle, 1996).

Nicholson (1986, 2000) investigated modes of spatial and temporal rainfall variability over Africa and found that large-scale climatic fluctuations occur across most of the continent. The interrelationship between spatial rainfall variability in tropical and extratropical regions is indicative of the strong inter-hemispheric teleconnections, which are apparent in African climate.

Figure 3.6 Gridded representation of mean monthly rainfall values for meteorological stations along a North-South gradient in East Africa. The sites transect ranges between longitudes of 28°65’ E (Ndola, Zambia) and 39°20’ E (Dar Es Salaam, Tanzania). Resulting data illustrates the bimodal nature of rainfall in equatorial regions coincident with the north- and south-ward passages of the ITCZ. Data compiled from GNIP database and interpolated using data meshing tool (IAEA/WMO, 2006).

3.4.2.2 Regional rainfall variability in East Africa

Annual rainfall variability in equatorial East Africa follows a bimodal distribution with maximum precipitation occurring during the months of April to June (‘long rains’) and October to November (‘short rains’), reflecting the passage of the sun, and the ITCZ, back and forth across the equator (figure 3.6) (Nicholson, 2000; Odada et al., 2003). The ‘short rains’ are coincident with a relatively rapid southward migration of the ITCZ creating a short yet intense period of rainfall. Contrastingly, the ‘long rains’ occur as the ITCZ moves northwards at a slower rate, thus inducing a longer period of rainfall (Goddard and Graham, 1999; Black et al., 2003). In addition to the strong linkages with the ITCZ, an element of North East African rainfall is also related to the westerly airstream of the African monsoon which is governed by the relative stability of the NE and SE monsoons. However, its effects are highly variable as a result of rift-related topography, and local rainfall distribution is highly regionally complex (deMenocal, 1995). Despite the complexity and variability of the local climatic regimes, interannual variations in rainfall are coherent and show strong teleconnections to the rest of the continent (Odada et al., 2003).

Recent investigations into the isotopic composition of water samples from Eastern and North-eastern Africa (Levin et al., 2009, 2010; Williams, 2010) have demonstrated that there is a significant isotopic enrichment of surface and subsurface waters in Ethiopia compared to waters from regions at similar altitudes elsewhere in tropical Africa. This reflects a regional difference in moisture source in Ethiopia during the ‘Kiremt’ rains (June- September) when precipitation is associated with westerly winds driven by the African monsoon which result in the precipitation of continentally transpired rainfall to the north of the Congo Air Boundary (Nicholson, 1996). The isotopic composition of water samples from Kenya mirrors that of those from Dar Es Salaam and is consistent with an Indian Ocean moisture source (Levin et al., 2009). This difference creates a regional divide which is reflected in the difference in isotopic composition of waters from Kenya and Ethiopia implying that the Kenyan section of the EARS is not subject to significant continental rainfall associated with the African monsoon (figure 3.7).

Tropical African rainfall variability is closely related to SST fluctuations in the Indian and Atlantic Oceans, and tends to be enhanced by ENSO-induced changes which primarily affect the ‘short rains’ (Clemens et al., 1991; Odada et al., 2003). Spatial variability in precipitation is closely related to regional topography with the exposed eastern shoulder of the rift valley generating orographic rainfall from the movement of the easterly trade winds.

valley floor receives less rainfall generated by convectional storms associated with the ITCZ. Large inland lakes such as Lake Turkana or Lake Tanganyika may also modify monsoonal flow and affect the influence of the Indian monsoon thus exerting a secondary control over spatial rainfall variability (Hastenrath, 1991).