Temporal and Spatial Patterns of Rainfall across Africa

The broad spatial and temporal distributions in rainfall over Africa are controlled to a large extent by global patterns of atmospheric circulation in the tropics.

Atmospheric circulation occurs as a result of pressure gradients that develop from unequal heating of the Earth’s surface (Taylor, 2004). Near the Equator, moisture-laden heated air expands, becomes less dense and rises at the inter-tropical convergence zone (ITCZ). When the air rises, it cools, and as cooler air is not able to hold as much moisture, forms condensate and rains. Thus this convergence zone is associated with heavy rainfall. As this air then flows poleward on either side of the Equator, it cools, becomes denser and eventually sinks towards the Earth’s surface at latitudes of approximately 30 °N and 30 °S (Taylor, 2004). The air descending after shedding precipitation is comparatively dry, and thus doesn’t deliver very much rainfall to these areas. The subsiding air then diverges so a portion returns to the low-pressure belt at the Equator and completes a cycle, known as the Hadley Cell (Taylor, 2004). These trade winds are dry at source but collect moisture as they blow towards the Equator before converging at the ITCZ (Taylor, 2004).

Figure 1. Diagram displaying Hadley Cell circulation and the formation of the ITCZ at the Equator. (Source) 

Temporal Patterns of Rainfall

As the world’s second largest continent, covering approximately 30.2 million km² (20.4% of Earth’s total land area) (Sayre, 1999) and extending from 37°N to 34°S (Lewin, 1924), Africa has a significant amount of landmass both above and below the Equator. Annual rainfall over tropical Africa is characterised by a strong seasonality with summer monsoonal rainfall (Ziegler et al., 2013) determined by migration of the ITCZ north and south over the continent as a result of latitudinal variations in solar radiation (Taylor, 2004). After July, the ITCZ moves southwards bringing heavy rainfall to progressively more southern latitudes until reaching its southernmost latitude in late December when solar radiation is at its peak in the Southern Hemisphere. The ITCZ then moves northward bringing heavy rainfall to progressively more northern latitudes until it reaches its northernmost latitude in July, before returning south (Taylor, 2004). This movement of the ITCZ across Africa is illustrated in Figure 2, which shows the southerly regions of Africa receiving more rainfall in austral summer, and more northerly latitudes receiving more rainfall in austral winter.

Figure 2. Rainfall variability over Africa in (a) January and (b) August. The colour bar indicates days per month with measurable rainfall. (Source)

This annual cycle delivers one distinct influx of moisture (a unimodal rainfall distribution) to latitudes at the southern (e.g. Sahel-Sudanian region) and northern (e.g. areas such as Zambia, Zimbabwe) limits of the ITCZ’s latitudinal course, which experience pronounced and often extreme wet and dry seasons (Taylor, 2004; UNEP, 2012). In contrast, lower latitudes experience two rainy seasons (a bimodal rainfall distribution) as a result of both northward and southward movements of the ITCZ between each solstice (Figure 3). So looking back at Figure 2 – essentially between those two red lines is humid Africa which receives rainfall throughout much of the year – and above and below those lines is semi-arid to arid Africa.

Figure 3. Diagram displaying the relationship between the annual migration of the ITCZ and the amount of rainfall received at different latitudes throughout the year. (Source)

Spatial Patterns of Rainfall

Latitudinal migration of the ITCZ determined not only seasonality in rainfall across tropical Africa, but also the broad spatial distribution. Figure 3 shows that environments at lower latitudes (between 10 °N – 10 °S) receive a greater amount of rainfall as they are influenced by heavy rainfall associated with the ITCZ for a greater proportion of the year due to its double passing (Taylor, 2004). This is further exemplified by Figure 2 showing countries such as the latitudinally central Democratic Republic of Congo, Uganda and Kenya substantial measurable rainfall in both austral summer (January) and winter (August). This rainfall pattern is broadly reflected in Figure 4, which shows the annual rainfall surplus (rainfall minus evapotranspiration) decreasing with increasing distance north and south from the Equator. Areas between the two red lines of Figure 2 are largely covered in blue, and areas outside are indicated in grey and yellow by a rainfall deficit. There is a latitudinal symmetry, with the deserts of the Kalahari in Southern African occurring where the pole-ward side of the Hadley Cells are coming down and delivering little moisture. The surplus is however constrained by evapotranspiration, which means 70-90% of rainfall across most parts of Africa is returned to the atmosphere. Approximately 66% of Africa is classified as arid/semi-arid, with extreme variability in rainfall (UNEP,2012).

Figure 4. Annual water balance: an estimate of available runoff after accounting for evapotranspiration. Yellow indicates areas of runoff deficit; blue indicates areas of runoff surplus. (Source)