Africa is a vast continent, and it experiences a wide variety of climate regimes. The location, size, and shape of the African continent play key roles in determining climate. The poleward extremes of the continent experience winter rainfall associated with the passage of mid-latitude airmasses. Across the Kalahari and Sahara deserts, precipitation is inhibited by subsidence virtually throughout the year. In contrast, moderate to heavy precipitation associated with the Inter-Tropical Convergence Zone (ITCZ) characterizes equatorial and tropical areas. Because the movement of the ITCZ follows the position of maximum surface heating associated with meridional displacement of the overhead position of the sun, near-equatorial regions experience two rain seasons, whereas regions further poleward experience one distinct rainfall season. The mean climate of Africa is further modified by the presence of large contrasts in topography (Semazzi and Sun, 1995) and the existence of large lakes in some parts of the continent.
Humans have adapted to patterns of climate variability through land-use systems that minimize risk, with agricultural calendars that are closely tuned to typical conditions and choices of crops and animal husbandry that best reflect prevailing conditions. Rapid changes in this variability may severely disrupt production systems and livelihoods. Interannual variability of the African climate is determined by several factors. The El Niño-Southern Oscillation (ENSO) is the most dominant perturbation responsible for interannual climate variability over eastern and southern Africa (Nicholson and Entekhapi, 1986). The typical rainfall anomaly associated with ENSO is a dipole rainfall pattern: Eastern Africa is in phase with warm ENSO episodes, whereas southern Africa is negatively correlated with these events (Nicholson and Kim, 1997). The 1997-1998 ENSO event resulted in extreme wet conditions over eastern Africa (see Boxes 10-1 and 10-2), and the 1999-2000 La Niña may have caused devastating floods in Mozambique. Modeling exercises indicate that climate change may increase the frequency of ENSO warm phases by increasing the warm pool in the tropical western Pacific or by reducing the efficiency of heat loss (Trenberth and Hoar, 1997; Timmerman et al., 1999).
In the Sahel and similar regions of west Africa, the problem is more complex. ENSO appears to influence year-to-year variations and reduces rainfall. Its influence appears to be greater within long dry intervals in the Sahel, but it is not the dominant factor controlling rainfall in this region (Ward, 1998).
Over northern Africa, the North Atlantic Oscillation (NAO) is a key factor
that is responsible for interannual variability of the climate (Lamb, 1978).
Across western Africa, year-to-year changes in seasonal climatic conditions
are determined primarily by the Atlantic Ocean, although the rest of the world's
oceans also play important roles. Low-lying islands and coastal regions receive
significant amounts of rainfall from tropical cyclone activity, which is sensitive
to interannual variability of SST conditions over adjacent ocean basins.
The climate of Africa also exhibits high interdecadal variability. Rainfall
variability in the Sahel derives from factors such as SST and atmospheric dynamics
(Lamb, 1978; Folland et al., 1986; Hulme and Kelly, 1997; Nicholson and
Kim, 1997) and is modulated by land surface effects related to soil moisture,
vegetation cover, dust, and so forth (Charney, 1975; Diedhiou and Mahfouf, 1996;
Xue, 1997; Zeng et al., 1999). Modeling evidence also suggests that orographic
control plays a significant role in promoting climate teleconnections between
global SST anomalies and west African interannual climate variability (Semazzi
and Sun, 1997).
Figure 10-1: Rainfall fluctuations, 1901-1998, expressed as regionally averaged standard deviation (departure from long-term mean divided by standard deviation) for the Sahel. |
There is emerging evidence that aerosols and dust also may be important factors in modulating the variability of the African climate (d'Almeida, 1986; Mohamed et al., 1992; Pinker et al., 1994). These studies provide overwhelming evidence of an extremely dense and deep (reaching up several kilometers) dust layer in the Sahel/Sudan during the main dust season from November to April.
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