10.1.3. Past to Present
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.
10.1.3.2. Interannual and Interdecadal Climate Variability
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).
|Box 10-1. The 1997-1998 ENSO Event
ENSO appears to play a major role in east Africa, but it masks
the perhaps more important role of the other oceans, particularly
the Indian Ocean. The 1961-1962 rains were spectacularly manifested
as rapid rises in the levels of east African lakes. Lake Victoria
rose 2 m in little more than a year (Flohn and Nicholson, 1980).
This was not an ENSO year, but exceedingly high sea-surface temperatures
(SSTs) occurred in the nearby Indian Ocean as well as the Atlantic.
Such high SSTs are associated with most ENSO events, and it is probably
SSTs in these regions, rather than the Pacific ENSO (Nicholson and
Kim, 1997), that have the largest influence on east African rainfall.
In another example, the dipole pattern anticipated to occur during
ENSO events did not occur during the 1997-1998 event. There
was a tremendous increase in rainfall in east Africa, but intense
drought conditions did not occur throughout southern Africa. The
reason appears to be an unusual pattern of SST in the Indian Ocean.
|Box 10-2. Drought Conditions in the Sahel
One of the most significant climatic variations has been the persistent
decline in rainfall in the Sahel since the late 1960s. The trend
was abruptly interrupted by a return of adequate rainfall conditions
in 1994. This was considered to be the wettest year of the past
30 and was thought to perhaps indicate the end of the drought. However,
by the standard of the whole century, rainfall in 1994 barely exceeded
the long-term mean. Also, the 1994 rainy season was unusual in that
the anomalously wet conditions occurred toward the end of the rainy
season and in the months following. Unfortunately, dry conditions
returned after 1994. The persistent drying trend has caused concern
among development planners regarding how to cope with losses of
food production, episodes of food insecurity, displacements of populations,
lack of water resources, and constraints on hydroelectricity.
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).
Besides ENSO, the NAO, and west African climate anomaly patterns,
other continental-scale and subcontinental climate anomalies play significant
roles in determining interannual and longer climate variability time scales (Nicholson
et al., 2000). For instance, the decade 1950-1959 was characterized
by above-normal precipitation over most of Africa, although rainfall deficiencies
prevailed over the near-equatorial region. Later, during the period 1960-1969,
this rainfall anomaly pattern dramatically reversed in sign, with rainfall deficits
observed for most of Africa while the equatorial region experienced widespread
abundance of rainfall. These two time periods also coincide with a reversal in
the sign of the Sahelian rainfall anomalies (Lamb and Peppler, 1992). More recently,
the pattern has been one of increased aridity throughout most of the continent.
Mean rainfall decreased by 20-49% in the Sahel between the periods 1931-1960
and 1968-1997 and generally 5-10% across the rest of the continent (see
Figure 10-1).In comparison with the period between 1950
and 1970, the average length of the rainy season has not changed significantly
during the dry period 1970-1990. Instead, the decrease in rainfall in July
and August explains most of the diminution of total annual rainfall over the Sahel
since 1970. The average number of rainy events in August was reduced by about
30% (Le Barbé and Lebel, 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.