Global Warming

Forecasting for the African Sahel region

The Sahel region of Africa forms a band about
500 km wide alongthe southern edgeof the Sahara
Desert that gets most of its rainfall during northern
hemisphere summer (particularly July to Septem-
ber). This region is known for its prolonged periods
of drought, which can have a devastating impact on
the local economy, an example of which occurred
in the 1970s and 1980s (Figure 5.10). The droughts
have been linked to fluctuations in several patterns
of sea surface temperature (SST). Figure 5.11 illus-

trates the difference in pattern of the mean world-

wide SST between the five wettest and five driest

Sahel years since 1901. This and other patterns form

the basis of the statistical and atmosphere global

circulation model seasonal forecasts that have been

made by the UK Meteorological Office for the Sahel

since 1986.^8 Long-lead forecasts are made in May

for the July–September rainfall season and updated

in early July.

1

0

-1

1900 1910 1920 1930 1940 1950 1960 1970 1980 1980 2000

Wetter

Anomoly (standardised units) drier

Figure 5.10Annual rainfall

differences from a long-term

average (anomalies) for the

Sahel in standardised units

(the standard deviation from

the average). The smoothed

curve is approximately a

decadal average.

A common difficulty with the forecasts has been
that changes in the SST patterns occur between the
time of the forecast and the main rainfall season. To
overcome this problem, an atmosphere–ocean cou-
pled model (see ‘The climate system’, next section)
is being used to forecast changes in SST in addi-
tion to changes in atmospheric circulation. Fore-
cast skill has improved although remaining modest

(correlation between observed and modelled mean

rainfall is about 0.37), in particular major changes

to drier conditions are being picked up.^9 Such mod-

els, additionally coupled to variations in the char-

acteristics of land surface vegetation and soil which

have also been shown to be important,^10 provide the

greatest promise for seasonal forecasts in the Sahel

and in many other regions of the world.

Figure 5.11Mean difference

in July–September SST (in◦C)

observed during the five driest

years in the Sahel since 1900

(1972, 1982, 1983, 1984,

1987) and during the five

wettest years (1927, 1950,

1953, 1954, 1958). Shaded

areas show the most

consistently (statistically

significant) different regions.