Geoscience Reference
In-Depth Information
Central America
Saharan Africa
India
Southeast Asia
200
200
200
300
20°N
20°N
0
0
0
0
-200
-200
-200
-300
North South America
Central Africa
East Indies
400
400
500
0°
0°
0
0
0
-400
-400
-500
Central South America
Southern Africa
Australia
200
200
250
20°S
0
0
0
20°S
-200
-200
-250
1900
1940
1980
1900
1920
1940
1980
1960
1900
1920
1940
1980
1900
1920
1940
1980
1960
1920
1960
1960
Figure 13.10
Variations of tropical and subtropical land area precipitation anomalies relative to 1961-1990. Nine-
point binomially smoothed curves are superimposed on the annual anomalies.
Source:Houghton et al.(1996). By permission of Cambridge University Press.
example of this complexity,
Figure 13.10
shows
variations in tropical and subtropical precipitation
over land areas through the mid-1990s. Since the
mid-twentieth century, decreases in precipitation
dominate much of the region from North Africa
eastward to Southeast Asia and Indonesia. Many
of the dry episodes are associated with El Niño
events. Equatorial South America and Australasia
also show ENSO influences. The Indian monsoon
area shows wetter and drier intervals; the drier
periods are evident in the early twentieth century
and during 1961-1990.
As a further example of complexity, West
African records for the twentieth century (
Figure
13.11
) show a tendency for both wet and dry
years to occur in runs of up to 10 to 18 years.
Precipitation minima were experienced in the
1910s, 1940s and post-1968, with intervening wet
years, in all of sub-Saharan West Africa. Through-
out the two northern zones outlined in
Figure
13.11
, means for 1970-1984 were generally less
than 50 percent of those for 1950-1959, with
deficits during 1981-1984 equal to or exceeding
those of the disastrous early 1970s' drought. The
deficits continued into the 1990s. It has been
suggested that the severe drought is related to
weakening of the tropical easterly jet stream and
limited northward penetration of the West
African southwesterly monsoon flow. However,
MT
1952-8
20°N
MT 1972
15°N
10°N
5°N
Sahelo-Sahara
0
-2
Sahel
0
-2
Soudan
0
-2
1901
1911
1921
1931 1941
1951
1961 1971
1981 1991
Figure 13.11
Rainfall variations (percentage of standard
departures) during 1901-1998 for the Sahelo-Sahara, Sahel
and Soudan zones of West Africa. The average positions of
the Monsoon Trough (MT) in northern Nigeria during
1952-1958 and in 1972 (El Niño year) are shown.
Source: From Nicholson (2000, p. 2630, fig. 3). Courtesy of American
Meteorological Society.
