Geoscience Reference
In-Depth Information
Figure 11.37 Typhoon paths over
East Asia during January to April,
May to June, July to September and
October to December related to the
mean latitude of the central ridge axis
line of the subtropical high-pressure
cell at 500 mb over the western
Pacific.
Sources : Compiled from various sources,
including Lin (1982) and Tao (1984).
Reproduced by permission of the
Geographical Society of China.
upper troposphere. Various wind and rainfall criteria
have been used to define monsoon onset. Based on the
occurrence of (weighted) surface to 500-mb westerly
winds, overlain by 300 to 100 mb easterlies at Darwin
(12.5°S, 131°E), the main onset date is 28 December
and retreat date 13 March. Despite an average duration
of seventy-five days, monsoon conditions lasted only
ten days in January 1961 and 1986 but 123 to 125 days
in 1985 and 1974. Active phases with deep westerlies
and rainfall each occur on just over half the days in a
season, although there is little overlap between them.
However, summer rainfall may also occur during deep
easterlies associated with tropical squall lines and
tropical cyclones. Active monsoon conditions typically
persist for four to fourteen days, with breaks lasting
about twenty to forty days.
northeasterly to its north (Figure 11.38). The major
difference between the circulations of the two regions
is due largely to the differing geography of the land to
sea distribution and to the lack of a large mountain range
to the north of West Africa. This allows the monsoon
trough to migrate regularly with the seasons. In general,
the West African monsoon trough oscillates between
annual extreme locations of about 2°N and 25°N (Figure
11.39). In 1956, for example, these extreme positions
were 5°N on 1 January and 23°N in August. The leading
edge of the monsoon trough is complex in structure
(see Figure 11.40B) and its position may oscillate
greatly from day to day through several degrees of
latitude. The classical model of a steady northward
advance of the monsoon has recently been called into
question. The rainy season onset in February at the coast
does propagate northward to 13°N in May, but then in
mid-June there is a sudden synchronous onset of rains
between about 9°N and 13°N. The mechanism is not yet
firmly established, but it involves a shift of the lower
tropospheric African Easterly Jet (AEJ) (see Figure
11.40B).
In winter, the southwesterly monsoon airflow over
the coasts of West Africa is very shallow (i.e. 1000 m)
with 3000 m of overriding easterly winds, which are
themselves overlain by strong (>20 m s -1 ) winds (see
Figure 11.41). North of the monsoon trough, the surface
E CENTRAL AND SOUTHERN AFRICA
1 The African monsoon
The annual climatic regime over West Africa has many
similarities to that over South Asia, the surface airflow
being determined by the position of the leading edge
of a monsoon trough (see Figure 11.2). This airflow is
southwesterly to the south of the trough and easterly to
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