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wind regime - northwesterly (southeasterly) in the
austral summer (winter). By contrast, in more stable
mid-latitude airflow, the rainfall maximum is related
closely to the topography (see Figure 4.19B for the
Swiss Alps).
1
Increases in the size and persistence of subtropical
high-pressure cells. The major droughts in the African
Sahel (see Figure 13.11) have been attributed to an
eastward and southward expansion of the Azores
anticyclone.
2
Changes in the summer monsoon circulation. This
may cause a postponement or failure of moist trop-
ical incursions in areas such as Nigeria or the Punjab
of India.
5 Drought
The term drought implies an absence of significant
precipitation for a period long enough to cause moisture
deficits in the soil through evapotranspiration and
decreases in stream flow, so disrupting normal bio-
logical and human activities. Thus a drought condition
may obtain after only three weeks without rain in
parts of Britain, whereas areas of the tropics regularly
experience many successive dry months. There is no
universally applicable definition of drought. Specialists
in meteorology, agriculture, hydrology and socio-
economic studies, who have differing perspectives, have
suggested at least 150 different definitions. All regions
suffer the temporary but irregularly recurring condition
of drought, but particularly those with marginal climates
alternately influenced by differing climatic mechanisms.
Causes of drought conditions include:
3
Lower ocean-surface temperatures produced by
changes in currents or increased upwelling of cold
waters. Rainfall in California and Chile may be
affected by such mechanisms (see p. 155), and
adequate rainfall in the drought-prone region of
northeast Brazil appears to be strongly dependent on
high sea-surface temperatures at 0 to 15°S in the
South Atlantic.
4
Displacement of mid-latitude storm tracks. This may
be associated with an expansion of the circumpolar
westerlies into lower latitudes or with the devel-
opment of persistent blocking circulation patterns in
mid-latitudes (see Figure 8.25). It has been suggested
that droughts on the Great Plains east of the Rockies
in the 1890s and 1930s were due to such changes in
Figure 4.20 Drought areas of central
USA, based on areas receiving less
than 80 per cent of the normal July to
August precipitation.
Source : After Borchert (1971).
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