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the general circulation. However, the droughts of the
1910s and 1950s in this area were caused by persistent
high pressure in the southeast and the northward
displacement of storm tracks (Figure 4.20).
precipitation. Rainfall for April to August 1995 over
England and Wales was only 46 per cent of average
(compared with 47 per cent in 1976), again associated
with a northward extension of the Azores anticyclone.
This deficit has an estimated return period in excess of
200 years! Nevertheless, the fifteen-year period 1983 to
1995 included six summers that were dry and also warm
relative to the central England temperature record for
1659 to 1975.
Persistent, severe droughts involve combinations
of several mechanisms. The prolonged drought in
the Sahel - a 3000
From May 1975 to August 1976, parts of northwest
Europe from Sweden to western France experienced
severe drought conditions. Southern England received
less than 50 per cent of average rainfall, the most severe
and prolonged drought since records began in 1727
(Figure 4.21). The immediate causes of this regime were
the establishment of a persistent blocking ridge of high
pressure over the area, displacing depression tracks 5 to
10° latitude northward over the eastern North Atlantic.
Upstream, the circulation over the North Pacific had
changed earlier, with the development of a stronger
high-pressure cell and stronger upper-level westerlies,
associated perhaps with a cooler than average sea
surface. The westerlies were displaced northward over
both the Atlantic and the Pacific. Over Europe, the
dry conditions at the surface increased the stability
of the atmosphere, further lessening the possibility of
700-km zone stretching along the
southern edge of the Sahara from Mauritania to Chad
- which began in 1969 and has continued with inter-
ruptions up to the present (see Figure 13.11), has been
attributed to several factors. These include an expansion
of the circumpolar westerly vortex, shifting of the
subtropical high-pressure belt towards the equator, and
lower sea-surface temperatures in the eastern North
Atlantic. There is no evidence that the subtropical high
pressure was further south, but dry easterly airflow was
stronger across Africa during drought years.
Figure 4.21 The drought of northwest Europe between May 1975 and August 1976. (A) Conditions of a blocking high pressure over
Britain, jet stream bifurcation and low sea-surface temperatures. (B) Rainfall over western Europe between May 1975 and August 1976
expressed as a percentage of a thirty-year average.
Source : From Doornkamp and Gregory (1980).
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