Geoscience Reference
In-Depth Information
In Chapter 1 , we defined deserts and desert margins as including the hyper-arid,
arid, semi-arid and dry subhumid regions of the world, which together make up
about 47 per cent of the total land area of the globe ( Chapter 1 , Tabl e 1 . 1 ). The dry
subhumid areas include the drier parts of the seasonally wet tropics (see Chapter 1 ,
Figure 1.1 ) and fall within the domain of the summer monsoon ( Figure 23.1 ). Year to
year variations in the summer monsoon reflect fluctuations in the heat budget over this
region modulated through the Hadley Circulation, which was discussed in Chapter 2 .
These fluctuations are in part controlled by the relative temperature differences
between land and sea in early summer. In the case of Asia, these fluctuations are
also linked to variations in the strength of the winter anticyclone over Siberia, which
controls the strength of the winter monsoon and the extent of snow cover over central
Asia (Diaz and Markgraf, 1992 ; Wang, 2006 ). Another major cause of interannual
variation stems from changes in sea surface temperatures over the equatorial Pacific
Ocean and adjacent Indian Ocean that are associated with the Southern Oscillation,
to which we now turn.
23.2 El Ni no-Southern Oscillation (ENSO) events and historic floods
and droughts
Sir Gilbert Walker first recognised and defined the Southern Oscillation when he was
seeking more effective ways of predicting Indian summer monsoon rainfall (Walker,
1924 ). The Southern Oscillation is a measure of the surface atmospheric pressure
differences between the western and eastern limbs of the equatorial Pacific. Walker
observed that when the pressure off the coast of Peru is below average, that at Jakarta
is above average, and vice versa ( Figure 23.1 ). The difference is expressed as an
index termed the Southern Oscillation Index (SOI), which is now generally taken to
be the pressure difference between Tahiti in the central equatorial Pacific and Darwin
in tropical northern Australia (Glantz et al., 1991 ; Diaz and Markgraf, 1992 ; Allan
et al., 1996 ; Williams and Balling, 1996 ; Power et al., 1998 ; Grove and Chappell,
2000 ; Diaz and Markgraf, 2000 ; Peel et al., 2002 ). Figure 23.1 shows the main areas
influenced by the Southern Oscillation, which coincide very broadly with the domain
of the summer monsoon. The changes in atmospheric circulation associated with the
Southern Oscillation are termed the Walker Circulation.
The relationships calculated by Walker have stood the test of time (Diaz and Mark-
graf, 1992 ), although they were at first criticised. When the SOI is strongly negative,
droughts tend to be common in certain parts of the world and are often synchronous
in regions as far apart as the Ethiopian Highlands, peninsular India, eastern China,
northern Thailand, Java, north-east Brazil and eastern Australia. Conversely, during
years when the SOI is strongly positive, major floods are common and are usually syn-
chronous in these same regions. Peruvian fisherman have long recognised that around
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