Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 18

temporary or permanent). The characteristics and causes of these forms of water
scarcity are listed in Table 2.1 after Chapman (1999).
Droughts are often caused by short-term climatic changes relating to lati-
tudinal shifts in the position of the intertropical convergence zone (ITCZ), the
Hadley cells, the jet stream and alternations in the El Nino Southern Oscillation
(ENSO). The ITCZ is the region usually occurring between the Tropics of Cancer
and Capricorn, depending on the season, where tropical air rises due to intense
heating from the Sun. The rising air spreads northward and southward towards
thepoles where it cools and descends at 20 ◦ --30 ◦ north and south of the equator.
Where the sinking air moves back towards the equator the circulation pattern is
known as the Hadley cell. Some air also moves towards the poles and is uplifted
by frontal systems associated with the subpolar low pressure systems. This circu-
lation is known as the Ferral cell. The jet stream is an upper troposphere wind
which often forms a looping pattern as it encircles the globe. In the northern
hemisphere, the jet stream forms a wave-like pattern of alternating high and
low pressures in a line between Siberia across the north Pacific to central North
America. This looping pattern is known as a Rosby wave and is quasi-stationary
and orographically fixed in position by the Tibetan Plateau and the Rocky Moun-
tains (Bryant, 2005). The amplitude of the Rosby wave is defined by the pressure
differences between the respective high and low pressure cells and its ampli-
tude increases with increasing pressure in the pressure cells. Rosby waves can
affect the latitudinal position of the jet stream, which in turn can cause the
passage of high pressure systems and reduced precipitation into areas that nor-
mally receive higher annual rainfall. Such changes to the looping pattern of the
jet stream can result in both short-term drought and semi-permanent climatic
change (Bryant, 2005)
Drought conditions are also strongly influenced by the El Nino Southern
Oscillation. El Nino events occur when sea surface temperatures along the coast
of South America increase over months to several years. Normally, warm surface
waters dominate the Western Pacific and cool surface waters occur in the Eastern
Pacific. This pattern is maintained by the generally easterly trending trade winds
(north-easterlies in the North and south-easterlies in the South Pacific Oceans).
These trade winds are related to the annual migration of the Hadley cells follow-
ing the seasonal movement of the sun southward and northward of the equator
(ecliptic). High pressure systems normally develop over the equatorial ocean west
of South America and over the Peruvian coast. Easterly trade winds occur when
unstable air rises over the Indonesian--Australian region and cooler air descends
over the central to eastern Pacific region. Surface waters are blown back across
thePacific where they pile up in the Coral Sea off northeastern Australia. During
an ENSO event this system breaks down with a weakening of the easterly trade