Droughts - Extreme Events: A Physical Reconstruction and Risk Assessment - page 20

winds. Convection of air increases in the central to eastern Pacific Ocean fol-
lowed by a rise in sea surface temperatures by as much as 6 ◦ Calong the coast of
South America. At the same time, sea surface temperatures decrease in the west-
ern equatorial Pacific and there is a reversal in the trade winds (they tend more
westerly). These processes result in a decrease of the normal atmospheric pres-
sure gradient between the subtropical high pressure cell in the eastern South
Pacific and the low pressure region found to the north of Australia. At these
times, below average rainfall and drought often occurs in northern and eastern
Australia, subequatorial Africa, Indonesia and Chile (Chapman, 1999). Normal
rainfall conditions return to both western and eastern Pacific regions when the
easterly trade winds regain dominance and cool sea surface temperatures return
to thewest coast of South America.
The Southern Oscillation plays a vital role in promoting drought throughout
substantial areas of the globe and has been estimated to be responsible for about
30% of annual global rainfall variations (Bryant, 2005). Droughts are most com-
mon duringElNino events; however, some regions experience drought during
the reversed condition known as a La Nina event. During such times cold water
moves northward along the South American coast. This enhances the easterly
trade winds across the Pacific Ocean. As a consequence, these La Nina events can
cause severe drought over the Great Plains of North America as stable high pres-
sure cells become established resulting in a northward shift in the jet stream.
At thesame time, the likelihood of floods and tropical cyclones increases in
eastern Australia as convection is enhanced in the western Pacific.
The occurrence of droughts has also been linked to the 11 and 22 year
sunspot cycles and the 18.6 year lunar cycle. Associations between droughts and
sunspot cycles are not as well defined compared to those established for the 18.6
year lunar cycle. The 18.6 year lunar cycle is defined by changes in the orbital
parameters of the moon (these parameters are related to the Sun's rather than
the Earth's equator). This lunar cycle affects tides which have been suggested
to affect air circulation and produce reduced precipitation to, amongst other
places, the lee side of the US Rocky Mountains. This supposedly affects the west-
erly jet stream between the Tibetan Plateau and the Rocky Mountains (Bryant,
2005).
Droughts and impacts
The Great Plains drought in the 1930s ranks as one of the greatest
weather disasters in US history. The region received the name 'Dust Bowl' due to
thesevere dust storms that occurred during the prolonged years of the drought.
At this time the Great Plains experienced severe aeolian erosion due to a lack of