Historic floods and droughts - Climatic Change in Deserts - page 460

An important study by Power and Smith ( 2007 ), published soon after the three
2007 IPCC reports (IPCC, 2007a ; IPCC, 2007b ; IPCC, 2007c ), has revealed that
from 1977 to 2006, average values of the June-December SOI were the lowest on
record, indicating a weakening of the Walker Circulation. At the same time, mean
sea level atmospheric pressure at Darwin was the highest recorded, equatorial surface
wind-stresses were at their weakest and tropical sea surface temperatures were the
highest on record. They concluded that global warming must be considered in defining
new ENSO indices and in assessing and using statistical correlations between ENSO
events and climatic variations across the globe.
At present, the past offers us some valuable guidance as to which regions of the
world are most influenced by ENSO, and this is unlikely to change very significantly
in the future (Collins et al., 2010 ), although Vecchi et al. ( 2006 ) considered that
the tropical Pacific atmospheric circulation would weaken, as would ENSO events.
Indeed, the IPCC authors were unable to discern any significant change in the pattern
of such events in space and time since observational records became available, and
concluded that they could not predict likely future changes in the magnitude and
frequency of ENSO events (IPCC, 2007a ).
23.3 Historic floods, droughts and the Indian Ocean Dipole
Although part of the year-to-year and decadal-scale variation in the Indian monsoon
is indeed linked to ENSO events, other influences are also at play. Saji et al. ( 1999 )
examined forty years of observations and were the first to identify what they termed
a dipole mode in the tropical Indian Ocean, which soon became known as the Indian
Ocean Dipole (IOD). They discovered that during years when the sea surface temper-
atures (SSTs) off Sumatra were unusually low, they were correspondingly high in the
western Indian Ocean (termed the positive phase by Li and Mu, 2001 ). One outcome
of these SST anomalies was drought in Indonesia coinciding with exceptionally heavy
rainfall in East Africa. More than a decade earlier, Flohn ( 1987 ) had described the
catastrophic rains of 1961 in tropical eastern Africa and the sudden increase in White
Nile floods. This was also a year of pronounced IOD activity. The converse also
applies, with drought in East Africa coinciding with low SSTs in the western Indian
Ocean and with high SSTs and very wet conditions in Indonesia (termed the negative
phase by Li and Mu, 2001 ).
Saji et al. ( 1999 ) also pointed out that because the dipole mode was strongly
dependent on the state of the circulation associated with the summer monsoon, there
would be interaction between the monsoon and the IOD. In the same issue of Nature
in which Saji et al. ( 1999 ) had identified the IOD, Webster et al. ( 1999 ) elaborated on
coupled ocean-atmosphere dynamics in the Indian Ocean, concluding that the 1997-
1998 IOD was independent of ENSO, and Anderson ( 1999 ) observed that the dipole
is reflected not only in SST variation but also in subsurface ocean temperatures in the