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In-Depth Information
IndianOcean. Later work byAshok et al. (
2003
) confirmed that the IOD is indeed inde-
pendent of ENSO. Depending on which mode of the IOD is operating, it may either
strengthen or nullify the impact of ENSO on the Indian monsoon and associated rain-
fall (Ashok et al.,
2001
).LiandMu(
2001
) extended the analysis and concluded that the
IOD had a significant impact on atmospheric circulation and climate in North America
and the southern Indian Ocean region, including Australia and southern Africa.
23.4 Longer-term records of ENSO activity
Other sources of proxy data provide a longer record of El Ni no variability over the past
5,000 years and more. Moy et al. (
2002
) analysed a long lake record from Ecuador
that was very sensitive to El Ni no and reported an increase in El Ni no activity from
about 4,800 years ago onwards, after a long interval of some 5,000 years during
which El Ni no events were rare. They also observed that from about 1,600 years ago
onwards, the frequency of El Ni no activity has declined. Woodroffe and Gagan (
2000
)
and Woodroffe et al. (
2003
) analysed the oxygen isotope ratios in
Porites
coral
microatolls from Christmas (Kiritimati) Island in the central equatorial Pacific and
obtained a high-resolution record of ENSO events for the last 3,800 years. They found
that ENSO-related interannual variations in sea surface temperature and precipitation
were less intense 3,800 to 2,800 years ago but became more pronounced 1,700 years
ago, which is about the time that Moy et al. (
2002
) report that the onset of a decline in El
Ni no frequency in Ecuador occurred. Woodroffe et al. (
2003
) proposed that the amp-
lification of ENSO events roughly 2,000 years ago could reflect stronger interactions
between the Southern Oscillation and the Intertropical Convergence Zone, leading
to stronger summer rainfall.
The magnitude and frequency of ENSO events during the Holocene show consid-
erable variability. Cobb et al. (
2013
) obtained a 7,000-year record of ENSO events
from fossil corals on the Christmas and Fanning islands in the tropical central Pacific.
They found that ENSO activity during that time was highly variable, with no discern-
ible trend, and that the relatively high twentieth-century variability was still within
the range of past fluctuations. They concluded that it will prove hard to detect signs
of anthropogenic, as opposed to natural, forcing of ENSO activity, given the large
internal variability evident during the past 7,000 years. Bacon et al. (
2010
) completed
a detailed study of late Holocene alluvial fan activity on the Muggins Mountains in the
Sonoran Desert near Yuma in south-west Arizona and found evidence of widespread
fan aggradation between 3.2 and 2.3 ka, which they attributed to rapid climate change
and more intense ENSO events. They found no evidence of any historic reactivation
of the alluvial fan surfaces, although the precipitation records did show numerous
above-average rainfall events correlated with the SOI.
These findings from the geologically recent record of El Ni no events reinforce
the earlier conclusion that both the magnitude and frequency of ENSO events are
