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different rates, and the combination of their various magnitudes and phases
has a profound effect on individual episodes. Interactions between these
signals can generate a large number of possible climatic sequences. Thus,
''protracted'' ENSO episodes not only vary in overall duration but also have
the potential to wax and wane in magnitude within the lifetime of each
individual episode as QB, LF, and quasi-decadal signals interact together.
In ''protracted'' episodes, this is observed as fluctuations between intervals of
lower frequency ''ENSO-like'' and more distinctly higher frequency QB or
LF ENSO characteristics. However, in the real world, the interplay between
such signals would not occur simply through linear interactions. So there is a
need for ''protracted'' ENSO episode dynamics to be explored further with
more sophisticated techniques and using climate models.
Thus, some aspects of ''protracted'' El Ni˜o and La Ni˜ a episodes are not
captured in any simple combination of the canonical sequences shown in
Figures 2.7 t o 2.9 (Plates 1 to 3 ) . As has been detailed in many studies of both
individual ''classical'' LF ENSO events or composites of events, the El Ni˜o
and La Ni˜ a components of the phenomenon are not exact mirror images of
one another. This is also evident when comparing and contrasting composites
of variables indicative of ''protracted'' El Ni˜o and La Ni˜a episodes. In
addition, composites also suggest that the quasi-decadal signal may tend to
impose more of a broader Pacific SST signature on ''protracted'' La Ni˜a
than on ''protracted'' El Ni˜o episodes. Even more revealing are the studies
of individual episodes, such as the 1990-5 ''protracted'' El Ni˜o and the
1998-2001 ''protracted'' La Ni˜ a, in Allan et al.( 2003 ). Such research is
giving new insights into the type of variations that can occur during the course
of particular episodes, especially the tendency for El Ni ˜ o and/or La Ni ˜ a
conditions to wax and wane as episodes evolve.
The wider impacts of ''protracted'' ENSO episodes are only just beginning
to be addressed. Agrawala et al.( 2001 ) and Barlow et al.( 2002 ) have shown
that the recent severe and widespread drought in central and southwestern
Asia was a consequence of anomalous climatic patterns generated by the
1998-2001 ''protracted'' La Ni˜ a episode. Potential impacts of the quasi-
decadal ''ENSO-like'' signal on the European sector have been suggested in
recent analyses by Allan. Thus, it is apparent that an understanding of
''protracted'' ENSO episodes is crucial if they are to be incorporated into
existing climate prediction schemes.
2.8.6 Global warming and ENSO
Concerns about possible modulations of the ENSO phenomenon by anthro-
pogenic climate warming have been debated widely in the scientific litera-
ture. Such work has essentially focused on possible changes in the LF ENSO
 
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