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signal under global warming conditions in state-of-the-art global climate
models, and from analyses of historical climate data (Meehl et al. 1993 ;
Knutson and Manabe 1998 ; Collins 2000 , Fedorov and Philander 2000 ;
Trenberth et al. 2002 ). In part, this research focus has also been fuelled by
evidence of a climate regime shift over at least the Pacific sector around
1976-7 (Gu and Philander 1995 ; McPhaden and Zhang 2002 ), and the
occurrence of the 1990-5 ''protracted'' El Ni ˜ o episode (Gu and Philander
1997 ; Webster and Palmer 1997 ).
Recent climatic events and studies have provided further insights into
the global warming and ENSO question. The occurrence of the ''protracted''
La Ni ˜a episode of 1998-2001 (Allan et al. 2003 ) provided a counterbalance to
concerns that global warming had caused a climatic regime change towards
more ''El Ni˜o-like'' conditions in the Pacific during the first half of the 1990s
- a period now known to have been dominated by the 1990-5 ''protracted'' El
Ni ˜o episode. A growing research focus on low-frequency climatic variability
in general, with an emphasis on phenomena such as the PDO/IPO (Mantua and
Hare 2002 ), and the AMO (Enfield et al. 2001 ), has indicated the importance of
understanding these natural climatic fluctuations in any efforts seeking to
isolate the global warming signature in the climate system. In fact, studies
such as Bratcher and Giese ( 2002 ) suggest that natural decadal-multidecadal
variability in the Pacific may have played the dominant role in the warming
signatures observed to date. They also indicate that current Pacific climate
conditions may be in a configuration which is opposite to that seen prior to the
1976-7 regime shift, and that a return to earlier conditions is imminent.
Nevertheless, as noted by Lau and Weng ( 1999 ), natural and anthropogenic
climate signals may well be inextricably bound up and difficult to unravel.
2.8.7 ENSO: summary
The ENSO phenomenon is a major component of the climate system.
Through its dynamics and patterns of teleconnections, this climatic fluctua-
tion is able to extend tropical Indo-Pacific influences to higher latitudes in
both hemispheres, resulting in near-global climatic impacts. Recent research
has indicated that ''ENSO-like'' fluctuations, operating on decadal to multi-
decadal time frames, also exist in the climate system and play a significant
role in modulating climate. In fact, this may occur frequently through inter-
actions between these ''ENSO-like'' features and the QB and interannual
LF components of ENSO.
There is widespread evidence for the impacts of QB, LF ENSO, and quasi-
decadal signals on climate, and their interactions to produce ''protracted''
ENSO episodes, all within an even broader envelope of lower frequency
''ENSO-like''
PDO/IPO
and
AMO
climatic
fluctuations.
The
picture
 
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