Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 38

teleconnections in the Atlantic sector. As noted, the AO is also a representation
of what earlier researchers called the Northern Hemisphere Index Cycle.
Further details about the AO and AAO and their impacts are included in
Chapter 5 . As more data and information become available the role of these
oscillations will certainly provide the basis for much continuing research and
potentially an area for scientific debate.
2.8 ESSAY: ENSO and related teleconnections
Robert Allan, Hadley Centre
2.8.1 Background
Of the natural fluctuations inherent in the global climate system, the
El Ni˜ o-Southern Oscillation (ENSO) phenomenon is the best known and
studied. Interactions between the oceans and the atmosphere across the
Indo-Pacific region are at the heart of this irregular climatic feature.
Climatic and environmental impacts that result from ENSO activity range
from direct influences on countries within and surrounding the Indo-Pacific
basin to more indirect near-global manifestations (Glantz 2001 ; Diaz et al.
2001 ; Trenberth et al. 2002 ; Allan 2003 ). The latter result from the dynamics
of the planetary atmosphere, which often allows ENSO effects to propagate to
higher latitudes in both hemispheres.
Scientific research has revealed much about the ENSO phenomenon, its
extremes of El Ni˜o and La Ni˜a events, the essential physical processes that
power it, its tendency to be locked to the seasonal cycle, and a nature that
embodies events which range in magnitude, spatial extent, duration, onset,
and cessation (see the overviews of Philander 1992 ; Allan et al. 1996 ; Glantz
2001 ). No two individual El Ni˜o or La Ni˜a events are exactly alike and
there is still much to be understood about ENSO (Larkin and Harrison 2002 ).
The structure of the ENSO phenomenon is also shaped by the interplay
between its quasi-biennial (QB) (2-2.5 year) and LF interannual (2.5-7 year)
components (Allan et al. 1996 ; Ribera and Mann 2003 ). However, what is now
emerging is that natural decadal-multidecadal modes in the climate system
(Navarra 1999 ) can provide lower frequency modulations of ENSO character-
istics. A quasi-decadal signal centering on 9-13 years is seen as the cause of
''protracted'' El Ni˜o and La Ni˜a episodes (Allan et al. 2003 ), while the PDO/
IPO (Mantua and Hare 2002 ) appears to modulate ENSO through its influence
on the LF component of the phenomenon. These influences are also manifest
through the changes they cause in the nature of the teleconnection patterns
linked to ENSO activity. Such interactions also raise the question: is the quasi-
decadal signal a separate ''ENSO-like'' fluctuation that interacts with QB and
LF ENSO signals, or is it an integral part of the ENSO phenomenon itself ?