Oscillations and teleconnections - The Global Climate System: Patterns, Processes, and Teleconnections - page 48

manifest in the 12-25 year band with PDO/IPO characteristics. The recent
analysis of White and Tourre ( 2003 ) concerning QB to interdecadal signals
over the global oceans resolved a signal in the 14-22 year band (peaking at
16.7 years in an MTM-SVD LFV spectrum), which they saw as indicative of
the PDO/IPO phenomenon. Other work has pointed to the phenomenon being
manifest across an even broader frequency range (approximately 20-60
years). If the MTM-SVD LFV spectrum analysis used to construct
Figure 2.6 is applied to the data for the 1900 to 1998 period it resolves a
significant 32-year PDO/IPO signal, which is more consistent with the
temporal nature of the phenomenon.
Studies such as Bove and O'Brien ( 2000 ) and Meinke et al.( 2001 )have
produced evidence for the modulation of ENSO events, and their climatic
impacts, by the PDO/IPO phenomenon over North America, Australia, and
Europe. More recently this has been extendedintootherfields,withstudies
of specific oceanic and hydrological responses to PDO/IPO extremes
(Hildalgo and Dracup 2003 ). Physically, the PDO/IPO SST pattern
(Figure 2.10a (Plate 4 a )) has greater SST variance in the northwestern
Pacific than in the tropical central to eastern Pacific. In contrast to the
dominant phases with zonal equatorial Pacific SST gradients in the QB,
LF ENSO or quasi-decadal sequences (Figures 2.7 to 2.9 (Plates 1 to 3 ) ), the
peak PDO/IPO SST pattern in Figure 2.10a (Plate 4 a )ismarkedbySSTsof
the same sign across the tropical Pacific. It has been suggested that this
PDO/IPO SST pattern could change the distribution of diabatic heating,
leading to more (less) rainfall in the eastern margins of the west Pacific
during the ''El Ni ˜ o-like'' (''La Ni ˜ a-like'') phase of the phenomenon. LF
ENSO events of either phase occurring during such extremes of the PDO/
IPO would undergo modulations of their characteristics and related tele-
connection patterns. An example of this is given in Figure 2.10b (Plate 4 b )
(after the work of Meinke et al. 2001 ) where the probability of exceeding
median October to December precipitation, given El Ni ˜ o conditions in the
previous September, differs considerably in some regions of the globe in an
''El Ni˜o-like'', as opposed to a ''La Ni˜a-like'', PDO/IPO phase. In this case,
western-central Europe and India show the most distinctly opposite precipita-
tion response under contrasting PDO/IPO regimes. Studies have indicated
that the QB component of the ENSO phenomenon (Allan 2000 ;Tourreet al.,
2001 ) may also be modulated by distinctly quasi-decadal fluctuations with
''ENSO-like'' characteristics that operate in the climate system.
2.8.4 Physical mechanisms
In studies such as Zhang et al.( 1997 ), Allan ( 2000 ), and White and Tourre
( 2003 ), it has been shown that a number of discrete decadal to multidecadal