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Patagonian grasslands. These coherent interhemispheric changes in annual and
decadal climate patterns associated with the Pacific Decadal Oscillation (PDO)
appear to have been driven by fundamental changes in the hydrologic cycle of the
tropical Pacific Ocean (Graham 1994 ; Evans et al. 2001a , b ; Villalba et al. 2001 ) .
Hemispheric-scale networks of instrumental and proxy climate data are needed
to document and help understand these changes in the ocean-atmosphere system and
their impact on the Americas.
Substantial recent effort has been devoted to the development of ocean-
atmospheric monitoring arrays in the tropical Pacific (e.g., TOGA/TAO,
TOPEX/POSEIDON; Wallace et al. 1998 ) . The cost of these arrays has already
been justified by the economic benefits provided by the long-lead climate forecast-
ing associated with recent ENSO warm events. However, there is clear instrumental
and paleoclimatic evidence that, for example, the frequency of warm and cold ENSO
events has been subject to substantial changes over the past several centuries. The
available instrumental meteorological records are simply too short to clearly define
the important temporal and spatial modes inherent in the low-frequency dynamics
of the Pacific and high-latitude major circulation systems. As these decade-scale
changes in atmospheric circulation have strong impacts on regional climates and
society, understanding these phenomena will improve the skill of long-range cli-
mate forecasting. There is increasing evidence (e.g., Gershunov and Barnett 1998 )
that they modulate the character of high-frequency ENSO teleconnections, produc-
ing more extreme and more predictable anomaly patterns when the two systems are
in phase.
The Western America Cordilleras provide a contiguous latitudinal transect of
mountainous terrain flanking the world's largest ocean that invites comparative stud-
ies of climate variations along the Americas. The American Cordilleras can provide
high-quality proxy climate records over most of their lengths. Tree rings provide the
most broadly distributed, annually resolved source of proxy climate data through-
out the Cordillera and thereby supply the comprehensive baseline data necessary to
evaluate natural climate variability on different temporal and spatial scales.
Progress in dendroclimatology across the Americas has been concerned with the
geographical expansion of the research from the local to regional and continen-
tal scales. The work of Harold Fritts ( 1976 , 1991 ) and co-workers in the 1970s
represented the first attempt to reconstruct the patterns of spatial variation in tem-
perature, precipitation, and atmospheric pressure across North America and the
Pacific Ocean, based on 65 ring width chronologies from the western United States.
Collaborative work between several research groups during the past 10 years (Meko
et al. 1993 ; Cook et al. 1999 , 2004 ) have extended this methodology by compiling
835 chronologies from Canada, the United States, and Mexico to reconstruct a grid-
ded network of 297 summer Palmer Drought Severity Indices (PDSIs) across North
America that spans the past 500-600 years (or longer) over much of the grid (Cook
et al. 2004 ) . Following these initiatives, new projects have continued to develop
databases of tree-ring chronologies that cover large areas in the Western Americas
(Fig. 7.1 ) .
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