Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 197

7.3.2 Precipitation-Sensitive Records
Tree-ring chronologies from precipitation-sensitive regions across the western
Americas, such as the southern United States, the Bolivian Altiplano, and central
Chile, reveal common interannual to decadal-scale oscillations in precipitation vari-
ations during the past centuries (Boxes 7.4 , 7.5 , 7.6 , 7.7 and 7.8 ). Spatial correlation
patterns between precipitation-sensitive records and SST also show that variations
in these records are strongly connected with SST anomalies in the equatorial Pacific
and off the western coast of the subtropical Americas (Villalba et al. 2001 ) .
Box 7.4 Spatial patterns of drought and wetness regimes
over western North America
The network of moisture-sensitive tree-ring chronologies now available for
North America has been used to reconstruct the summer (June-July-August,
JJA) Palmer Drought Severity Index (PDSI) for 1200 years on 286 grid
points extending from southern Mexico across the United States into southern
Canada (Cook et al. 2004 ) . These reconstructions have high temporal and spa-
tial fidelity when compared with instrumental PDSIs on annual and decadal
timescales. The average of all 286 grid points for North America indicates
that the driest single year in the past 500 years occurred in 1864, and the
wettest single year occurred in 1833 (Box Figs. 7.7 and 7.8 ) . The reconstruc-
tions indicate that the twentieth century was relatively moist compared with
the past 500 years, the severe Dust Bowl and 1950s droughts notwithstanding
(Box Fig. 7.7 ) .
The long-range climate influence of the El Niño/Southern Oscillation
(ENSO) over western North America during the late nineteenth and twen-
tieth centuries has been strongest in the Texas-Mexican sector of northern
Mexico and the southwestern United States, with drought during La Niña
events and wetness during El Niño events. The epicenter of reconstructed
decadal drought was often located in the ENSO teleconnection province over
the Southwest, implicating the tropical Pacific in these decadal dry regimes.
The pluvials of the past 500 years were spatially heterogeneous and did not
tend to recur in the ENSO teleconnection region. The notable exception was
the early twentieth-century pluvial (Box Fig. 7.7 ) , one of most extremely
wet decades in 500 years, and which was concentrated in the drainage basin
of the Colorado River. This period of exceptional wetness inflated expec-
tations of surface water supplies in the Southwest, and provides a modern
demonstration of the significant environmental and socioeconomic impacts
associated with these decadal droughts and pluvials. The seventeenth-century
Pueblo Drought lasted at least six years over the same region impacted by
the twentieth-century pluvial (Box Fig. 7.7 ) , and provides a compelling con-
trast to the pluvial and a strong analog for the recent multiyear drought