Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 219

interactions in the past. Based on our present knowledge of climate variations during
the twentieth century, we intended this contribution to provide an interhemispheric
view of interactions between large-scale modes of variability and climate along
the American Cordilleras over several centuries prior to the period of instrumen-
tal observations. The striking symmetries in SSTs across the equatorial Pacific and
in sea level pressure (SLP) patterns at high latitudes in both hemispheres induce
similar patterns of climate variability in widely separated regions of the western
Americas (Dettinger et al. 2001 ) .
Comparisons of fire histories between the southwestern United States and
northern Patagonia over the past several centuries provide additional evidence for
similarities in past climate variations across the extratropical Americas (Kitzberger
et al. 2001 ) . The synchrony of fire regimes in these two distant regions has tenta-
tively been interpreted as a response to decadal-scale changes in ENSO and PDO
activities. For example, a period of decreased fire occurrence in both regions from
about 1780 to 1830 was attributed to decreased amplitude and/or frequency of
ENSO events.
Strong interhemispheric symmetries of ENSO and ENSO-like variations of
Pacific climate on decadal timescales produce similar patterns of temperature vari-
ations in Patagonia and the Gulf of Alaska. However, the comparison of these
extratropical temperature reconstructions with the Raratonga temperature-sensitive
coral record from the tropical Pacific indicates that over the last 200 years,
interdecadal SST variations in the Pacific alternated between times of more geo-
graphically widespread interdecadal changes—such as the shift in the mid-1970s
that was recorded across the entire Pacific basin—and times of less geographically
organized interdecadal changes shared by the tropics and the north or south Pacific
Ocean. Our observations that interdecadal variations in SST in the tropical Pacific
were more strongly connected to the north Pacific in the twentieth century than in
the mid-1800s agree with previous studies by Evans et al. ( 2001a ) and Labeyrie
et al. ( 2003 ) .
In addition to tropical forcings, similarities in decadal- to century-scale climate
variations also result from changes in high-latitude modes of climate variability
that simultaneously affect the extratropical regions of North and South America.
Instrumental records show a simultaneous intensification of the AO and AAO during
recent decades (Thompson and Solomon 2002 ) .
As was shown in Section 7.3.3 , the annular modes are strongly coupled with
surface air temperatures over high latitudes in both hemispheres (Thompson and
Wallace 2000 ) . Sustained positive trends in both modes in recent decades may
be linked to large-scale warming, particularly in Eurasia and northern Canada
in the Northern Hemisphere and across Patagonia in the Southern Hemisphere
(Thompson and Wallace 2000 ; Thompson and Solomon 2002 ; Ogi et al. 2004 ) .
D'Arrigo et al. ( 2003 ) presented a first reconstruction of a warm season AO tem-
perature index during the interval 1650-1975. Values during the middle twentieth
century, overlapping with the anthropogenic increase in trace gases, equal or exceed
those in the prior record. Lower values are reconstructed for several colder periods,
including the early nineteenth-century interval. Trends in the AO temperature index