Dendroclimatology from Regional to Continental Scales: Understanding Regional Processes to Reconstruct Large-Scale Climatic Variations Across theWestern Americas - Dendroclimatology - page 193

the major contributors to the common variance between these records. Waveforms
were extracted from the original reconstructions by using singular spectrum anal-
ysis (SSA), basically a statistical technique related to EOF analysis, to determine
oscillatory modes in the time domain (Vautard and Ghil 1989 ) . The reconstructed
waveforms, representing oscillations >30 years and approximately 10 years, reveal
interesting changes in amplitude during the past 350 years. As was previously noted
(Villalba et al. 2001 ) , the temporal evolution of these components is more closely
related in amplitude and intensity from 1640 to approximately 1850. After 1850,
relationships between waveforms are weaker. The most remarkable feature in the
long-term oscillations is the positive amplitudes during the past 100 years, reflecting
the warming in the twentieth century.

7.3.1.2 Tropical Pacific Ocean

Although the temperature reconstructions from the Gulf of Alaska and northern
Patagonia provide insight into the temporal evolution of the relationships between
the tropical ocean and higher latitudes in the Americas, it is important to note that
the variability in the records is related to tropical teleconnections along the west-
ern coasts of the Americas and not to direct forcing from the equatorial Pacific.
In a first attempt to connect the extratropical tree-ring records from North and
South America with climate variability in the tropical Pacific, we compared the
temperature reconstructions with high-resolution coral records in the Pacific Ocean.
Long-lived corals provide continuous, high-resolution records of tropical Pacific
climate that supplement the instrumental record of climate from this key region.
Modern coral records from the central tropical Pacific are several centuries in length
and have yielded insights into the recent history of tropical Pacific climate variabil-
ity on a variety of timescales. The

18 O and Sr/Ca time series from corals in the
subtropical Pacific at Rarotonga (21 ◦ 14 S and 159 ◦ 49 W), have recently been com-
pared with indices of climate variability in the north Pacific, suggesting some degree
of cross-hemispheric symmetry of interdecadal oceanographic variability in the past
centuries (Linsley et al. 2004 ) . A tree-ring reconstruction of the north Pacific index,
a measure of the intensity of the large-scale atmospheric circulation related to the
Aleutian low-pressure cell, correlates significantly during the twentieth century with
both instrumental tropical climate indices and a coral-based reconstruction of an
optimal tropical index for the Indian and Pacific Oceans, supporting evidence for
a tropical/north Pacific link that extends as far west as the western Indian Ocean
(D'Arrigo et al. 2005 ) .
The coral skeletal Sr/Ca at Rarotonga appears to be related to SST variability on
annual through at least decadal timescales based on correlation with instrumental
SST (Linsley et al. 2004 ) . The coral record, which covers the period 1726-1997,
is significantly correlated with both the Gulf of Alaska ( r

δ

=− 0.32, p < 0.01)

and northern Patagonia ( r 0.01) temperature reconstructions

(Fig. 7.5 ) . The low-pass fraction for each time series was isolated by using SSA
(Vautard 1995 ) , and all reconstructed components with mean frequencies longer
than 20 years were summed. The results are shown in Fig. 7.7 . The subtropical

=− 0.34, p <