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FIGURE 11 Amplitudes of the first principal components (PCs) (a) of temperature-sensitive chronologies for northern Patagonia (thick line) and coastal Alaska (thin line), and their most significant decadal-scale oscillatory modes (b-d) extracted by singular spectrum analysis (SSA). The oscillatory modes have periods of (b) longer than 50 years, (c) ca. 13 years, and (d) 9 years. Percentages of the original variance contributed by each of the Patagonia and Alaska waveforms are indicated in the upper and lower left corners of the figures, respectively. In the upper right corners, r is Pearson's correlation coefficient between the Patagonia and Alaska series. Major changes in the oscillatory modes are observed at about 1850 (vertical dotted line). Correlation coefficients for the intervals 1592-1849 and 1850-1988 between the northern Patagonia and the Alaska oscillatory modes are shown at the left and right of the vertical dotted line, respectively.

coherence at the decadal scale between the records during the most recent interval (Fig. 12).

SSA of the precipitation-sensitive records for central Chile and the Midwest-southern United States in the time and frequency domains also show important shifts in the oscillatory modes at ca. 1825-50 (Fig. 13). Two major temporal patterns, centered at ca. 7.8 and 10-18 years, show contrasting relationships before and after 1825 and 1850, respectively, as indicated by changes in the correlation coefficients between these records during the early and late intervals. The 10- to 18-year waveforms are highly correlated from 1700 to 1849, whereas the 7.8-year modes are strongly correlated from 1825 to 1978.

BT spectral analyses of the precipitation-sensitive records also indicate important changes in the concen tration of spectral power at ca. 1850 (Fig. 14). For the early interval 1700-1849, both records show that much of the variability in tree rings was confined principally to frequency bands longer than 5 years. In contrast, the 3.2- to 3.8-year oscillation was the dominant mode in central Chile during the most recent interval, without any significant peak at lower frequencies. Cross-spectral analyses also reveal a shift in the coherence frequency bands from decadal modes at 8.3 and > 17 years during the early interval to shorter scale modes at 3.2 and 6.7-9.4 years during the most recent interval (Fig. 14).

Following a procedure that is similar to that used by Zhang et al. (1997), we examined the temporal evolution of the leading decadal-scale oscillatory mode in the Pacific SST field back to 1856. Monthly SST tempera

Interval 1592-1849 Interval 1850-1988

Interval 1592-1849 Interval 1850-1988

0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Frequency (cycles/ year) Frequency (cycles/year)

FIGURE 12 Blackman-Tukey (BT) power spectra (thick solid line) of temperature-sensitive records for (a and d) northern Patagonia and (b and e) the Gulf of Alaska estimated over two independent intervals: 15921849 (left) and 1850-1988 (right). The 95% confidence limits (dotted line) are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectra between these records are shown for the intervals (c) 1592-1849 and (f) 1850-1988. Note the changes in squared coherency between the two intervals.

0.0 0.1 0.2 0.3 0.4 0.5 0.0 0.1 0.2 0.3 0.4 0.5 Frequency (cycles/ year) Frequency (cycles/year)

FIGURE 12 Blackman-Tukey (BT) power spectra (thick solid line) of temperature-sensitive records for (a and d) northern Patagonia and (b and e) the Gulf of Alaska estimated over two independent intervals: 15921849 (left) and 1850-1988 (right). The 95% confidence limits (dotted line) are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectra between these records are shown for the intervals (c) 1592-1849 and (f) 1850-1988. Note the changes in squared coherency between the two intervals.

ture anomalies for 583 points over the Pacific were taken from Kaplan et al. (1997). Low-pass-filtered time series were generated by applying to the monthly mean SST anomalies a cubic spline (Cook and Peters, 1981) designed to reduce 50% of the variance in a sine wave with a periodicity of 10 years. This procedure preserved 25% of the variance on a timescale up to 7.6 years and only 10% of the variance on timescales up to 5.8 years. Because of our interest in examining decadal-scale modes of variation in SST, the use of a 10-year cubic spline for filtering the SST series was a reasonable choice. Using the Varimax criterion, a rotated PC (RPC) analysis (Richman, 1986) was applied to the low-pass SST series to relieve the orthogonality constraints imposed on unrotated factors. Figure 15a (see color insert) shows the leading Varimax rotated SST factor for the interval 1856-1991. The abrupt change toward warmer SSTs at ca. 1976 is the most obvious feature of the decadal-scale oscillatory mode over the Pacific during the past 130 years. In contrast, low energy appears to be related to the decadal oscillatory frequencies over the whole Pacific from ca. 1910-1976 (Fig. 15a). The spatial pattern associated with the leading PC is characterized by positive departures in the tropical central and western Pacific and along the subtropical coasts of North and South America. Anomalies of opposite polarity occur in the extratropical central North and South Pacific (Fig. 15b [see color insert]), but they are more pronounced in the North Pacific.

This analysis of both instrumental and tree-ring records shows the occurrence of major changes in the temporal evolution of climate modes over the Pacific and along the western Americas. These records suggest that the patterns of climatic variability over the Pacific have been characterized by interactions between inter-annual and interdecadal oscillatory modes. An intensification of the interannual ENSO-type forcing from the mid-nineteenth century to ca. 1976, associated with more energy concentrated at high-frequency modes of oscillation, may have played a major role in masking or modifying the decadal-scale mode of oscillation that prevailed in the Pacific during the previous centuries.

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