Spectral Properties Of The Temperatureand Precipitationsensitive Treering Record

Climatically sensitive tree-ring records have been used to identify similar modes of climatic variations in the extratropical regions of North and South America during past centuries. The spatial patterns of correlation between SSTs over the Pacific Ocean and the tree-ring records reveal the importance of the Pacific Ocean in forcing these climatic changes, which simultaneously affect the extratropical Americas. In this section, we use cross-spectral analysis to characterize the oscillatory modes that have contributed, in a large degree, to the occurrence of simultaneous climatic changes in western North and South America.

We used Blackman-Tukey (BT; Jenkins and Watts, 1968) spectral analysis to establish the significant dominant periods at which variance occurs in the tree-ring records. The BT spectrum was estimated from 60 and 80 lags of the autocorrelation function for the precipitation- and temperature-sensitive records, respectively. With this number of lags (~25% of the series lengths), we set a reasonable balance between high resolution and moderate stability. The number of degrees of freedom per spectral estimate is 10. The 95% confidence level of the spectrum was estimated from a red noise first-order Markov null continuum based on the lag-1 autocorrelation of the time series (Mitchell et al., 1966).

Figure 9 shows the BT spectra for the temperature-sensitive records from the coast of Alaska and northern Patagonia based on a common period of 397 years (1592-1988). In both sets of records, a large portion of the spectral variance is concentrated at lower (decadal) frequencies. For northern Patagonia, peaks that exceed the 95% confidence limit are observed at 6.5, 8.4-9.4, and 11.4-13.3 years (Fig. 9). The BT spectra of the Gulf of Alaska record also show a large part of the spectral variance concentrated at similar frequencies: 6.7, 810.7, and 15.8-20 years (Fig. 9). Although not statistically significant, obvious peaks are also observed in both sets of temperature-sensitive records at about 50 years. Cross-spectral analysis was used to identify coherent peaks in the Alaska and Patagonia records. Coherent oscillatory modes between the Alaska and northern Patagonia records are observed at 4.8, 5.3, 6.8, 9.0, and 11.1-15.4 years and at temporal scales longer than 50 years (Fig. 9).

The BT spectra for the precipitation-sensitive records for central Chile and the United States (17001978) show that both sets of records have peaks in spectral variance at similar frequencies (Fig. 10). Peaks that exceed the 95% confidence limit are observed at 3.23.5, 5.1, 7.5-8, and 10.9 years in central Chile and at 44.1, 5.7-6.3, 7.5-8, and 10-10.9 years in the Midwest-southern United States records. Cross-spectral analysis of the central Chile and United States records indicates significant coherence at 2.9-3.3, 4.4, 7.4-8, 10, 14-20,

FIGURE 9 Blackman-Tukey (BT) power spectra of temperature-sensitive records for (a) northern Patagonia and (b) the Gulf of Alaska estimated over the interval 1592-1988. The 95% confidence limits are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectrum between these two records is shown in (c). Records are highly coherent at decadal-scale wavelengths at ca. 9, 11.1-15.4, and longer than 50 years.

FIGURE 9 Blackman-Tukey (BT) power spectra of temperature-sensitive records for (a) northern Patagonia and (b) the Gulf of Alaska estimated over the interval 1592-1988. The 95% confidence limits are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectrum between these two records is shown in (c). Records are highly coherent at decadal-scale wavelengths at ca. 9, 11.1-15.4, and longer than 50 years.

FIGURE 10 Blackman-Tukey (BT) power spectra of precipitation-sensitive records for (a) central Chile and (b) the Midwest-southern United States estimated over the interval 1700-1978. The 95% confidence limits are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectrum between these two records is shown in (c). Records are highly coherent at decadal-scale wavelengths at ca. 10, 14-20, and 50 years.

FIGURE 10 Blackman-Tukey (BT) power spectra of precipitation-sensitive records for (a) central Chile and (b) the Midwest-southern United States estimated over the interval 1700-1978. The 95% confidence limits are based on a first-order Markov null continuum model. The periods are given in years for each significant peak. The coherency spectrum between these two records is shown in (c). Records are highly coherent at decadal-scale wavelengths at ca. 10, 14-20, and 50 years.

and 50 years (Fig. 10). These spectral analyses basically reaffirm the existence of common oscillatory modes in temperature- and precipitation-sensitive records for the western coasts of North and South America.

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