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North-to-south comparative studies along the western Americas, which integrate instrumental and proxy records, provide a unique opportunity for interpreting present and past climatic changes owing to the predominant influence of a single climatic forcing mechanism: the Pacific Ocean. Patterns of modern climate circulation can be used to interpret the dynamics of climatic changes preserved in the paleorecords. On the other hand, high-resolution proxy records allow us to examine changes in phase relationships and magnitude of the dynamic components of the Pacific climate system as they change over time.

The most striking results of this study are the significant relationships between North and South American climate-sensitive records extending back to 1600. Tree-ring records show the occurrence of decadal-scale climatic variations that have simultaneously affected the extratropical western coast of the Americas during the past four centuries. It is very difficult to conceive of any hypothesis to explain these significant relationships other than the records are responding to large-scale, common climatic variability. This hypothesis also leads to the implication that the temperature- and precipitation-sensitive chronologies have recorded this climatic variability with reasonable fidelity throughout the past four centuries.

It has been shown that decadal-scale climate changes in the western Americas during recent decades have been influenced by the Pacific Ocean. This has been simultaneously reflected in instrumental records and circulation indices such as the Southern Oscillation, Pacific-North America Oscillation (PNA), and the Pacific Decadal Oscillation (PDO) (Trenberth and Hur-rell, 1994; Mantua et al., 1997). Many studies suggest that low-frequency climate variability in the North Pacific is caused by tropical forcing (Nitta and Yamada, 1989; Trenberth, 1990; Graham, 1994; Graham et al., 1994; Trenberth and Hurrell, 1994; Lau and Nath, 1994). However, some simulations from global coupled ocean-atmosphere general circulation models (GCMs) sug

FIGURE 14 Blackman-Tukey (BT) power spectra (thick solid line) of precipitation-sensitive records from (a and d) central Chile and (b and e) the Midwest-southern United States estimated over two independent intervals: 1700-1849 (left) and 1850-1978 (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) 1700-1849 and (f) 1850-1978. Note the changes in squared coherency between the two intervals.

FIGURE 14 Blackman-Tukey (BT) power spectra (thick solid line) of precipitation-sensitive records from (a and d) central Chile and (b and e) the Midwest-southern United States estimated over two independent intervals: 1700-1849 (left) and 1850-1978 (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) 1700-1849 and (f) 1850-1978. Note the changes in squared coherency between the two intervals.

gest that the atmosphere-ocean system over the extratropical North Pacific may generate interdecadal climatic changes by itself (Latif and Barnett, 1994; Robertson, 1996). Recent studies show that the tropical Pacific strongly influences decadal variability in SST within the subtropical gyre, whereas the variability within the subpolar gyre may respond, in a large degree, to a self-maintaining mechanism inherent to ocean-atmosphere interactions in the North Pacific (Nakamura et al., 1997). Significant correlations between tree-ring records for distant sites located in North and South America suggest that the tropical Pacific may have played a major role in forcing common decadal-scale climatic variations in extratropical regions of both North and South America during the past three to four centuries. Additional support for this idea comes from the similarity in the spatial correlation patterns derived from tree-ring records for both North and South America and SSTs over the Pacific. These patterns are characterized by high positive values centered on the tropical western Pacific with up- and downstream regions of positive values along the western coasts of the Amer icas. Presently, it is extremely difficult to conceive of a system of atmospheric interactions in which long-term climatic changes in the North Pacific act to force changes in the South Pacific, or vice versa. Although our results support the idea of a common tropical forcing of decadal-scale climate oscillations along the western Americas, Zhang et al. (1998) have recently presented observational evidence that the Pacific warming and the decadal change in ENSO after 1976 may have originated from the midlatitude North Pacific Ocean. In consequence, there is a possibility that thermal anomalies in the North Pacific may indirectly affect the South Pacific by forcing changes in tropical Pacific SSTs, thus leading us to question our hypothesis of a common tropical forcing mechanism for climate variations along the western Americas.

The dominant modes of variability in the tropics during the last few decades have been recently investigated by several authors (Kawamura, 1994; Latif et al., 1997; Zhang et al., 1997). These studies show that the tropical SST variability can be characterized by an in-terannual mode (ENSO cycle related) and a decadal mode (residual independent mode). The presence of a trend or unresolved ultra-low-frequency variability has also been reported by Latif et al. (1997). The spatial structure of the interannual mode is characterized by an equatorial maximum in the eastern Pacific narrowly confined along the equator. In contrast, the spatial signature of the decadal mode is less confined to equatorial regions in the eastern tropical Pacific, with the strongest SST anomalies in the western equatorial Pacific extending to the northeast and southeast into the western American subtropics. Sea surface temperature anomalies (SSTAs) in the extratropical central North and South Pacific are also more prominent in the decadal mode (Latif et al., 1997; Zhang et al., 1997).

The spatial signatures, which result from correlating SST over the Pacific with both the temperature- and precipitation-sensitive records, resemble those of the leading decadal modes of SST variability presented by Latif et al. (1997) and Zhang et al. (1997). Indeed, positive correlations between SST and the tree-ring records are stronger over the western Pacific and along the west coast of the Americas than over the eastern tropical Pacific. In agreement with our spatial correlation patterns, Latif et al. (1997) showed that the SST field associated with the decadal mode of variability over the Pacific originates in the subtropics at the west coast of North and South America (Fig. 10a in Latif et al., 1997), where the variances explained by this mode are the highest (Fig. 10c in Latif et al., 1997). Interestingly, the rotation period proposed by Latif et al. (1997) for this SST variability mode is 13 years, consistent with the 12.8-year oscillation common to the temperature-sensitive records for both Alaska and northern Patagonia during the past 400 years. These observations strongly suggest that in-terdecadal variations of climate, modulated by Pacific SSTs, appear to be one of the most solid climatic forcing mechanism influencing temperature and precipitation variations along the extratropical western coasts of the Americas.

Additional support indicating the existence of a decadal oscillatory mode associated with the Pacific Ocean comes from a 100-year global spatiotemporal analysis of temperature anomalies by Mann and Park (1994), who detected an interdecadal mode of temperature variability centered on a 15- to 18-year period. The spatial pattern associated with this oscillation resembles that of ENSO, and the time domain signal suggests that the warmth after 1976 was consistent with a large positive excursion of this interdecadal oscillation.

Variations in the temporal evolution of climate-sensitive records for extratropical North and South America show a consistent change in the relationships between modes of decadal-scale climatic variability at about the mid-nineteenth century. Certainly, major shifts in climate at about the 1850s have been suggested many times and in many places. Starting at ca. 1850, the Arctic warmed to the highest temperatures in four centuries (D'Arrigo and Jacoby, 1993; Overpeck et al., 1997). Since ca. 1850, widespread retreats of mountain glaciers have been reported for the Canadian Rockies (Luckman, 1996), the European Alps (Haeberli et al., 1989), the New Zealand Alps (Gellatly and Norton, 1984), and the tropical Andes (Hastenrath, 1981). Based on the documented relationships between the Pacific SSTs and climate along the western coast of the Americas, we speculated that the recorded changes in the principal modes of extratropical climatic variability at ca. 1850 reflect a major reorganization in the circulation of the Pacific Ocean at that time. This reorganization might have changed the relationships between interan-nual and interdecadal modes of atmospheric circulation over the Pacific, which in turn may have altered the extratropical teleconnections in time and frequency domains.

Additional support indicating the existence of major changes at ca. 1850 over the tropical Pacific comes from totally independent long records of tropical ocean variability. These records are also related to changes in extratropical teleconnections. Whetton and Rutherfurd (1994) used a set of proxy records from the Eastern Hemisphere (Indonesia, Northeast Africa, North China, and India) to identify interrelationships characteristic of ENSO and how these relationships may have varied over the past 500 years. Based on the information that emerged from the analysis of these proxy records, these researchers noted that ENSO teleconnec-tions, as we know them today, were less active from 1550 -1850 than they were during the later period. However, the authors largely attributed the inconsistency between proxy records during the early period to a poorer quality of the data. Documentary records of ENSO manifestations in Peru, central Chile, northeastern Brazil, and central México, in combination with a coral record from the Galápagos, also show poor agreement between ENSO events during the sixteenth, seventeenth, and eighteenth centuries (Ortlieb, 2000). Correspondence between events within the South America sites is better after 1814 and almost perfect in the last quarter of the nineteenth century. Ortlieb (2000) suggests that the changes in event correspondence between documentary records reveal a distinct mode of climatic variability over the Pacific, which in turn might be associated with different ENSO teleconnec-tions before and after the mid-nineteenth century. A composite tree-ring reconstruction of the SOI since 1706 (Stahle et al., 1998) also suggests a significant increase in the interannual ENSO-related oscillatory modes, more frequent cold events, and a stronger sea level pressure

(SLP) gradient across the equatorial Pacific from the mid-nineteenth century. A more energetic interannual mode of ENSO since 1850 may have affected the amplitudes of the decadal-scale oscillatory mode in the Pacific, which according to our analysis has been weaker and less coherent from the mid-nineteenth century to the mid-1970s.

Interactions between different modes of tropical SST variability have also been reported on the basis of instrumental observations (Ji et al., 1996; Latif et al., 1997). In their analysis of the climatic conditions prevailing during the 1990s, Latif et al. (1997) concluded that those anomalies were caused mainly by a prominent decadal mode with some weak interannual fluctuations superimposed on it. In contrast, they noted that the amplitude of the interannual (ENSO) mode was stronger than that of the decadal mode before 1990. Our results suggest that the decadal mode of variability over the Pacific was more prominent from 1600 to ca. 1850; after that, the interannual mode has dominated. Based on instrumental SST records for the past 130 years, the Pacific decadal mode was apparently weaker during most of the twentieth century (Fig. 15a). The widely reported transition from cold to warm winter conditions in the tropical Pacific during 1976 is the most significant decadal-scale oscillation in the instrumental record.

These observations have important implications in relation to our knowledge of ENSO and ENSO-related climate variability. Have the Pacific modes of variability always been dominated by oscillations of between 3 and 7 years as has been indicated by the traditional analysis of the instrumental records during the past 130 years? Or is the decadal mode, of which the shift in the mid-1970s is the most significant manifestation in this century, another important mode of variability inherent to the Pacific? If the idea of a decadal mode of oscillation were incorporated into the natural climatic variability of the Pacific Ocean, it would not be necessary to refer to the 1980s and 1990s as periods of permanent or extended El Niño conditions, nor would it be necessary to invoke anthropogenic influences (Tren-berth and Hoar, 1996) to explain anomalous ENSO such as the long 1991-94 event. Although the impact of the decadal mode on the interannual (ENSO) mode is still unclear, Latif et al. (1997) suggest that the decadal mode is a source of irregularities in the interannual (ENSO) mode of variability. Certainly, the short-lived warmings in the tropical Pacific in the early 1990s were poorly simulated by both statistical and dynamic forecast models, which were developed for predicting the strong interannual variability in the tropical Pacific during the previous decades (Ji et al., 1996).

Although tree-ring records 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 western coasts of the Americas and not to direct forcing from the equatorial Pacific. Additional instrumental and proxy records will aid efforts to understand the interactions between different modes of variability over the Pacific Ocean and the adjacent western Americas. As a follow-up step to investigate temporal changes in the oscillatory modes over the whole Pacific, we suggest comparing tree-ring and historical records along the western Americas with high-resolution coral records from the tropical Pacific.


This research was supported by a Lamont-Doherty postdoctoral fellowship to Ricardo Villalba by grant ATM-96-16975 of the Climate Dynamics Program of the National Science Foundation (NSF), by National Oceanic and Atmospheric Administration (NOAA) grant NA56 GPO235, by the Argentinean Agency for the Promotion of Science (PICT 0703093), and by the Argentinean Council of Research and Technology (CONICET). Lamont-Doherty Earth Observatory Contribution Number 5916.


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