Exactly dated tree-ring chronologies from the western coasts of the Americas have been used to track decadal-scale climatic variations which have simultaneously impacted the extratropical regions of North and South America during the past four centuries. Significantly-correlated records from the coast of Alaska and northern Patagonia show the existence of common oscillatory modes for temperature variations at 9, 13, and > 50 years. Tree-ring chronologies from precipitation-sensitive regions also reveal the occurrence of decadal-scale oscillations, centered at 8 and 10-18 years, which have simultaneously influenced climatic conditions in the southern United States and Central Chile. Spatial correlation patterns between tree-ring records and sea-surface temperature (SST) over the Pacific show that variations in climatic-sensitive records are strongly connected with SST anomalies in the equatorial Pacific and off the western subtropical Americas. Correlations of opposite sign occur for the extratropical central North and South Pacific. These correlation patterns resemble the spatial signature of the decadal mode of SST variability over the Pacific, which has recently been detected on instrumental records. Whereas the spectral decomposition of the tree-ring estimates suggests the existence of decadal-scale oscillations common to the climates of western North and South America, the spatial correlation patterns between tree-ring and SSTs indicate that the tropical Pacific is the major forcing of these synchronous oscillations. It is likely that climate variations over the Pacific Ocean result from interactions between interannual, interdecadal and secular modes of climatic variability. A detailed analysis in time and frequency domains of the temperature- and precipitation-sensitive records from North and South America indicates a major change in the decadal oscillatory mode around 1850. In general, the decadal mode of variability was more energetic from 1600 to 1850. During this interval, the North and South American records were more strongly correlated. The leading modes of the Pacific SST variability indicate that, with the exception of the last two decades, most of the 20th century has been dominated by the interannual mode of variability. A major reorganization in the Pacific Ocean around 1850 may have altered a persistent decadal mode of oscillation and given rise to a dominant interannual mode. Consequently, what we know of tropical Pacific variability through the analysis of instrumental records has largely been based on a period of predominant interannual variability. The "anomalies" observed in SSTs over the Pacific during the last two decades may represent a recovery of the decadal mode of SST variability which prevailed in the Pacific before 1850