Time series analyses of tree ring chronologies from Pacific North America: evidence for sub-century climate oscillations

Tree ring-width chronologies from the west coast of North America were obtained from on-line data archives, maintained by the National Oceanic and Atmospheric Administration (NOAA). Data covered the period 1650–1983, for trees sampled at 43 sites between Dawson Creek, British Columbia (55°N) and Yosemite Park, CA (37°N). These time series were analyzed using Fourier transformation methods. Spyglass Transform software allowed the resulting power spectra to be stacked and compared, based on characteristics such as topography and continentality, latitude and species ecological niche. Results suggested that several regular or quasi-regular cycles are occurring over various time periods. There are some similarities, though not an exact match, with El Niño periodicities noted by earlier authors. These patterns appear to be widespread, affecting trees at various elevations and distances from the coast, and from different species. A separate analysis of the oldest trees revealed sub-century cycles. In particular, the longer fluctuations at 50–65 years are consistent with recent findings on north Pacific climate by other authors. A Euclidean cluster analysis of individual years identified unusual year groupings between 1924–1946, a period associated with positive temperature anomalies and generally higher precipitation. These findings appear to be consistent with other basin-scale climate cycles, like the Southern Oscillation, and the recently-proposed Pacific Interdecadal Oscillation (PDO).