TREE-RING INFERENCES ON SPATIAL COHERENCE OF PRECIPITATION IN THE SOUTHWESTERN USA ON THE MILLENNIAL TIME SCALE

The global network of tree-ring sites for exploring the evolution of moisture patterns in time and space is nowhere denser than in the southwestern United States, but even there sample coverage is marginal on the millennial time scale. Regions with outstanding precipitation sensitivity of ring-width indices as well as unusually strong sample depth to before A.D. 1200 are the Colorado Plateau and the southern Sierra Nevada of California. The covariation of tree growth in these regions is summarized by cross-spectral analysis of a regional-average tree-ring series from the Four Corners area and a composite tree-ring chronology of foxtail pine (Pinus balfouriana) from the Kern River Basin, California. These series are interpreted primarily as indicators for cool-season precipitation.

For their full overlap, A.D. 1000-1988, the two series are significantly correlated (r=0.30, N=988, p<1E-15), and coherency is statistically significant at wavelengths greater than about 4 years. Six synchronous multi-decadal droughts, defined as smoothed (17-year Gaussian) series simultaneously in their lowest decile, are identified in the long-term record, and all occurred before the mid-1800s. Time-dependency in the spectra and coherency are also evident from a cross-spectral analysis of the two tree-ring series in a sliding 301-year window. The results are consistent with synoptic-scale controls over moisture variations in the Southwest at low and high frequencies, and emphasize the need for long records to understand the full range of variability of climate related to regional drought history.