Tree-ring-based reconstruction of precipitation in the Bighorn Basin, Wyoming, since 1260 AD

Cores and cross sections from 79 Douglas fir ( Pseudotsuga menziesii) and limber pine ( Pinus flexilis) trees at four sites in the Bighorn Basin of north-central Wyoming and south-central Montana were used to develop a proxy for annual ( June - June) precipitation spanning 1260 - 1998 A. D. The reconstruction exhibits considerable nonstationarity, and the instrumental era (post-1900) in particular fails to capture the full range of precipitation variability experienced in the past; 750 years. Both single-year and decadal-scale dry events were more severe before 1900. Dry spells in the late thirteenth and sixteenth centuries surpass both magnitude and duration of any droughts in the Bighorn Basin after 1900. Precipitation variability appears to shift to a higher-frequency mode after 1750, with 15 - 20-yr droughts becoming rare. Comparisons between instrumental and reconstructed values of precipitation and indices of Pacific basin variability reveal that precipitation in the Bighorn Basin generally responds to Pacific forcing in a manner similar to that of the southwestern United States ( drier during La Nina events), but high country precipitation in areas surrounding the basin displays the opposite response ( drier during El Nino events).