Interdecadal Drought and Wetness Patterns in the Northwest United States, 1675-2000

PAUL F. HESSBURG and ELLEN E. KUHLMANN

USDA Forest Service, Pacific Northwest Research Station, 1133 N. Western Avenue, Wenatchee, WA 98801

THOMAS W. SWETNAM

Laboratory of Tree-Ring Research, University of Arizona, Tucson, AZ 85721, USA.

ABSTRACT

Reconstructions of the Palmer Drought Severity Index (PDSI) from a network of tree-ring chronologies (1675-1978) and instrumental records (1895-1995) were used to evaluate temporal patterns of drought and wetness in the northwest United States. Pattern analysis of tree-ring PDSI time series yielded 4 unique drought/wetness signals occurring in 10 discrete time periods. The instrumental reconstruction corroborated temporal patterns based on tree-rings with minor differences in period boundaries. We compared patterns of drought and wetness from both sets of PDSI reconstructions with regime shifts of the Pacific (inter) Decadal Oscillation (PDO) using intervention and cross correlation analyses. Signal changes beginning about 1922-24, 1944-48, 1977, and 1992 were strongly supported by the PDO, but change in 1987 was not. Eigenvector and intervention analyses of the instrumental time series revealed that the 1987 shift was significant despite the fact that it was not supported by the PDO. Our results suggest that the PDO contributes a primary forcing mechanism for observed Northwest drought/wetness signal changes, and that short-lived regime shifts may go unnoticed if evaluations rely solely on indices of Pacific climate. If temporal patterns from tree-ring reconstructed PDSI are used as a proxy for the period that predates the PDO record, we infer that the PDO remained in a cool phase from 1766-1921, but completed 3 cycles from 1714-1766 and from 1921-1999. Furthermore, while the climate of the Northwest has been mostly moderate and equitable over the last 325 years, that of the last 3 quarters of the 20th century has been droughty, and evidence of return to moderate conditions is lacking.