Precipitation Variation in the Pacific Northwest (1675–1975) as Reconstructed from Tree Rings

Long-term records of precipitation variation for three regions within the Pacific Northwest are reconstructed based on ring-width data from drought-sensitive conifers. Precipitation reconstructions are derived using multiple regression models that predict variation in annual precipitation as a function of standardized and prewhitened tree-ring chronologies. The precipitation reconstructions indicate that droughts similar in magnitude and duration to those observed in the 1920s and 1930s have occurred frequently, at least once per century, in the past. The timing of drought episodes varies spatially, most notably during the nineteenth century. During the first half of the nineteenth century, precipitation was above average in Washington and northern Oregon but below average in southern Oregon and northern California. During the latter half of the nineteenth century, southern Oregon and northern California experienced above average precipitation while Washington and northern Oregon experienced repeated droughts. In contrast, severe, single-year drought events (1973, 1929, 1899, 1839, 1739, 1721, 1717) have affected the Pacific Northwest as a whole, reflecting the scale and persistence of the circulation features that cause such extreme events.