Instrumental and Reconstructed Water Year Precipitation for the Western United States: A 440-Year Perspective

Abstract:

We use a suite of reconstructed water year (WY; October through September) streamflow for 12 river basins in the western United States to reconstruct gridded water year precipitation over a similar western US domain. The streamflow values were reconstructed based on moisture sensitive tree ring records and we used values published in the peer-reviewed literature. A set of Principal Components (PC) of the streamflow reconstructions were calibrated against a complementary PC set from an ~120-year long instrumental record of WY precipitation west of ~90˚W to yield a 407 year reconstruction at each gridpoint. We were motivated to assess the rarity of the current extreme 3-year drought in the Southwest US—particularly in California. Hence, our analysis focuses largely on the moisture side of the water balance, although of course, streamflow is an integrator of both evapotranspiration and precipitation. Nevertheless, the strong calibration and validation statistics obtained in our study (and notably for California and Nevada) suggest that WY streamflow in much of the West is mainly driven by precipitation variability. The current WY drought (2013-14) and three year (2012-2014) extended drought is ranked in the instrumental record for California itself as the driest such period since 1895. In our longer reconstructed record of California and adjacent western Nevada, the current dry episode—exceeding one year's mean total in three years—is similar to only three other such occurrences over the past 440 years, centered on 1654, 1795, and 1930. Analyses of anomalous mid-tropospheric (700 hPa) circulation features over the instrumental period (since 1895) show the presence of persistent anticyclonic flow over, near, and to the west of the US Pacific coast during the driest WYs. Some differences in the location and spatial structure of the anomalous Northeast Pacific ridge are evident and these are discussed in the context of low-frequency climate variability and change.