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Franco Biondi

Abstract

Preparing for future hydroclimatic variability greatly benefits from long (i.e., multicentury) records at seasonal to annual time steps that have been gridded at kilometer-scale spatial intervals over a geographic region. Kriging is commonly used for optimal interpolation of environmental data, and space–time geostatistical models can improve kriging estimates when long temporal sequences of observations exist at relatively few points on the landscape. A network of 22 tree-ring chronologies from single-leaf pinyon (Pinus monophylla) in the central Great Basin of North America was used to extend hydroclimatic records both temporally and spatially. First, the line of organic correlation (LOC) method was used to reconstruct October–May total precipitation anomalies at each tree-ring site, as these ecotonal environments at the lower forest border are typically moisture-limited areas. Individual site reconstructions were then combined using a hierarchical model of spatiotemporal kriging that produced annual anomaly maps on a 12 km × 12 km grid during the period in common among all chronologies (1650–1976). Hydroclimatic episodes were numerically identified using their duration, magnitude, and peak. Precipitation anomalies were spatially more variable during wet years than during dry years, and the evolution of drought episodes over space and time could be visualized and quantified. The most remarkable episode in the entire reconstruction was the early 1900s pluvial, followed by the late 1800s drought. The 1930s Dust Bowl drought was among the top 10 hydroclimatic episodes in the past few centuries. These results directly address the needs of water and natural resource managers with respect to planning for worst-case scenarios of drought duration and magnitude at the watershed level.

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Franco Biondi, Alexander Gershunov, and Daniel R. Cayan

Abstract

Climate in the North Pacific and North American sectors has experienced interdecadal shifts during the twentieth century. A network of recently developed tree-ring chronologies for Southern and Baja California extends the instrumental record and reveals decadal-scale variability back to 1661. The Pacific decadal oscillation (PDO) is closely matched by the dominant mode of tree-ring variability that provides a preliminary view of multiannual climate fluctuations spanning the past four centuries. The reconstructed PDO index features a prominent bidecadal oscillation, whose amplitude weakened in the late l700s to mid-1800s. A comparison with proxy records of ENSO suggests that the greatest decadal-scale oscillations in Pacific climate between 1706 and 1977 occurred around 1750, 1905, and 1947.

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