干旱区生态环境知识资源中心

We examine instrumental meteorological records to compare recent precipitation regimes in the eastern and western Uinta Mountains region of Utah. The comparison although the summer monsoon contributes a higher proportion of demonstrates that, the two regions are significantly correlated in terms of annual precipitation in the east, the two regions are significantly correlated in terms of precipitation variations including summer precipitation. Major droughts, such as the 1930s Dustbowl event, the 1976-1977 event, and the 1987-1989 event, are largely typified by strong decreases in winter precipitation, although deficits can extend into summer. Droughts generally impact the entire region when they occur. Unlike the Southwest and the Pacific Northwest, year-to-year precipitation variability in the Uinta Mountains region does not appear correlated with the El Nino-Southern Oscillation or the Pacific Decadal Oscillation. However, severe prolonged droughts such as the 1976-1977 and 1986-1987 events are related to decreases in eastern Pacific Ocean sea surface temperatures. We extend the record of hydroclimatic variability for the Uinta Mountains by using tree-ring chronologies from Pinus edulis (two-needle pinyon) to reconstruct the summer Palmer Drought Severity Index (PDSI) back to A.D. 1405. The analysis demonstrates that extreme droughts (PDSI < -4) and extended multiannual to multidecadal and periods (PDSI < 0 to < -2) are a recurrent feature of the Uinta hydroclimatic regime. Extreme droughts of PDSI < -4 typically occur two to five times per century, with an annual probability of occurrence of similar to 4.4%. In the context of prolonged severe droughts, the 20th century enjoyed relatively moist conditions compared to the 17th, 18th, and 19th centuries. The most prominent of these earlier arid periods was an apparent 'mega-drought' between ca. A.D. 1625 and 1670 that is evident in prolonged depression of PDSI values in the Uinta Mountains region, decreased discharge of the Ashley River, and decreased precipitation in the adjacent Uinta Basin. There was a general depression of the Pacific Decadal Oscillation during this time, suggesting some linkage between conditions in the Pacific Ocean and prolonged aridity. Wavelet analysis demonstrates the presence of multidecadal variability in aridity, but the frequency and power of long-term modes of variability are inconsistent over the past 600 years.