Forcing of hydroclimatic variability in the northwestern Great Plains since AD 1406

The 20th century hydroclimatology of northwestern North America has been linked to naturally recurring large-scale climate patterns such as the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). Few observed hydroclimatic records from this region exceed in length the ∼60-year periodicity of the lower frequency climate oscillations; however, tree-ring proxy data from semi-arid western North America document natural hydroclimate variation over centennial to millennial scales. We reconstructed the summer Palmer Drought Severity Index (PDSI) over the western Canadian Prairies, providing a record of drought for the past 600 years. This long reconstruction was examined for information about the severity, intensity, and duration of positive (wet) and negative (dry) summer moisture anomalies during the different phases of the PDO and the ENSO, as reconstructed by other researchers from independent proxy datasets. As well, by comparing our PDSI reconstruction to other researchers' independent regional summer temperature reconstruction, we were able to identify warm versus cold droughts, and warm versus cold wet periods. The reconstruction was also compared to 500 hPa geopotential heights. These results suggest that summer moisture conditions are associated with the North Pacific Ocean and tropical Pacific Ocean variability. Summer droughts generally have occurred during the positive PDO phase; however, ENSO conditions have varied between El Niño and La Niña. The most severe droughts, such as those of AD 1858–1872 and AD 1930–1941, were commonly associated with higher summer temperatures, the positive phase of the PDO, and increased ENSO variability.