Abstract
One of the most familiar and well-documented patterns of drought across western North American is the dipole pattern between the US northern Rocky Mountains/Pacific Northwest and the southwestern US. Principle components analysis (PCA) of gridded Palmer Drought Severity Index (PDSI) values across North America demonstrates the consistency of a northwest/southwest dipole as a leading pattern of drought in the instrumental PDSI data, and in paleoclimatic data over the past six centuries as well as in the medieval period. Correlation maps of PC loadings, and sea surface temperatures and 500 mb heights suggest an association with ENSO-like circulation, and indeed, ENSO is commonly associated with the dipole pattern. Although this pattern of drought and its relationship with ocean/atmospheric circulation provide an understanding of one of the main influences on drought in the Pacific Northwest and southwestern US, this information is less useful for understanding drought in regions that lie in between the dipole centers. We have investigated the boundary between the two signs of the dipole, a dynamic feature that varies over time. In particular, the upper Colorado River basin, which lies in this boundary region, is the focus of an analysis in which we explore how shifts in the dipole boundary result in different types of droughts in the basin. Colorado River annual streamflow at Lees Ferry, from the gage record and tree-ring based reconstructions of streamflow for the past 12 centuries, is assessed along with the spatial and temporal variability in the dipole pattern to make inferences about the dipole’s role in Colorado River droughts. Results of this analysis suggest that droughts in the upper Colorado River basin correspond with droughts that impact the southwestern US in some years, particularly in association cool ENSO events, while in other years, upper Colorado River basin droughts are in common with those impacting the Pacific Northwest. This variability, which is often a reflection of the changing dipole boundary, is also evident in the paleoclimatic record.
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