Author
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Mauget, Steven |
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Submitted to: Trade Journal Publication
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 8/12/2015 Publication Date: 8/26/2015 Citation: Mauget, S.A. 2015. Optimal ranking regime analysis of TreeFlow dendrohydrological reconstructions. Clim.Past, 11, 1107-1125, doi:10.5194/cp-11-1107-2015, 2015. Interpretive Summary: The growth of western U.S. population and agricultural production centers in the second half of the 20th century spurred demand for better estimates of drought duration and frequency than were available from streamflow gauge records. However, existing gauge records were usually not long enough to estimate the return periods and intensity of extended drought and wet regimes. A common method of generating streamflow records during periods before gauge data was available involves estimating annual flow values from tree-ring growth patterns in the stream or river’s watershed. In meeting the need for these extended streamflow records, reconstructions of flow variation during past centuries have been conducted over a number of western U.S. watershed regions. Here, the Optimal Ranking Regime (ORR) time series analysis method was used to identify 6-100 year time windows containing significant drought and wet periods in 55 western U.S. streamflow reconstructions, and reconstructions of the level of the Great Salt Lake and San Francisco Bay salinity during 1500-2007. The method’s ability to identify the most significant runs of low and high ranked values in a reconstructed time series allows it to re-express the series as a simple sequence of periods of low or high streamflow, lake level, or salinity. Those ORR sequences, referred to here as Z-lines, can be plotted to identify consistent drought or wet regime patterns in the analysis of many reconstructions. The Z-lines for the 57 reconstructions evaluated here show a common pattern of IMD cycles of drought and wet periods during the late 16th and 17th centuries, a period with relatively few drought or wet periods during the 18th century, and the reappearance of alternating dry and wet periods during the 19th and early 20th centuries. This pattern suggests the possibility of similarly active and inactive oceanic modes in the North Pacific and North Atlantic, but similar patterns are not evident in the ORR analyses of reconstructed Pacific Decadal Oscillation (PDO), El Niño-Southern Oscillation, and North Atlantic seas-surface temperature time series. However, given the inconsistency in the analyses of four PDO reconstructions the possible role of ocean mechanisms is uncertain. In future research the ORR method might be applied to streamflow reconstructions around the Pacific Basin to try to resolve this uncertainty. Whereas the early emphasis was on the dating of prehistoric native American structures in the U.S. southwest (Nash 1999; Creasman et al. 2012), subsequent work has tried to better estimate hydrological variability over important watershed areas of the arid American west. Although the first streamflow reconstructions based on tree-ring data were conducted in the 1930s (see Meko and Woodhouse, 2011 for a review), the growth of western population and agricultural production centers in the latter half of the 20th century spurred demand for better estimates of hydrological variability than were available from gauged streamflow data records. Technical Abstract: The Optimal Ranking Regime (ORR) method was used to identify 6-100 year time windows containing significant ranking sequences in 55 western U.S. streamflow reconstructions, and reconstructions of the level of the Great Salt Lake and San Francisco Bay salinity during 1500-2007. The method’s ability to identify optimally significant and non-overlapping runs of low and high rankings allows it to re-express a reconstruction time series as a simplified sequence of regime segments marking intra- to multi-decadal (IMD) periods of low or high streamflow, lake level, or salinity. Those ORR sequences, referred to here as Z-lines, can be plotted to identify consistent regime patterns in the analysis of numerous reconstructions. The Z-lines for the 57 reconstructions evaluated here show a common pattern of IMD cycles of drought and pluvial periods during the late 16th and 17th centuries, a relatively dormant period during the 18th century, and the reappearance of alternating dry and wet IMD periods during the 19th and early 20th centuries. Although this pattern suggests the possibility of similarly active and inactive oceanic modes in the North Pacific and North Atlantic, such centennial-scale patterns are not evident in the ORR analyses of reconstructed Pacific Decadal Oscillation (PDO), El Niño-Southern Oscillation, and North Atlantic seas-surface temperature variation. But given the inconsistency in the analyses of four PDO reconstructions the possible role of centennial-scale oceanic mechanisms is uncertain. In future research the ORR method might be applied to climate reconstructions around the Pacific Basin to try to resolve this uncertainty. |
