Late-Holocene teleconnections between tropical Pacific climatic variability and precipitation in the western USA: evidence from proxy records

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Abstract
Decadal to multidecadal fluctuations between drought and relatively more moist conditions are a defining characteristic of precipitation variability in the western USA. In an attempt to better understand the forcing mechanisms responsible for producing this variability, relationships between proxy indices of tropical climatic variability and precipitation-related reconstructions in the western USA are examined. The tropical proxies include the δ18O record from the Quelccaya ice cap (southern subtropical Andes) and the lacustrine sedimentation record from Laguna Pallcacocha (near-equatorial Andes). The precipi tation-related records are tree-ring reconstructions of river flow, drought severity and precipitation over the contiguo USA. One new finding is a consistent relation between decadal variability in El Niñno activity and precipitation-related proxies through much of the western USA during the past 1200 years. Surpris ingly, the proxy records do not show a systematic link between inferred El Niñno activity and precipitation in the interior Southwest USA. A second new result is the existence of relationships at decadal to multidecadal timescales between fluctuations in Quelccaya δ18O, precipitation-related proxies from the western USA, and proxy records from the Pacific Basin. This finding links the Quelccaya record to the transhemi spheric Pacific decadal-scale climatic variability documented from proxy records by Villalba et al. (2001), and suggests that similar changes in the large-scale circulation changes accompany both the decadal and multidecadal signals. A composite constructed using gridded proxy precipitation constructed on the basis of decadal-scale fluctuations in inferred-El Niñno activity shows a pattern rather different to Niñno-based teleconnection maps for the instrumental record. In the proxy-based composite, positive values cover much of the extreme western, west central and southeastern USA, with negative values in the central Mis sissippi Valley. The interior Southwest shows essentially no response. A second composite based on multi decadal variability in Quelccayaδ18O shows a well-developed dipole pattern across the western USA with opposing centres in the interior Southwest and Northwest. It is suggested that this dipole pattern results from multidecadal variability in tropical Pacific SSTs and resulting changes in the strength of the Northeast Pacific High and the latitudinal distribution of cool season westerlies across the eastern North Pacific and western USA.