R. D'Arrigo á R. Villalba á G. Wiles Tree-ring estimates of Paci®c decadal climate variability Received: 15 September 2000 /Accepted: 30 March 2001 Abstract Decadal-scale oscillatory modes of atmo- sphere-ocean variability have recently been identi®ed in instrumental studies of the Paci®c sector. The regime shift around 1976 is one example of such a ¯uctuation, which has been shown to have signi®cantly impacted climate and the environment along the coastline of the western N and S Americas. The length of meteorological data for the Paci®c and western Americas critically limits analyses of such decadal-scale climate variability. Here we present reconstructions of the annual Paci®c Decadal Oscillation PDO) index based on western North American tree-ring records which account for up to 53% of the instrumental variance and extend as far back as AD 1700. The PDO reconstructions indicate that deca- dal-scale climatic shifts have occurred prior to the period of instrumental record. Evaluation of temperature and precipitation-sensitive tree-ring series from the northeast Paci®c as well as these reconstructions reveals evidence for a shift towards less pronounced interdecadal vari- ability after about the middle 1800s. Our analyses also suggest that sites from both the northeast Paci®c coast as well as the subtropical Americas need to be included in proxy data sets used to reconstruct the PDO. 1 Introduction The existence of Paci®c interdecadal variability has now been documented in a number of instrumental Tren- berth 1994; Latif et al. 1997; Mantua et al. 1997; Zhang et al. 1997), paleoclimatic Wiles et al. 1996, 1998; Biondi et al. 1999, 2001) and environmental Ebbes- meyer et al. 1991) records. The Paci®c Decadal Oscilla- tion PDO) index, characterized as the dominant decadal mode of North Paci®c sea surface temperature SST), is one measure of this variability Mantua et al. 1997). Recent observations indicate that the PDO is shifting into a negative mode, with possible implications for cooler conditions in western North America in the coming decades Mantua 2000). Strong linkages between PDO-related ¯uctuations and the climate of the United States and other locations, and environmental variables such as salmon production, have been identi®ed Gra- ham 1994; Nigam et al. 1999; Mantua 2000). The rela- tive importance of extratropical and tropical Paci®c SST in driving decadal ¯uctuations in climate in the USA and elsewhere in the circum-Paci®c sector is currently under investigation Graham 1994; Latif and Barnett 1996; Nigam et al. 1999). Instrumental records for the Paci®c are only available for the past century or less in many areas, limiting studies of decadal variability. High-resolution tree-ring records, hundreds of years in length, can be used to assess the nature of this variability on longer time scales. Here we describe decadal-scale modes of variation as re¯ected in tree-ring records from the northeast Paci®c sector. We also reconstruct the annual PDO index as far back as AD 1700 and use this information as an addi- tional means of investigating Paci®c variations over this interval. 2 Materials and methods 2.1 Northeast Paci®c tree-ring studies Wiles et al. 1996, 1998) utilized tree-ring records from coastal Alaska and vicinity to evaluate temperature variability for the Gulf of Alaska and its relation to North Paci®c decadal modes over the past four centuries. Similarities were seen between these tree-ring records and time series of the Aleutian Low as well as salmon Climate Dynamics 2001) 18: 219±224 Ó Springer-Verlag 2001 R. D'Arrigo &) Tree-Ring Laboratory, Lamont-Doherty Earth Observatory, P.O. Box 1000, Route 9W, Palisades, NY 10964, USA E-mail: druidrd@ldeo.columbia.edu R. Villalba Departamento de DendrocronologõÂa e Historia Ambiental, IANIGLA ± CRICYT, C.C. 330, 5500) Mendoza, Argentina G. Wiles The College of Wooster, Wooster, OH 44691, USA