Climate of the Past www.clim-past.net 1814-9324 1814-9332 6 1 2010 10.5194/cp-6-1-2010 http://www.clim-past.net/6/1/2010/ http://www.clim-past.net/6/1/2010/cp-6-1-2010.html http://www.clim-past.net/6/1/2010/cp-6-1-2010.pdf 1 17 2010-01-05 A unified proxy for ENSO and PDO variability since 1650 S. McGregor shaynemc@hawaii.edu A. Timmermann O. Timm IPRC, SOEST, University of Hawaii at Manoa, Hawaii, USA In this manuscript we have attempted to consolidate the common signal in previously defined proxy reconstructions of the El Niño-Southern Oscillation into one individual proxy titled the Unified ENSO Proxy (UEP). While correlating well with the majority of input reconstructions, the UEP provides better representation of observed indices of ENSO, discrete ENSO events and documented historical chronologies of ENSO than any of these input ENSO reconstructions. Further to this, the UEP also provides a means to reconstruct the PDO/IPO multi-decadal variability of the Pacific Ocean as the low-pass filtered UEP displays multi-decadal variability that is consistent with the 20th century variability of the PDO and IPO. The UEP is then used to describe changes in ENSO variability which have occurred since 1650 focusing on changes in ENSOs variance, multi-year ENSO events, PDO-like multi-decadal variability and the effects of volcanic and solar forcing on ENSO. We find that multi-year El Niño events similar to the 1990–1995 event have occurred several times over the last 3 1/2 centuries. Consistent with earlier studies we find that volcanic forcing can induce a statistically significant change in the mean state of ENSO in the year of the eruption and a doubling of the probability of an El Niño (La Niña) event occurring in the year of (three years after) the eruption. 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