EXPERIMENTAL DENDROCLIMATIC RECONSTRUCTION OF THE SOUTHERN OSCILLATION

Exactly dated tree-ring chronologies from ENSO-sensitive regions in su btropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been use d to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the inst rumental winter SOI during the boreal cool season (December-February) and was verified in the time, space, and frequency domains by comparis ons with independent instrumental SOI and sea surface temperature (SST ) data. The large-scale SST anomaly patterns associated with ENSO in t he equatorial and North Pacific during the 1879-1977 calibration perio d are reproduced in detail by this reconstruction. Cross-spectral anal yses indicate that the reconstruction reproduces over 70% of the instr umental winter SOI variance at periods between 3.5 and 5.6 yr, and ove r 88% in the 4-yr frequency band. Oscillatory modes of variance identi fied with singular spectrum analysis at similar to 3.5, 4.0, and 5.8 y r in both the instrumental and reconstructed series exhibit regimelike behavior over the 272-yr reconstruction. The tree-ring estimates also suggest a statistically significant increase in the interannual varia bility of winter SOI, more frequent cold events, and a slightly strong er sea level pressure gradient across the equatorial Pacific from the mid-nineteenth to twentieth centuries. Some of the variability in this reconstruction must be associated with background climate influences affecting the ENSO teleconnection to subtropical North America and may not arise solely from equatorial ENSO forcing. However, there is some Limited independent support for the nineteenth to twentieth century c hanges in tropical Pacific climate identified in this reconstruction a nd, if substantiated, it will have important implications to the low-f requency dynamics of ENSO.