Experimental Dendroclimatic Reconstruction of the Southern Oscillation

Exactly dated tree-ring chronologies from ENSO-sensitive regions in subtropical North America and Indonesia together register the strongest ENSO signal yet detected in tree-ring data worldwide and have been used to reconstruct the winter Southern Oscillation index (SOI) from 1706 to 1977. This reconstruction explains 53% of the variance in the instrumental winter SOI during the boreal cool season (December–February) and was verified in the time, space, and frequency domains by comparisons with independent instrumental SOI and sea surface temperature (SST) data. The large-scale SST anomaly patterns associated with ENSO in the equatorial and North Pacific during the 1879–1977 calibration period are reproduced in detail by this reconstruction. Cross-spectral analyses indicate that the reconstruction reproduces over 70% of the instrumental winter SOI variance at periods between 3.5 and 5.6 yr, and over 88% in the 4-yr frequency band. Oscillatory modes of variance identified with singular spectrum analysis at ~3.5,4.0, and 5.8 yr 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 variability of winter SOI, more frequent cold events, and a slightly stronger 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 changes in tropical Pacific climate identified in this reconstruction and, if substantiated, it will have important implications to the low-frequency dynamics of ENSO.