The El Niñio-Southern Oscillation (ENSO) is correlated with climate and tree growth over northern Mexico and the southern Great Plains of the USA. Warm events favor moist-cool conditions from October through March (event years 0 and +1), and subsequent tree growth (year +1) in the region tends to be above average. The opposite climate and tree growth conditions prevail with less consistency during cold events. ENSO-sensitive tree-ring chronologies from this region were selected to develop two reconstructions of the Southern Oscillation index (SOI) back to 1699. For the first reconstruction, a multiple regression-based calibration equation between prewhitened and regionally averaged tree-ring data from Mexico and Oklahoma and a prewhitened winter (DJF) SO index during the period 1900–71 were used to estimate the winter SOI for each year from 1699 to 1971. The tree-ring predictors account for 41% of the winter SOI variance from 1900 to 1971, and the reconstructed SO indices are significantly correlated with independent winter SO indices available from 1866 to 1899.

Because correlation analyses indicate that SO extremes have a much stronger influence on climate and tree growth over Mexico and the southern United States than near-normal SO indices, a second reconstruction of just winter SOI extremes was also developed. Discriminant function analysis based on the growth anomalies at eight individual tree-ring sites located in Mexico, Texas, and Oklahoma was used to classify 56 years from 1699 to 1965 into two opposite winter SOI “extremes” (i.e., ≥0.5 or ≤ −0.5). Using a high posterior probability of membership in either extreme category (P ≥ 0.65), and ignoring classifications of near normal winter SO indices, the discriminant functions identified 27 years as winter SOI extremes during the calibration period from 1866 to 1965 (56 extremes actually occurred; reliability = 48%). However, 5 of the 27 classified extremes were actually near-normal indices (post agreement = 81% ). With a reasonably stationary relationship between Mexico/Southem Plains climate and winter SOI, validation tests indicate that the reliability of the classification-based reconstruction is about 50% , with a post agreement of at least 70%. Thew calibration and validation results suggest that the 56 extremes classified from 1699 to 1965 represent about half of the true number of extremes during this 267-year period, and that each reconstructed extreme has up to a 70% chance of representing a true winter SOI extreme.

The most accurate estimates of past winter SOI extremes may be achieved in those years when the regression and classification methods of reconstruction agree, but comparisons with the instrumental data indicate that evidence for a past extreme cannot be disregarded when based on only one method. Both reconstructions indicate an increase in the frequency of winter SOI extremes after ca. 1850. Because the regression and classification errors are randomly distributed through time, these and other reconstructed changes in event frequency may reflect real changes in the extratropical influence of the SO over Mexico and the southern United States, if not in the SO itself.

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