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David W. Stahle and Malcolm K. Cleaveland

Abstract

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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David W. Stahle and Malcolm K. Cleaveland

Abstract

A selected group of nine climate-sensitive tree-ring chronologies from old post oak trees are used to reconstruct the June Palmer Drought Severity Index (PDSI) from 1698 to 1980 for two large regions in northern and southern Texas. Analysis of tree growth and monthly climate variables indicate that the June PDSI is the most robust climate signal evident in these chronologies, and principal component analysis (PCA) reveals a north-south geographic pattern in the relationships between the regional tree-ring time series. Serially random amplitude series from the first two significant eigenvectors of tree growth, which explain 65% of the total variance in the tree-ring data, were entered into stepwise multiple regression as predictors of regionally averaged June PDSI in north and south Texas for the common interval 1931–80. The regression models explain 59% and 60% of the variance in north and south Texas June PDSI, respectively, and both reconstructions are well verified against independent June PDSI data available on a statewide basis from 1888 to 1930. The weak persistence present in the observed June PDSI series was added to the serially random tree-ring reconnections prior to verification, using autoregressive modeling procedures.

The mean and variance of June PDSI during the 50-yr period of meteorological observation (1931–80) appear to be representative of the last 283 yr, but significant changes in average June PDSI for Texas appear to have occurred over both 30 and ∼90-yr time intervals. Moderate or more severe June droughts (PDSI ≤ −2.0) have an estimated recurrence probability of over 90% each decade, and the risk of extreme June drought (PDSI ≤ −4.0) is estimated at over 50% every 15 yr in north Texas and every 10 yr in south Texas. The reconstructions faithfully reproduce the frequency domain properties of the actual June PDSI, and marginally significant spectral peaks are present at 2.3 yr and between 14 and 18.67 yr in both reconstructions. Significant interannual persistence of June moisture extremes apparent in the statewide June temperature, precipitation, and PDSI data from 1888 to 1982 is also present in both regional reconstructions from 1698 to 1980. The reconstructions indicate that the risk for below average June moisture conditions increases to at least 65% in north and south Texas in the summer following a June drought (PDSI ≤ −2.0). Interannual persistence is also indicated for June wetness anomalies and may have some modest value in statistical forecasts of growing season moisture conditions in Texas.

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David W. Stahle and Malcolm K. Cleaveland

Tree-ring chronologies can provide surprisingly accurate estimates of the natural variability of important climate parameters such as precipitation and temperature during the centuries prior to the Industrial Revolution. Bald cypress tree-ring chronologies have been used to reconstruct spring rainfall for the past 1000 years in North Carolina, South Carolina, and Georgia. These rainfall reconstructions explain from 54% to 68% of the spring rainfall variance in each state, and are well verified against independent rainfall measurements. In fact, these tree-ring data explain only 6% to 13% less statewide rainfall variance than is explained by the same number of instrumental raingage records. The reconstructions indicate that the spring rainfall extremes and decade-long regimes witnessed during the past century of instrumental observation have been a prominent feature of southeastern United States climate over the past millennium. These spring rainfall regimes are linked in part to anomalies in the seasonal expansion and migration of the subtropical anticyclone over the North Atlantic. The western sector of the Bermuda high often ridges strongly westward into the southeastern United States during dry springs, but during wet springs it is usually located east of its mean position and well offshore. Similar anomalies in the western sector of the Bermuda high occurred during multidecadal regimes of spring rainfall over the Southeast. During the relatively dry springs from 1901 to 1939, the high often ridged into the Southeast, but the western periphery of the high was more frequently located offshore during the relatively wet period from 1940 to 1980. Spring and summer rainfall extremes and decade-long regimes over the Southeast are frequently out of phase, and the tendency for wet (dry) springs to be followed by dry (wet) summers also appears to reflect anomalies in the zonal position of the Bermuda high during spring and summer.

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Edward R. Cook, David M. Meko, David W. Stahle, and Malcolm K. Cleaveland

Abstract

The development of a 2° lat × 3° long grid of summer drought reconstructions for the continental United States estimated from a dense network of annual tree-ring chronologies is described. The drought metric used is the Palmer Drought Severity Index (PDSI). The number of grid points is 154 and the reconstructions cover the common period 1700–1978. In producing this grid, an automated gridpoint regression method called “point-by-point regression” was developed and tested. In so doing, a near-optimal global solution was found for its implementation. The reconstructions have been thoroughly tested for validity using PDSI data not used in regression modeling. In general, most of the gridpoint estimates of drought pass the verification tests used. In addition, the spatial features of drought in the United States have been faithfully recorded in the reconstructions even though the method of reconstruction is not explicitly spatial in its design.

The drought reconstructions show that the 1930s “Dust Bowl” drought was the most severe such event to strike the United States since 1700. Other more local droughts are also revealed in the regional patterns of drought obtained by rotated principal component analysis. These reconstructions are located on a NOAA Web site at the World Data Center-A in Boulder, Colorado, and can be freely downloaded from there.

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Carlos Le Quesne, David W. Stahle, Malcolm K. Cleaveland, Matthew D. Therrell, Juan Carlos Aravena, and Jonathan Barichivich

Abstract

An expanded network of moisture-sensitive tree-ring chronologies has been developed for central Chile from long-lived cypress trees in the Andean Cordillera. A regional ring width chronology of cypress sites has been used to develop well-calibrated and verified estimates of June–December precipitation totals for central Chile extending from a.d. 1200 to 2000. These reconstructions are confirmed in part by historical references to drought in the seventeenth and eighteenth centuries and by nineteenth-century observations on the position of the Río Cipreses glacier. Analyses of the return intervals between droughts in the instrumental and reconstructed precipitation series indicate that the probability of drought has increased dramatically during the late nineteenth and twentieth centuries, consistent with selected long instrumental precipitation records and with the general recession of glaciers in the Andean Cordillera. This increased drought risk has occurred along with the growing demand on surface water resources and may heighten socioeconomic sensitivity to climate variability in central Chile.

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Daniel L. Druckenbrod, Michael E. Mann, David W. Stahle, Malcolm K. Cleaveland, Matthew D. Therrell, and Herman H. Shugart

This study presents two independent reconstructions of precipitation from James Madison's Montpelier plantation at the end of the eighteenth century. The first is transcribed directly from meteorological diaries recorded by the Madison family for 17 years and reflects the scientific interests of James Madison and Thomas Jefferson. In his most active period as a scientist, Madison assisted Jefferson by observing the climate and fauna in Virginia to counter the contemporary scientific view that the humid, cold climate of the New World decreased the size and number of its species. The second reconstruction is generated using tree rings from a forest in the Montpelier plantation and connects Madison's era to the modern instrumental precipitation record. These trees provide a significant reconstruction of both early summer and prior fall precipitation. Comparison of the dendroclimatic and diary reconstructions suggests a delay in the seasonality of precipitation from Madison's era to the mid-twentieth century. Furthermore, the dendroclimatic reconstructions of early summer and prior fall precipitation appear to track this shift in seasonality.

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