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Ian M. Howard and David W. Stahle

Abstract

Mean daily to monthly precipitation averages peak in late July over eastern Colorado and some of the most damaging Front Range flash floods have occurred because of extreme 1-day rainfall events during this period. Tree-ring chronologies of adjusted latewood width in ponderosa pine from eastern Colorado are highly correlated with the highest 1-day rainfall totals occurring during this 2-week precipitation maximum in late July. A regional average of four adjusted latewood chronologies from eastern Colorado was used to reconstruct the single wettest day observed during the last two weeks of July. The regional chronology was calibrated with the CPC 0.25° × 0.25° Daily U.S. Unified Gauge-Based Analysis of Precipitation dataset and explains 65% of the variance in the highest 1-day late July precipitation totals in the instrumental data from 1948 to 1997. The reconstruction and instrumental data extend fully from 1779 to 2019 and indicate that the frequency of 1-day rainfall extremes in late July has increased since the late eighteenth century. The largest instrumental and reconstructed 1-day precipitation extremes are most commonly associated with the intrusion of a major frontal system into a deep layer of atmospheric moisture across eastern Colorado. These general synoptic conditions have been previously linked to extreme localized rainfall totals and widespread thunderstorm activity over Colorado during the summer season. Chronologies of adjusted latewood width in semiarid eastern Colorado constitute a proxy of weather time-scale rainfall events useful for investigations of long-term variability and for framing natural and potential anthropogenic forcing of precipitation extremes during this 2-week precipitation maximum in a long historical perspective.

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Max C. A. Torbenson and David W. Stahle

Abstract

Land surface feedbacks impart a significant degree of persistence between cool and warm season moisture availability in the central United States. However, the degree of correlation between these two variables is subject to major changes that appear to occur on decadal to multidecadal time scales, even in the relatively short 115-yr instrumental record. Tree-ring reconstructions have extended the limited observational record of long-term soil moisture levels, but such reconstructions do not resolve the seasonal differences in moisture conditions. We present two separate 331-yr-long seasonal moisture reconstructions for the central United States, based on sensitive subannual and annual tree-ring chronologies that have strong and separate seasonal moisture signals: an estimate of the long-term May soil moisture balance and a second estimate of the short-term June–August atmospheric moisture balance. The predictors used in each seasonal reconstruction are not significantly correlated with the alternate season target. Both reconstructions capture over 70% of the interannual variance in the instrumental data for the calibration period and also share significant decadal and multidecadal variability with the instrumental record in both the calibration and validation periods. The instrumental and reconstructed moisture levels are both positively correlated between spring and summer strongly enough to have potential value in seasonal prediction. However, the relationship between spring and summer moisture exhibits major decadal changes in strength and even sign that appear to be related to large-scale ocean–atmosphere dynamics associated with the Atlantic multidecadal oscillation.

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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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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

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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Falko K. Fye, David W. Stahle, and Edward R. Cook

Abstract

Three great moisture anomalies were observed during the twentieth century over the western United States: a pluvial from 1905 to 1917, the Dust Bowl drought (1929–40), and the Southwestern drought of 1946–56. A composite analysis of the concurrent Pacific sea surface temperature (SST) field is used to infer the atmospheric circulation that may have been associated with these objectively defined decadal dry and wet periods. The early-twentieth-century pluvial occurred during a 13-yr SST regime with unusually cold water in the northern and northwestern North Pacific and in the eastern North Pacific. This pattern would favor a “Pineapple Express–like” mean storm track into the west. Warm ENSO-like conditions also observed during the pluvial would have favored an enhanced subtropical jet stream into the southwestern United States. The 11-yr Dust Bowl drought occurred during a poorly defined Pacific SST regime, although unusually cold water was present in the far western North Pacific. Weak warm SST conditions were also noted in the extreme northeastern North Pacific. This cold west–warm east SST pattern, although weak for the full 11-yr interval, may have contributed to positive atmospheric geopotential heights over the western and central United States during the Dust Bowl drought. Cooler SSTs in the eastern equatorial Pacific during some of the Dust Bowl years (e.g., 1934, 1935, 1938, and 1939) suggest a possible La Niña influence. La Niña conditions definitely seemed to have contributed to the 1950s drought, but the most anomalous SSTs for the 11-yr average were observed in the west-central North Pacific. The overall Pacific SST field during the 1946–56 drought was consistent with the cool phase of the Pacific decadal oscillation, and the warm SSTs in the west-central North Pacific would have favored a trough over the central North Pacific and a ridge over western North America in the upper-tropospheric flow.

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Dorian J. Burnette, David W. Stahle, and Cary J. Mock

Abstract

A continuous record of 65 987 daily-mean temperature observations since 1828 has been developed for Manhattan, Kansas, by screening and correcting original station records of the U.S. Army Surgeon General, the Smithsonian Institution, and the Signal Service. Hourly, minimum, and maximum temperature observations from seven discontinuous historical stations and four modern stations in Kansas, Missouri, and Oklahoma were used to compile this unbroken record of daily-mean temperature. The historical temperature data were linked with the modern temperature record for Manhattan after these data were adjusted for time of observation differences, station movements, and changes in the environment around the station. The new daily-mean temperature reconstruction for Manhattan now extends with confidence back to 1 July 1855 and with more uncertainty back to 1 July 1828. The uncertainty prior to 1855 is due to instrumentation changes in 1843 and changes in observation practices in 1855 that occurred at many stations. The error estimates reported in this paper do not reflect these potential inhomogeneities and should be considered lower limits. Nonetheless, this new daily record indicates significant warming in all seasons; in heating and cooling degree-days; in the warmest and coldest days of the year; in extremes above the 90th percentile and below the 10th percentile; in the frequency of winter cold waves and summer heat waves; and in the overall annual-mean temperature, which has warmed by 1.57° ± 0.23°C since 1855 (1.27° ± 0.23°C since 1829). The warm Dust Bowl event in the summer of the 1930s and cold winters of the 1870s and 1880s dominate the reconstruction and included some of the warmest and coldest daily extremes, respectively, of the last 154–180 yr. This new reconstruction is currently the longest unbroken daily corrected record in the Americas. These data indicate that the nineteenth century was fundamentally cooler than the twentieth and early twenty-first century.

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Matthew D. Therrell, David W. Stahle, and Rodolfo Acuña Soto

Sixteenth-century Aztec codices preserve a record of at least 13 drought years in central Mexico during the prehispanic and early colonial period. Climate-sensitive tree-ring records recently developed for Mexico confirm 9 of the 13 Aztec drought dates, including the extended drought related to the infamous “famine of One Rabbit” in 1454. One Rabbit is the first year of the 52-yr Aztec calendar cycle, and folklore suggests that famine and catastrophe accompany its return. The Mexican treering data indicate that severe drought occurred immediately before 10 of the 13 One Rabbit years during and before the Aztec era a.d. 882–1558. This relationship between drought and the year preceding One Rabbit is statistically significant and suggests a real climatic origin for the “curse of One Rabbit.”

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Falko K. Fye, David W. Stahle, and Edward R. Cook

Instrumental Palmer Drought Severity Indexes (PDSI) averaged over the western United States and Great Plains document three major decadal moisture regimes during the twentieth century: the early twentieth-century pluvial, the Dust Bowl drought, and the 1950s drought. Tree-ring reconstructed PDSI for the contiguous Unites States replicates these three twentieth-century moisture regimes, and have been used to search for possible analogs over the past 500 yr. The early twentieth-century wet regime from 1905 to 1917 appears to have been the wettest episode across the West since A.D.1 500, but similar pluvials occurred in the nineteenth, seventeenth, and sixteenth centuries. The Dust Bowl drought (1929–40) was most severe over the northern Plains to the northern Rockies. No close analogs are found for the full severity and geographical focus of the Dust Bowl drought over the past 500 yr. The 1950s drought (1946–56) was concentrated over the Southwest and was replicated by some 12 droughts of similar spatial coverage and duration over the past 500 yr. One of these analogs, the sixteenth-century mega-drought, was also focused over the Southwest and appears to have surpassed the Dust Bowl drought in coverage, duration, and severity.

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David Meko, Edward R. Cook, David W. Stahle, Charles W. Stockton, and Malcolm K. Hughes

Abstract

A network of 248 tree-ring chronologies in the conterminous United States is assembled and analyzed by rotated principal components analysis (RPCA) to delineate “regions” of common tree-growth variation during the period 1705–1979. Spatial continuity of the tree-ring data is summarized by variogram analysis, and tree-ring data are gridded before RPCA to reduce effects of site clustering. Principal component drought information is evaluated by comparing PC scores and primary pattern coefficients with Palmer Drought Severity Index (PDSI) data from instrumental records.

High PC pattern coefficients group geographically into regions coinciding roughly with nine drought regions delineated by RPCA of PDSI by other researchers. The drought signal as measured by the correlation between tree-ring PC scores and July PDSI, 1929–79, is strongest in the South and the interior West (r>0.7), and weakest in the Northeast and Pacific Northwest (r<0.16). A count of years with large negative PC scores in multiple regions marks the 1950s as the extreme in widespread drought across the southern United States to 1705.

Tree-growth regions are sensitive to whether tree-ring data are gridded before RPCA. Principal components on ungridded tree-ring data tend to center on dense clusters of sites. The importance of site density is most noticeable in the RPCA results for the southeast, where the gridded data yield a PC centered on a group of climate-sensitive but widely spaced bald cypress chronologies. Cross-validation indicates that gridding of tree-ring anomalies over different species for drought reconstruction is more appropriate in the semiarid southwest than in cooler, moister regions—especially the northeast and the Pacific Northwest. Our results endorse the large-scale chronology network as a long-term proxy for the spatial and temporal patterns of past drought across the United States.

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