Malcolm K. Cleaveland

Drought Reconstructions for the Continental United States

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 Bow...

Experimental Dendroclimatic Reconstruction of the Southern Oscillation

Abstract 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 ana...

Tree‐ring data document 16th century megadrought over North America

The two most severe, sustained droughts in the continental United States during the 20th century occurred in the 1930s and 1950s. The 1950s drought was most extreme over the southwest and southern Great Plains, where ecological consequences are still evident on the landscape [Swetnam and Betancourt], 1998].The Dust Bowl,vividly recounted in John Steinbecks The Grapes of Wrath, was the nations most severe, sustained,and widespread drought of the past 300 years, according to tree-ring reconstructions of the Palmer drought severity index (PDSI) across the continental United States [Cook et al., 1999] (http://www.ngdc.noaa.gov/paleo/pdsiyear.html. Droughts during the 1750s, 1820s, and 1850s–1860s estimated from tree rings were similar to the 1950s drought in terms of magnitude, persistence, and spatial coverage, but these earlier episodes do not appear to have surpassed the severity or extent of the Dust Bowl drought. However, longer tree-ring reconstructions of PDSI for the United States and precipitation for northwestern Mexico and western Canada indicate that the “megadrought” of the 16th century far exceeded any drought of the 20th century (Figure 1) [also see Wood-house and Overpeck, 1998], and is considered to be the most severe prolonged drought over much of North America for at least the last 500 years [Meko et al., 1995].

Texas drought history reconstructed and analyzed from 1698 to 1980

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 PDS...

TREE-RING RECONSTRUCTED WINTER PRECIPITATION AND TROPICAL TELECONNECTIONS IN DURANGO, MEXICO

Earlywood width chronologies from Douglas-fir tree rings were used to reconstruct winter (November–March) precipitation for more than 600 years over Durango, Mexico. The tree-ring data were obtained from two sites of long-lived Douglas-fir in northern and southern Durango and the seasonal climatic precipitation data were regionally averaged from five weather stations well distributed across the state. The averaged earlywood chronology accounted for 56% of the variance in instrumental November–March precipitation 1942–1983. We validated the reconstruction against independent precipitation records. The worst winter drought of the 20th century in Durango occurred 1950–1965. However, the reconstruction indicates droughts more severe than any witnessed in the 20th century, e.g., the 1850s–1860s, and the megadrought in the mid- to late-16th century. Reconstructed winter precipitation 1540–1579 shows 33 of 40 years were dry. Persistent drought may be linked to extended La Niña episodes. The Tropical Rainfall Index (TRI) correlates well with instrumental and reconstructed winter precipitation (r = 0.49 and 0.55, respectively), reflecting the strong ENSO modulation of cool season climate over northern Mexico. The ENSO teleconnection varies through time, with TRI-reconstructed precipitation correlations ranging from 0.78 to 0.27 in five periods 1895–1993. The 1942–1983 winter observed and reconstructed Durango data correlate well with the corresponding seasonalization of the All-Mexico Rainfall Index (AMRI; r=0.68, P<0.0001 and r=0.70, P<0.001, respectively), indicating that both the observed and the reconstructed precipitation often reflect broad-scale precipitation anomalies across Mexico. New long Douglas-fir and baldcypress tree-ring chronologies are now available for central and southern Mexico near major population centers, allowing the exploration of relationships between drought, food scarcity, and social and political upheaval in Mexican history.

Southern Oscillation Extremes Reconstructed from Tree Rings of the Sierra Madre Occidental and Southern Great Plains

Abstract The El Ninio-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 correl...

Ancient Austrocedrus Tree-Ring Chronologies Used to Reconstruct Central Chile Precipitation Variability from A.D. 1200 to 2000

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 Rio 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.

Tree-ring reconstructed rainfall over the southeastern U.S.A. during the medieval warm period and little ice age

A 1053-year reconstruction of spring rainfall (March-June) was developed for the southeastern United States, based on three tree-ring reconstructions of statewide rainfall from North Carolina, South Carolina, and Georgia. This regional reconstruction is highly correlated with the instrumental record of spring rainfall (r = +0.80; 1887-1982), and accurately reproduces the decade-scale departures in spring rainfall amount and variance witnessed over the Southeast during the past century. No large-magnitude centuries-long trends in spring rainfall amounts were reconstructed over the past 1053 years, but large changes in the interannual variability of spring rainfall were reconstructed during portions of the Medieval Warm Period (MWP), Little Ice Age (LIA), and the 20th century. Dry conditions persisted at the end of the 12th century, but appear to have been exceeded by a reconstructed drought in the mid-18th century. High interannual variability, including five extremely wet years were reconstructed for a 20-yr period during the late 16th and early 17th centuries, and may reflect amplified atmospheric circulation over eastern North America during what appears to have been one of the most widespread cold episodes of the Little Ice Age.

Iowa climate reconstructed from tree rings, 1640–1982

Tree ring indices from an expanded network of 17 white oak (Quercus alba} sites in eastern and central Iowa were used to reconstruct state average July Palmer hydrological drought index (PHDI), annual precipitation (previous August to current July), and other climate variables for 1640–1982. We removed nonclimatic variance trends caused by changing sample size and senescent growth. July PHDI correlated better with tree growth than annual precipitation. Occurrence of prolonged droughts throughout the reconstruction suggests that decades like the 1930s occur about twice per century in Iowa. Iowa climate is correlated with the Southern Oscillation Index (SOI) from June in the year of El Nino onset (Yr0) through the next February (Yr+1), with negative SOI (El Nino) associated with wetter conditions. When the June (Yr0) to February (Yr+1) average SOI reaches extremes ≥ + 1.0 or ≤−1.0, it correlates significantly with observed and reconstructed July PHDI (r = −0.37 and −0.56, respectively). Climate during solar cycles centered on sunspot minima alternates between wet and dry regimes that differ by an average of 1.21 units of observed July PHDI and 46.7 mm of annual precipitation for 1877–1982. The solar relationship has been stable since 1640. Combining solar and SOI influences in forecasts may improve prediction of Iowa climate.

A 963-year reconstruction of summer (JJA) streamflow in the White River, Arkansas, USA, from tree-rings.

An average of three baldcypress (Taxodium distichum) ring-width chronologies was used to recon struct total summer (JJA) mean daily flow of the White River at Clarendon, Arkansas, for years 1023–1985. A quadratic transformation of the tree-ring data accounted for 68% (R2 adjusted for degrees of freedom lost) when regressed against total summer flow 1931–1985. The distribution of the quadratic reconstruction matched the gauged distribution much better than a simple linear model which only accounted for 62% of the variance. The model was validated by comparing regression estimates against independent data. Years with summer flow below the 25th percentile occur nonrandomly, i.e., they tend to cluster, in both the reconstructed and gauged data. Hydrologic regimes have apparently varied considerably in the past on annual to century time-scales, with extended dry and wet periods that exceeded anything in the modern record. The frequency of both wet and dry extremes has varied considerably over the last millennium. The eleventh through thirteenth cen turies were not analysed due to reduced replication, but the well-replicated fourteenth and twentieth centuries both have large numbers of extremes. The twentieth century appears to have more extreme low flows than the previous centuries and also to have a large number of high flows. The practical consequences for society of variation in extremes and persistence of low flows may be considerable. Climatic change or anthropogenic changes to the watershed (e.g., widespread upland clearing for agriculture and logging of bottomland forests) may be responsible for the change in hydrologic regime during the twentieth century.