David M. Meko

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...

North-South Precipitation Patterns in Western North America on Interannual-to-Decadal Timescales

The overall amount of precipitation deposited along the West Coast and western cordillera of North America from 258 to 558N varies from year to year, and superimposed on this domain-average variability are varying north‐south contrasts on timescales from at least interannual to interdecadal. In order to better understand the north‐south precipitation contrasts, their interannual and decadal variations are studied in terms of how much they affect overall precipitation amounts and how they are related to large-scale climatic patterns. Spatial empirical orthogonal functions (EOFs) and spatial moments (domain average, central latitude, and latitudinal spread) of zonally averaged precipitation anomalies along the westernmost parts of North America are analyzed, and each is correlated with global sea level pressure (SLP) and sea surface temperature series, on interannual (defined here as 3‐7 yr) and decadal (7 yr) timescales. The interannual band considered here corresponds to timescales that are particularly strong in tropical climate variations and thus is expected to contain much (%) (%) p ¯

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].

A 1,200-year perspective of 21st century drought in southwestern North America

A key feature of anticipated 21st century droughts in Southwest North America is the concurrence of elevated temperatures and increased aridity. Instrumental records and paleoclimatic evidence for past prolonged drought in the Southwest that coincide with elevated temperatures can be assessed to provide insights on temperature-drought relations and to develop worst-case scenarios for the future. In particular, during the medieval period, ∼AD 900–1300, the Northern Hemisphere experienced temperatures warmer than all but the most recent decades. Paleoclimatic and model data indicate increased temperatures in western North America of approximately 1 °C over the long-term mean. This was a period of extensive and persistent aridity over western North America. Paleoclimatic evidence suggests drought in the mid-12th century far exceeded the severity, duration, and extent of subsequent droughts. The driest decade of this drought was anomalously warm, though not as warm as the late 20th and early 21st centuries. The convergence of prolonged warming and arid conditions suggests the mid-12th century may serve as a conservative analogue for severe droughts that might occur in the future. The severity, extent, and persistence of the 12th century drought that occurred under natural climate variability, have important implications for water resource management. The causes of past and future drought will not be identical but warm droughts, inferred from paleoclimatic records, demonstrate the plausibility of extensive, severe droughts, provide a long-term perspective on the ongoing drought conditions in the Southwest, and suggest the need for regional sustainability planning for the future.

A New Assessment of Possible Solar and Lunar Forcing of the Bidecadal Drought Rhythm in the Western United States

Abstract A new drought area index (DAI) for the United States has been developed based on a high-quality network of drought reconstructions from tree rings. This DAI is remarkably similar to one developed earlier based on much less data and shows strong evidence for a persistent bidecadal drought rhythm in the western United States since 1700. This rhythm has in the past been associated with possible forcing by the 22-yr Hale solar magnetic cycle and the 18.6-yr lunar nodal tidal cycle. The authors make a new assessment of these possible forcings on DAI using different methods of analysis. In so doing, they confirm most of the previous findings. In particular, there is a reasonably strong statistical association between the bidecadal drought area rhythm and years of Hale solar cycle minima and 18.6-yr lunar tidal maxima. The authors also show that the putative solar and lunar effects appear to be interacting to modulate the drought area rhythm, especially since 1800. These results do not eliminate the pos...

Evidence of a 22-year Rhythm of Drought in the Western United States Related to the Hale Solar Cycle since the 17th Century

Families of Drought Area Indices (DAI) have been derived from tree-ring data for the entire U.S. west of the Mississippi River, for each year back to either 1700 or 1600 A.D., depending on the data base used. Each DAI is expressed in terms of the relative area in which the Palmer Drought Severity Index (PDSI) lies below a specified threshold value between −1 (mild drought) and −4 (extreme drought). Three families of DAI are considered in the analysis reported in this paper. Each DAI family is based on reconstructions from a selection of between 40 and 65 tree-ring sites ranging from Canada to Mexico and from the west coast to the Plains states. Variance spectrum analysis of the DAI series shows a concentration of variance at periods near 22 yr, at significance levels ranging from 5 to 0.1% (relative to a pink noise continuum). Band-pass filters tuned to periods near 22 yr are used in a form of harmonic dial analysis proposed by G. W. Brier to verify the extent of phase locking between the drought area variations and the Hale sunspot cycle since 1700 A.D. Phase locking is confirmed at significance levels of order 1 to 0.1% for all DAI families and all drought severity limits except extreme drought (PDSI < −4). A tendency is found for the amplitude of the 22 yr drought rhythm to vary systematically in parallel with the amplitude (envelope) of the Hale sunspot cycle, on the Gleissberg time scale of about 90 yr. This relationship is statistically significant between the 5 and 1% levels, and is independent of the phase locking found within the Hale cycle. The DAI series that extend back to 1600 A.D. reflect a well-defined 22 yr drought rhythm during the early stages of the Maunder Minimum of solar activity, but a very weak rhythm near the end of the Maunder Minimum. The average areal extent of drought was relatively low, and remained low for a prolonged period, during the Maunder Minimum. This analysis strongly supports earlier evidence of a 22 yr drought rhythm, or “cycle,” in the U.S. and suggests that the drought rhythm is in some manner controlled by long-term solar variability directly or indirectly related to solar magnetic effects. The solar control is best described as a modulation of terrestrial drought-inducing mechanisms, such that it alternately encourages and discourages the development of major continental droughts which are set up by evolutionary climatic processes unrelated to solar activity.

Dendroclimatic Reconstruction with Time Varying Predictor Subsets of Tree Indices

Abstract Tree-ring site chronologies, the predictors for most dendroclimatic reconstructions, are essentially mean-value functions with a time varying sample size (number of trees) and sample composition. Because reconstruction models are calibrated and verified on the most recent, best-replicated part of the chronologies, regression and verification statistics can be misleading as indicators of long-term reconstruction accuracy. A new reconstruction method is described that circumvents the use of site chronologies and instead derives predictor variables from indices of individual trees. Separate regression models are estimated and cross validated for various time segments of the tree-ring record, depending on the trees available at the time. This approach allows the reconstruction to extend to the first year covered by any tree in the network and yields direct evaluation of the change in reconstruction accuracy with tree-ring sample composition. The method includes two regression stages. The first is to ...

Spatial Patterns of Tree-Growth Anomalies in the United States and Southeastern Canada

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 r...

Assessing the risk of persistent drought using climate model simulations and paleoclimate data

Projected changes in global rainfall patterns will likely alter water supplies and ecosystems in semiarid regions during the coming century. Instrumental and paleoclimate data indicate that natural hydroclimate fluctuations tend to be more energetic at low (multidecadal to multicentury) than at high (interannual) frequencies. State-of-the-art global climate models do not capture this characteristic of hydroclimate variability, suggesting that the models underestimate the risk of future persistent droughts. Methods are developed here for assessing the risk of such events in the coming century using climate model projections as well as observational (paleoclimate) information. Where instrumental and paleoclimate data are reliable, these methods may provide a more complete view of prolonged drought risk. In the U.S. Southwest, for instance, state-of-the-art climate model projections suggest the risk of a decade-scale megadrought in the coming century is less than 50%; the analysis hereinsuggests that the risk is at least 80%, and may be higherthan 90% in certain areas. The likelihood of longer-lived events (.35yr) is between 20% and 50%, and the risk of an unprecedented 50-yr megadrought is nonnegligible under the most severe warming scenario (5%‐10%). These findings are important to consider as adaptation and mitigation strategies are developed to cope with regional impacts of climate change, where population growth is high and multidecadal megadrought—worse thananythingseenduringthelast2000years—wouldposeunprecedentedchallengestowaterresourcesinthe region.

Drought Recurrence in the Great Plains as Reconstructedfrom Long-Term Tree-Ring Records

Abstract Recently collected tree-ring data were used to reconstruct drought from 1700 to the present in four regionsflanking the Great Plains. Regions were centered in Iowa, Oklahoma, eastern Montana and eastern Wyoming.Reconstructions derived by multiple linear regression explained from 44 to 56% of the variance in regionallyaveraged annual precipitation from 1933 to 1977. Years of widespread severe drought clustered into droughtepochs lasting 5-10 years. A weighted mean of the four regional reconstructions pointed out the severity ofthe 1930s drought; the years 1934, 1936 and 1939 ranked among the driest 10 of 278 years. When droughtconditions were averaged over periods of three or more years, the 1930s drought was equaled or surpassedin severity by droughts in the 1750s, 1820s and 1860s. Spectral analysis of the 1700-1977 reconstructionindicated that precipitation averaged over the four regions had a penodicity of 16-19 years, but reconstructions for the individual regions deviated considerably fr...