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Edward R. Cook
and
Gordon C. Jacoby

Abstract

A 248-year reconstruction of the low-flow (July, August and September) period of the Potomac River indicates that the prolonged drought of the 1960s may have been the most severe since 1730. However, there appear to have been several long periods of about 50 years in length when flow was generally above or below the long-term median flow. The period from 1900 through 1950, which comprises most of the measured flow period, was generally above median. Long-period climatic shifts can have important water resource implications.

The Potomac River streamflow at Point of Rocks, Maryland was reconstructed by using tree-ring chronologies from sites in or near the river basin. Canonical regression analysis was used to reconstruct simultaneously July, August and September discharge after screening all the tree-ring predictors. Verification statistics and cross-spectral analysis indicate that the average reconstruction of these three months is most reliable for periods longer than about six years and shorter than about three years. Spectral analysis of the reconstruction indicates the presence of a 15.7-year periodicity that warrants verification through examination of meteorological data, as well as through additional streamflow reconstructions in the region.

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Michelle Ho
,
Upmanu Lall
, and
Edward R. Cook

Abstract

Evolving patterns of droughts and wet spells in the conterminous United States (CONUS) are examined over 555 years using a tree-ring-based paleoclimate reconstruction of the modified Palmer drought severity index (PDSI). A hidden Markov model is used as an unsupervised method of classifying climate states and quantifying the temporal evolution from one state to another. Modeling temporal variability in spatial patterns of drought and wet spells provides the ability to objectively assess and simulate historical persistence and recurrence of similar patterns. The Viterbi algorithm reveals the probable sequence of states through time, enabling an examination of temporal and spatial features and associated large-scale climate forcing. Distinct patterns of sea surface temperature that are known to enhance or inhibit rainfall are associated with some states. Using the current CONUS PDSI field the model can be used to simulate the space–time PDSI pattern over the next few years, or unconditional simulations can be used to derive estimates of spatially concurrent PDSI patterns and their persistence and intensity across the CONUS.

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Celine Herweijer
,
Richard Seager
,
Edward R. Cook
, and
Julien Emile-Geay

Abstract

Drought is the most economically expensive recurring natural disaster to strike North America in modern times. Recently available gridded drought reconstructions have been developed for most of North America from a network of drought-sensitive tree-ring chronologies, many of which span the last 1000 yr. These reconstructions enable the authors to put the famous droughts of the instrumental record (i.e., the 1930s Dust Bowl and the 1950s Southwest droughts) into the context of 1000 yr of natural drought variability on the continent. We can now, with this remarkable new record, examine the severity, persistence, spatial signatures, and frequencies of drought variability over the past milllennium, and how these have changed with time.

The gridded drought reconstructions reveal the existence of successive “megadroughts,” unprecedented in persistence (20–40 yr), yet similar in year-to-year severity and spatial distribution to the major droughts experienced in today’s North America. These megadroughts occurred during a 400-yr-long period in the early to middle second millennium a.d., with a climate varying as today’s, but around a drier mean. The implication is that the mechanism forcing persistent drought in the West and the Plains in the instrumental era is analagous to that underlying the megadroughts of the medieval period. The leading spatial mode of drought variability in the recontructions resembles the North American ENSO pattern: widespread drought across the United States, centered on the Southwest, with a hint of the opposite phase in the Pacific Northwest.

Recently, climate models forced by the observed history of tropical Pacific SSTs have been able to successfully simulate all of the major North American droughts of the last 150 yr. In each case, cool “La Niña–like” conditions in the tropical Pacific are consistent with North American drought. With ENSO showing a pronounced signal in the gridded drought recontructions of the last millennium, both in terms of its link to the leading spatial mode, and the leading time scales of drought variability (revealed by multitaper spectral analysis and wavelet analysis), it is postulated that, as for the modern day, the medieval megadroughts were forced by protracted La Niña–like tropical Pacific SSTs. Further evidence for this comes from the global hydroclimatic “footprint” of the medieval era revealed by existing paleoclimatic archives from the tropical Pacific and ENSO-sensitive tropical and extratropical land regions. In general, this global pattern matches that observed for modern-day persistent North American drought, whereby a La Niña–like tropical Pacific is accompanied by hemispheric, and in the midlatitudes, zonal, symmetry of hydroclimatic anomalies.

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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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Edward R. Cook
,
David M. Meko
, and
Charles W. Stockton

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 possibility that the drought area rhythm is, in fact, internally forced by coupled ocean–atmosphere processes. Recent modeling results suggest that unstable ocean–atmosphere interactions in the North Pacific could be responsible for the drought rhythm as well. However, the results presented here do not easily allow for the rejection of the solar and lunar forcing hypotheses either.

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Edward R. Cook
,
Rosanne D. D'Arrigo
, and
Michael E. Mann

Abstract

A new, well-verified, multiproxy reconstruction of the winter North Atlantic Oscillation (NAO) index is described that can be used to examine the variability of the NAO prior to twentieth century greenhouse forcing. It covers the period a.d. 1400–1979 and successfully verifies against independent estimates of the winter NAO index from European instrumental and noninstrumental data as far back as 1500. The best validation occurs at interannual timescales and the weakest at multidecadal periods. This result is a significant improvement over previous proxy-based estimates, which often failed to verify prior to 1850, and is related to the use of an extended reconstruction model calibration period that reduced an apparent bias in selected proxies associated with the impact of anomalous twentieth century winter NAO variability on climate teleconnections over North Atlantic sector land areas. Although twentieth century NAO variability is somewhat unusual, comparable periods of persistent positive-phase NAO are reconstructed to have occurred in the past, especially before 1650.

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Hui Shi
,
Bin Wang
,
Edward R. Cook
,
Jian Liu
, and
Fei Liu

Abstract

Sparse long-term Asian monsoon (AM) records have limited our ability to understand and accurately model low-frequency AM variability. Here we present a gridded 544-yr (from 1470 to 2013) reconstructed Asian summer precipitation (RAP) dataset by weighted merging of two complementary proxies including 453 tree-ring-width chronologies and 71 historical documentary records. The RAP dataset provides substantially improved data quality when compared with single-proxy-type reconstructions. Skillful reconstructions are obtained in East and North China, northern India and Pakistan, the Indochina Peninsula, midlatitude Asia, the Maritime Continent, and southern Japan. The RAP faithfully illustrates large-scale regional rainfall variability but has more uncertainties in representing small-scale local rainfall anomalies. The RAP reproduces a realistic climatology and captures well the year-to-year rainfall variability averaged over monsoon Asia, arid central Asia, and all of Asia during the twentieth century. It also shows a general agreement with other proxies (speleothems and ice cores) during the period of 1470–1920. The RAP captures the remarkably abrupt change during the 1600s recorded in the upwelling proxy over the Arabian Sea. Four major modes of variability of the Asian summer precipitation are identified with the long record of the RAP, including a biennial El Niño–Southern Oscillation (ENSO) mode, a low-frequency ENSO mode, a central Pacific El Niño–like decadal mode, and an interdecadal mode. In sum, the RAP provides a valuable dataset for study of the large-scale Asian summer precipitation variability, especially the decadal–centennial variability that is caused by external forcing and internal feedback processes within the Earth climate system.

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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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Jason E. Smerdon
,
Benjamin I. Cook
,
Edward R. Cook
, and
Richard Seager

Abstract

Potential biases in tree-ring reconstructed Palmer drought severity index (PDSI) are evaluated using Thornthwaite (TH), Penman–Monteith (PM), and self-calibrating Penman–Monteith (SC) PDSI in three diverse regions of the United States and tree-ring chronologies from the North American drought atlas (NADA). Minimal differences are found between the three PDSI reconstructions and all compare favorably to independently reconstructed Thornthwaite-based PDSI from the NADA. Reconstructions are bridged with model-derived PDSI_TH and PDSI_PM, which both closely track modeled soil moisture (near surface and full column) during the twentieth century. Differences between modeled moisture-balance metrics only emerge in twenty-first-century projections. These differences confirm the tendency of PDSI_TH to overestimate drying when temperatures exceed the range of the normalization interval; the more physical accounting of PDSI_PM compares well with modeled soil moisture in the projection interval. Remaining regional differences in the secular behavior of projected soil moisture and PDSI_PM are interpreted in terms of underlying physical processes and temporal sampling. Results demonstrate the continued utility of PDSI as a metric of surface moisture balance while additionally providing two recommendations for future work: 1) PDSI_PM (or similar moisture-balance metrics) compare well to modeled soil moisture and are an appropriate means of representing soil-moisture balance in model simulations and 2) although PDSI_PM is more physically appropriate than PDSI_TH, the latter metric does not bias tree-ring reconstructions of past hydroclimate variability and, as such, reconstructions targeting PDSI_TH can be used with confidence in data–model comparisons. These recommendations and the collective results of this study thus provide a framework for comparing hydroclimate variability within paleoclimatic, observational, and modeled data.

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Benjamin I. Cook
,
Jason E. Smerdon
,
Richard Seager
, and
Edward R. Cook

Abstract

Regional droughts are common in North America, but pan-continental droughts extending across multiple regions, including the 2012 event, are rare relative to single-region events. Here, the tree-ring-derived North American Drought Atlas is used to investigate drought variability in four regions over the last millennium, focusing on pan-continental droughts. During the Medieval Climate Anomaly (MCA), the central plains (CP), Southwest (SW), and Southeast (SE) regions experienced drier conditions and increased occurrence of droughts and the Northwest (NW) experienced several extended pluvials. Enhanced MCA aridity in the SW and CP manifested as multidecadal megadroughts. Notably, megadroughts in these regions differed in their timing and persistence, suggesting that they represent regional events influenced by local dynamics rather than a unified, continental-scale phenomena. There is no trend in pan-continental drought occurrence, defined as synchronous droughts in three or more regions. SW, CP, and SE (SW+CP+SE) droughts are the most common, occurring in 12% of all years and peaking in prevalence during the twelfth and thirteenth centuries; patterns involving three other regions occur in about 8% of years. Positive values of the Southern Oscillation index (La Niña conditions) are linked to SW, CP, and SE (SW+CP+SE) droughts and SW, CP, and NW (SW+CP+NW) droughts, whereas CP, NW, and SE (CP+NW+SE) droughts are associated with positive values of the Pacific decadal oscillation and Atlantic multidecadal oscillation. While relatively rare, pan-continental droughts are present in the paleo record and are linked to defined modes of climate variability, implying the potential for seasonal predictability. Assuming stable drought teleconnections, these events will remain an important feature of future North American hydroclimate, possibly increasing in their severity in step with other expected hydroclimate responses to increased greenhouse gas forcing.

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