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Cool- and warm-season precipitation totals have been reconstructed on a gridded basis for North America using 439 tree-ring chronologies correlated with December–April totals and 547 different chronologies correlated with May–July totals. These discrete seasonal chronologies are not significantly correlated with the alternate season; the December–April reconstructions are skillful over most of the southern and western United States and north-central Mexico, and the May–July estimates have skill over most of the United States, southwestern Canada, and northeastern Mexico. Both the strong continent-wide El Niño–Southern Oscillation (ENSO) signal embedded in the cool-season reconstructions and the Arctic Oscillation signal registered by the warm-season estimates faithfully reproduce the sign, intensity, and spatial patterns of these ocean–atmospheric influences on North American precipitation as recorded with instrumental data. The reconstructions are included in the North American Seasonal Precipitation Atlas (NASPA) and provide insight into decadal droughts and pluvials. They indicate that the sixteenth-century megadrought, the most severe and sustained North American drought of the past 500 years, was the combined result of three distinct seasonal droughts, each bearing unique spatial patterns potentially associated with seasonal forcing from ENSO, the Arctic Oscillation, and the Atlantic multidecadal oscillation. Significant 200–500-yr-long trends toward increased precipitation have been detected in the cool- and warm-season reconstructions for eastern North America. These seasonal precipitation changes appear to be part of the positive moisture trend measured in other paleoclimate proxies for the eastern area that began as a result of natural forcing before the industrial revolution and may have recently been enhanced by anthropogenic climate change.

The great rivers that flow from the southern Tibetan Plateau (TP) affect billions of people in the downstream countries of Asia. Understanding of the hydrological variability of these rivers is still limited, however, because of the lack of long-term streamflow records. Tree-ring width chronologies from six sites are applied to reconstruct annual streamflow of the Salween River, the last remaining large free-flowing transboundary river draining the southern TP, and a critical water source for countries of Southeast Asia. Response function analysis shows that precipitation is the main factor limiting the radial growth of the sampled trees. Linear regression of annual (September–June) Salween River streamflow on the first principal component of tree-ring chronologies explains 53.4% of the streamflow variance, 1958–2011, and yields a reconstruction for the interval 1500–2011 CE. A tally of droughts and wet periods emphasizes the severity of droughts before the start of the gauged records, and a tendency toward wetter conditions in recent decades. Regional temperature is negatively associated with the reconstructed streamflow. Cold wet summers controlled by the Asian summer monsoon are responsible for an increasing trend in streamflow over the last decades. Reconstructed hydrological change is linked to the history of mainland Southeast Asia through the impact of water shortages on Burma society. In particular, prolonged periods of low flow of the Salween River coincide with the falls of the Toungoo Empires and the First Anglo-Burmese War. This tree-ring reconstruction provides a long-term perspective on hydrological changes in the Upper Salween River Basin that can give insight for sustainable water management on the TP and in Myanmar.

The Lhasa River is the largest tributary of the Yarlung Zangbo River, provides up to 85% of the water supply for the city of Lhasa, and has a high ecologic and economic importance. Annual streamflow of the Lhasa River is reconstructed from Juniperus tibetica tree rings. The streamflow reconstruction developed for the southern Tibetan Plateau goes back to 1546 CE and has a R2 of 0.485. Spectral and wavelet analysis indicates the existence of decadal (34 and 16 years) and interannual (8.1, 5.7, 4.8, 3.8, 3.5, 3.2, 2.7, 2.3 and 2.1 years) cycles that may reflect climate forcings. Lhasa River streamflow is significantly correlated with precipitation over a vast part of the Yarlung Zangbo River basin, and represents streamflow of the upper Yarlung Zangbo River to a certain extent. Lhasa River variation is linked with large-scale climate circulation features, such as the El Nino-Southern Oscillation (ENSO) and Asian summer monsoon. Some years of abnormally high and low streamflow are related to ENSO events. Based on the reconstruction, the probability that annual streamflow does not exceed a target defined as the instrumental-period mean was lower (48% non-exceedance probability) than during the instrumental period. This study indicates that the instrumental record does not contain the full range of streamflow, especially the lowest streamflow events, and the reconstruction makes up for this shortcoming. Future water resource supply planning based on the instrumental record and streamflow reconstructions will be able to effectively reduce the risks posed by climate change.

This study analyses dynamics of the hydrological regime of Bele-Shira closed basin and evaluates the potential for using radial growth of Siberian larch (Larix sibirica) for its assessment. We investigated the relationships between different characteristics of the water level variation of Lake Shira, precipitation amount and long-term regional chronologies developed from 56 living trees and 32 dead trees on three sites across this basin. Graphical and correlation analysis indicate that the interannual change (June minus previous June) of the water level of Lake Shira is strongly positively related to the annual sum of precipitation from July to June and the radial growth of larch. It was shown that this hydrological characteristic integrates the current dynamics of the regional precipitation and moisture regime as a whole of the Bele-Shira closed basin on interannual and decadal scales. The water level of Lake Shira fluctuates on a multi-year timescale in synchrony with the cumulative sum of the tree-ring chronology and also has strong positive long-term trend, probably driven by the continual groundwater inflow from neighboring Lake Itkul. Delayed relationships of precipitation and radial growth with the Lake Shira level change are interpreted with reference to a water balance model of the closed basin. Results offer the possibility of reconstructing interannual and decadal variation of the hydrological regime during the last few centuries through regression models using tree-ring chronologies or the dynamics of climatic variables recovered from them.

Recent Mediterranean droughts have highlighted concerns that climate change may be contributing to observed drying trends, but natural climate variability in the region is still poorly understood. We analyze 900 years (1100–2012) of Mediterranean drought variability in the Old World Drought Atlas (OWDA), a spatiotemporal tree-ring reconstruction of the June–July–August self calibrating Palmer Drought Severity Index. In the Mediterranean, the OWDA is highly correlated with spring precipitation (April–June), the North Atlantic Oscillation (January–April), the Scandinavian Pattern (January–March), and the East Atlantic Pattern (April–June). Drought variability displays significant east-west coherence across the basin on multi-decadal to centennial time scales and north-south anti-phasing in the eastern Mediterranean, with a tendency for wet anomalies in the Black Sea region (e.g., Greece, Anatolia, the Balkans, etc) when coastal Libya, the southern Levant, and the Middle East are dry, possibly related to the North Atlantic Oscillation. Recent droughts are centered in the Western Mediterranean, Greece, and the Levant. Events of similar magnitude in the Western Mediterranean and Greece occur in the OWDA, but the recent 15-year drought in the Levant (1998–2012) is the driest in the record. Estimating uncertainties using a resampling approach, we conclude there is an 89% likelihood this drought is drier than any comparable period of the last 900 years and a 98% likelihood it is drier than the last 500 years. These results confirm the exceptional nature of this drought relative to natural variability in recent centuries, consistent with studies that have found evidence for anthropogenically forced drying in the region.

Drought is one of the main natural factors in declining tree-ring growth and the production of agricultural crops in Algeria. This paper addresses the variability of growing conditions for wheat in Algeria with climatic data and a tree-ring reconstruction of January–June precipitation from ten Pinus halepensis tree-ring chronologies. A regression-based reconstruction equation explains up to 74% of the variance of precipitation in the 1970–2011 calibration period and cross validates well. Classification of dry years by the 30% percentile of observed precipitation (131 mm) yields a maximum length of drought of five years (1877–1881) and increasing frequency of dry years in the late 20th and early 21st centuries. A correlation-based sensitivity analysis shows a similar pattern of dependence of tree-growth and wheat production on monthly and seasonal precipitation, but contrasting patterns of dependence on temperature. The patterns are interpreted by reference to phenology, growth phases, and – for wheat – agricultural practices.

The goal of this research report is to describe annual precipitation reconstruction from Pinus sylvestris trees on three sites in the Abakan region, located in the Minusinsk Depression, at the confluence of the Yenisei and Abakan Rivers, Russia. The study was performed during the 4th annual international summer course “Tree Rings, Climate, Natural Resources and Human Interaction” held in Abakan, 5-19 August 2013. The reconstruction, for the 12-month total precipitation ending in July of the growth year, is based on a reliable and replicable statistical relationship between precipitation and tree-ring growth, and shows climate variability on both interannual and interdecadal time scales. The regional tree-ring chronology accounts for 56% of the variance of observed annual precipitation in a linear regression model, with the strongest monthly precipitation signal concentrated in May and June of the current growing season. Composite 500 mb height-anomaly maps suggest that the tree-ring data from thi...

Tree-ring carbon and oxygen isotope ratios have been used to understand past dynamics in forest carbon and water cycling. Recently, this has been possible for different parts of single growing seasons by isolating anatomical sections within individual annual rings. Uncertainties in this approach are associated with correlated climate legacies that can occur at a higher frequency, such as across successive seasons, or a lower frequency, such as across years. The objective of this study was to gain insight into how legacies affect cross-correlation in the δ13 C and δ18 O isotope ratios in the earlywood (EW) and latewood (LW) fractions of Pinus ponderosa trees at thirteen sites across a latitudinal gradient influenced by the North American Monsoon (NAM) climate system. We observed that δ13 C from EW and LW has significant positive cross-correlations at most sites, whereas EW and LW δ18 O values were cross-correlated at about half the sites. Using combined statistical and mechanistic models, we show that cross-correlations in both δ13 C and δ18 O can be largely explained by a low-frequency (multiple-year) mode that may be associated with long-term climate change. We isolated, and statistically removed, the low-frequency correlation, which resulted in greater geographical differentiation of the EW and LW isotope signals. The remaining higher-frequency (seasonal) cross-correlations between EW and LW isotope ratios were explored using a mechanistic isotope fractionation-climate model. This showed that lower atmospheric vapor pressure deficits associated with monsoon rain increase the EW-LW differentiation for both δ13 C and δ18 O at southern sites, compared to northern sites. Our results support the hypothesis that dominantly unimodal precipitation regimes, such as near the northern boundary of the NAM, are more likely to foster cross-correlations in the isotope signals of EW and LW, potentially due to greater sharing of common carbohydrate and soil water resource pools, compared to southerly sites with bimodal precipitation regimes.

A May–July precipitation nested reconstruction for the period AD 1415–2010 was developed from multi-century tree-ring records of Pinus nigra, Pinus brutia, and Cedrus brevifolia for Cyprus. Calibration and verification statistics for the period 1917–2010 show a good level of skill, and split-sample validation over 1917–2010 supports temporal stability of the tree-ring signal for precipitation. Smoothed annual time series of reconstructed precipitation and a tally of drought events in a moving time window indicate that the calibration period is not representative of the full range of drought variability. While convective precipitation in the warm season may be driven strongly by local factors, composite maps of geopotential height anomaly for dry years and wet years support large-scale atmospheric-flow influence related to height anomalies over the broader region of northeast Africa and the eastern Mediterranean. Emerging positive trend in reconstruction residuals may be an early sign of exacerbation of drought stress on trees by recent warming in May–July. Future warming expected from increases in greenhouse gases poses a threat to forest resources in Cyprus and elsewhere in the Mediterranean.

This study reports two multi-century regional reconstructions of annual precipitation based on Pinus ponderosa and P. edulis from four sites in central northern Arizona. It compares standard regional and time-nested methods to generate reconstructions from 1581–2016 C.E. and 1529–2016 C.E., respectively. The strongest climate relationship is a positive correlation between total ring width and 12-month total precipitation ending in July of the growth year. The chronologies account for 50% of the variance of observed annual precipitation in the regional model and 59%, 60%, and 47% and 35% in the nested models. The two reconstructions are highly correlated (Pearson's correlation r > 0.97, p < 0.001) demonstrating that the reconstructions are highly similar over the period common to both reconstructions, with the nested-models’ advantage of extending the range of the reconstruction. The precipitation reconstructions are significantly correlated (r = 0.66, p < 0.001) with the North American Drought Atlas (NADA).

[1] Tree ring data are analyzed for a multicentury record of drought history in the Sierra San Pedro Martir (SSPM) of Baja California, Mexico. Climatic variation in the study area is of particular interest because the SSPM is a rich biotic environment at the southern limit of the California floristic province and the southern limit of the planetary jet stream. Future shifts in the jet stream would be expected to have amplified effect on this marginal environment. The study applies linear regression to tree ring indices of earlywood-width of Abies concolor to estimate a 353 year (1658–2010 C.E.) record of cool-season (October–April) precipitation, P, in SSPM. Time-nested regression models account for more than half the variance of grid point P in calibration periods of length 50–65 years. Cross-spectral analysis indicates strong tracking of observed P by the reconstruction over a broad range of frequencies. Robustness of the reconstruction is supported by synchrony of reconstructed P with tree ring variations in other tree species from SSPM. The reconstruction emphasizes the severity of the 1950s drought in a long-term context and the single-year intensity of droughts in the last decade: 2007 stands out as the driest reconstructed year, with a high percentage of missing rings in A. concolor. The reconstruction identifies the early twentieth century pluvial as the wettest epoch in the last 353 years in the SSPM. High-elevation tree species in SSPM may be especially well-suited to sensing snowpack-related moisture variations associated with a southerly branched jet stream and the types of weather systems active in the pluvial.

Ancient blue oak trees are still widespread across the foothills of the Coast Ranges, Cascades, and Sierra Nevada in California. The most extensive tracts of intact old-growth blue oak woodland appear to survive on rugged and remote terrain in the southern Coast Ranges and on the foothills west and southwest of Mt. Lassen. In the authors' sampling of old-growth stands, most blue oak appear to have recruited to the canopy in the middle to late nineteenth century. The oldest living blue oak tree sampled was over 459 years old, and several dead blue oak logs had over 500 annual rings. Precipitation sensitive tree-ring chronologies up to 700 years long have been developed from old blue oak trees and logs. Annual ring-width chronologies of blue oak are strongly correlated with cool season precipitation totals, streamflow in the major rivers of California, and the estuarine water quality of San Francisco Bay. A new network of 36 blue oak chronologies records spatial anomalies in growth that arise from latitudinal changes in the mean storm track and location of landfalling atmospheric rivers. These long, climate-sensitive blue oak chronologies have been used to reconstruct hydroclimatic history in California and will help to better understand and manage water resources. The environmental history embedded in blue oak growth chronologies may help justify efforts to conserve these authentic old-growth native woodlands.

Summary This work presents the first tree-ring reconstructions of hydroclimate for the Upper Klamath River basin, which stretches from northern California into southern Oregon. The extended record provides a centuries-long perspective on the region’s hydroclimatic variability and context for water-related political issues that have erupted in recent years. Reconstructions of water-year precipitation for Klamath Falls, Oregon (extending 1564–2004 and 1000–2010 CE) were developed to compare past drought severity with drought severity of the instrumental record (extending 1896–2011). The reconstructions suggest that variability exhibited during the instrumental period captures extremes of moderate-to-long-duration (6-, 10-, and 20-year) droughts, but not of short (single-year and 3-year) and very long (50-year) droughts, which were more severe during the 11th–13th centuries. The late-16th-century “mega drought” is present in the Klamath River basin, though with less strength than in the neighboring Sacramento River basin. Cool-season storm tracks appear to be a direct driver of hydroclimatic variability, leading to instances of see-saw like relationships with neighboring regions, such as in the mid-14th century. In contrast, the larger area of drought in the 12th century is suggestive of a long-term northward shift in cool-season storm tracks.

[1] The North American monsoon is a major focus of modern and paleoclimate research, but relatively little is known about interannual- to decadal-scale monsoon moisture variability in the pre-instrumental era. This study draws from a new network of subannual tree-ring latewood width chronologies and presents a 470-year reconstruction of monsoon (June–August) standardized precipitation for southwestern North America. Comparison with an independent reconstruction of cool-season (October–April) standardized precipitation indicates that southwestern decadal droughts of the last five centuries were characterized not only by cool-season precipitation deficits but also by concurrent failure of the summer monsoon. Monsoon drought events identified in the past were more severe and persistent than any of the instrumental era. The relationship between winter and summer precipitation is weak, at best, and not time stable. Years with opposing-sign seasonal precipitation anomalies, as noted by other studies, were anomalously frequent during the mid to late 20th century.

[1] Agriculture and ranching in semiarid regions often rely on local precipitation during the growing season as well as streamflow from runoff in distant headwaters. Where snowpack and reservoir storage are important, this pattern of reliance leads to vulnerability to multiseason drought. The lower Rio Grande basin in New Mexico, used as a case study here, has experienced drought conditions over the past 12 years characterized both by low local summer monsoon precipitation and by reduced availability of surface water supplies from the upper Rio Grande. To place this drought in a long-term context, we evaluate the covariability of local warm-season and remote cool-season hydroclimate over both the modern period and past centuries. We draw on a recently developed network of tree-ring data that allows an assessment of preinstrumental warm-season variations in precipitation over the southwest. Both instrumental and paleoclimatic data suggest that low runoff followed by a dry monsoon is not unusual, although over the full reconstruction period (1659–2008), years with wet or dry conditions shared in both seasons do not occur significantly more often than unshared conditions. Low flows followed by dry monsoon conditions were most persistent in the 1770s and 1780s; other notable periods of shared seasonal droughts occurred in the 1660s and 1950s. The recent drought does not yet appear to be unusually severe in either the instrumental or paleoclimatic context.

We use the process-based VS (Vaganov-Shashkin) model to investigate whether a regional Pinus halepensis tree-ring chronology from Tunisia can be simulated as a function of climate alone by employing a biological model linking day length and daily temperature and precipitation (AD 1959–2004) from a climate station to ring-width variations. We check performance of the model on independent data by a validation exercise in which the model's parameters are tuned using data for 1982–2004 and the model is applied to generate tree-ring indices for 1959–1981. The validation exercise yields a highly significant positive correlation between the residual chronology and estimated growth curve (r=0.76 pn=23). The model shows that the average duration of the growing season is 191 days, with considerable variation from year to year. On average, soil moisture limits tree-ring growth for 128 days and temperature for 63 days. Model results depend on chosen values of parameters, in particular a parameter specifying a balance ratio between soil moisture and precipitation. Future work in the Mediterranean region should include multi-year natural experiments to verify patterns of cambial-growth variation suggested by the VS model.

Tree-ring studies have demonstrated that conifer latewood measurements contain information on long-term North American monsoon (NAM) variability, a hydroclimatic feature of great importance to plants, animals, and human society in the US Southwest. This paper explores data-treatment options for developing latewood chronologies aimed at NAM reconstruction. Archived wood samples for five Douglas-fir (Pseudotsuga menziesii, Mirb. Franco) sites in southeastern Arizona are augmented with new collections. The combined dataset is analyzed along with time series of regionally averaged observed precipitation to quantify the strength of regional precipitation signal in latewood time series and to identify ways of increasing the signal strength. Analysis addresses the signal strength influences of including or excluding “false” latewood bands in the nominal “latewood” portion of the ring, the necessary adjustment of latewood width for statistical dependence on antecedent earlywood width, and tree age. Resul...

Drought is a recurring phenomenon in the American Southwest. Since the frequency and severity of hydrologic droughts and other hydroclimatic events are of critical importance to the ecology and rapidly growing human population of this region, knowledge of long-term natural hydroclimatic variability is valuable for resource managers and policy-makers. An October–June precipitation reconstruction for the period AD 824–2007 was developed from multi-century tree-ring records of Pseudotsuga menziesii (Douglas-fir), Pinus strobiformis (Southwestern white pine) and Pinus ponderosa (Ponderosa pine) for the Jemez Mountains in Northern New Mexico. Calibration and verification statistics for the period 1896–2007 show a high level of skill, and account for a significant portion of the observed variance (>50%) irrespective of which period is used to develop or verify the regression model. Split-sample validation supports our use of a reconstruction model based on the full period of reliable observational data (1896–2007). A recent segment of the reconstruction (2000–2006) emerges as the driest 7-year period sensed by the trees in the entire record. That this period was only moderately dry in precipitation anomaly likely indicates accentuated stress from other factors, such as warmer temperatures. Correlation field maps of actual and reconstructed October–June total precipitation, sea surface temperatures and 500-mb geopotential heights show characteristics that are similar to those indicative of El Nino–Southern Oscillation patterns, particularly with regard to ocean and atmospheric conditions in the equatorial and north Pacific. Our 1184-year reconstruction of hydroclimatic variability provides long-term perspective on current and 20th century wet and dry events in Northern New Mexico, is useful to guide expectations of future variability, aids sustainable water management, provides scenarios for drought planning and as inputs for hydrologic models under a broader range of conditions than those provided by historical climate records. Copyright © 2010 Royal Meteorological Society

Long-term information on gradients in precipitation-anomaly over tens to hundreds of km is important to hydroclimatology for improved understanding of the spatiotemporal variability of moisture-delivery systems and runoff. Site-centered reconstructions of cool-season (Nov–Apr) precipitation at 36 Quercus douglasii tree-ring sites in the Central Valley of California, USA, are generated, regionalized, and evaluated for ability to track north–south gradients in precipitation-anomaly. Event series are constructed for overall-wet (W), overall-dry (D), wetter-to-north (W/D) and wetter-to-south (D/W) conditions, 1557–2001. Interesting features of the event series are clustering of W events in the 1780–1790s and a three-year run of D/W events in 1816–1818 (coincidentally following the eruption of Tambora in 1815). The most recent 25 years of the event series stand out for a high frequency of W and D events and low frequency of events associated with strong gradients in precipitation-anomaly. The five strongest W events in this period and seven of the nine W events since 1934 match El Nino years. Recent changes in the event series may be a Central-Valley footprint of a well-documented post-1976 change in the atmosphere-ocean climate system over the North Pacific. Similar studies may prove useful in other geographical areas where networks of tree-ring data sufficiently sensitive to precipitation are available.

We examine how the seasonality of precipitation signals embedded within the North American Drought Atlas varies across the continent. Instrumental records of average summer (JJA) Palmer Drought Severity Index (PDSI) are characterized by major regional differences in the relative importance of precipitation during summer and winter (DJF). The Atlas, which is based on a network of drought-sensitive tree-ring records, is able to reproduce the main geographic patterns of these biases, but tree-ring reconstructions exaggerate the influence of seasonal precipitation anomalies in the southwestern United States and northern Mexico (towards a stronger winter signal) and western Canada (towards a stronger summer signal). Drought reconstructions from the Southwest and Tex-Mex regions are tuned mainly to winter precipitation and display strong teleconnections to both El Niño and La Niña. In contrast, winter precipitation signals are either weak or absent in drought reconstructions from northwestern North America, and tree-ring estimates of PDSI show a much less robust association with the El Niño-Southern Oscillation. Geographical differences in the relative strength of seasonal precipitation signals are likely due to (i) local factors that influence tree growth but are not incorporated into the PDSI algorithm and (ii) real differences in regional climatology. These seasonal biases must be taken into account when comparing drought reconstructions across North America, when comparing tree-ring PDSI to drought records developed from other proxies or when attempting to use the Drought Atlas to link past droughts to potential forcing mechanisms.

Despite the extensive network of moisture-sensitive tree-ring chronologies in western North America, relatively few are long enough to document climatic variability before and during the Medieval Climate Anomaly (MCA) ca. AD 800-1300. We developed a 2300-yr tree-ring chronology extending to 323 BC utilizing live and remnant Douglas-fir (Pseudotsuga menziesii) from the Tavaputs Plateau in northeastern Utah. A resulting regression model accounts for 70% of the variance of precipitation for the AD 1918–2005 calibration period. Extreme wet and dry periods without modern analogues were identified in the reconstruction. The MCA is marked by several prolonged droughts, especially prominent in the mid AD 1100s and late 1200s, and a lack of wet or dry single-year extremes. The frequency of extended droughts is not markedly different, however, than before or after the MCA. A drought in the early AD 500s surpasses in magnitude any other drought during the last 1800 yr. A set of four long high-resolution records suggests this drought decreased in severity toward the south in the western United States. The spatial pattern is consistent with the western dipole of moisture anomaly driven by El Niño and is also similar to the spatial footprint of the AD 1930s "Dust Bowl" drought.

Precipitation over the southwestern United States exhibits distinctive seasonality, and contrasting ocean–atmospheric dynamics are involved in the interannual variability of cool- and warm-season totals. Tree-ring chronologies based on annual-ring widths of conifers in the southwestern United States are well correlated with accumulated precipitation and have previously been used to reconstruct cool-season and annual precipitation totals. However, annual-ring-width chronologies cannot typically be used to derive a specific record of summer monsoon-season precipitation. Some southwestern conifers exhibit a clear anatomical transition from the earlywood and latewood components of the annual ring, and these exactly dated subannual ring components can be measured separately and used as unique proxies of cool- and warm-season precipitation and their associated large-scale ocean–atmospheric dynamics. Two 2139-yr-long reconstructions of cool- (November–May) and early-warm season (July) precipitation have been developed from ancient conifers and relict wood at El Malpais National Monument, New Mexico. Both reconstructions have been verified on independent precipitation data and reproduce the spatial correlation patterns detected in the large-scale SST and 500-mb height fields using instrumental precipitation data from New Mexico. Above-average precipitation in the cool-season reconstruction is related to El Niño conditions and to the positive phase of the Pacific decadal oscillation. Above-average precipitation in July is related to the onset of the North American monsoon over New Mexico and with anomalies in the 500-mb height field favoring moisture advection into the Southwest from the North Pacific, the Gulf of California, and the Gulf of Mexico. Cool- and warm-season precipitation totals are not correlated on an interannual basis in the 74-yr instrumental or 2139-yr reconstructed records, but wet winter–spring extremes tend to be followed by dry conditions in July and very dry winters tend to be followed by wet Julys in the reconstructions. This antiphasing of extremes could arise from the hypothesized cool- to early-warm-season change in the sign of large-scale ocean–atmospheric forcing of southwestern precipitation, from the negative land surface feedback hypothesis in which winter–spring precipitation and snow cover reduce surface warming and delay the onset of the monsoon, or perhaps from an interaction of both large-scale and regional forcing. Episodes of simultaneous interseasonal drought (“perfect” interseasonal drought) persisted for a decade or more during the 1950s drought of the instrumental era and during the eighth- and sixteenth-century droughts, which appear to have been two of the most profound droughts over the Southwest in the past 1400 yr. Simultaneous interseasonal drought is doubly detrimental to dry-land crop yields and is estimated to have occurred during the mid-seventeenth-century famines of colonial New Mexico but was less frequent during the late-thirteenth-century Great Drought among the Anasazi, which was most severe during the cool season.

An October–June precipitation reconstruction was developed from a Pinus halepensis regional tree-ring chronology from four sites in northwestern Tunisia for the period of 1771–2002. The reconstruction is based on a reliable and replicable statistical relationship between climate and tree-ring growth and shows climate variability on both interannual and interdecadal time scales. Thresholds (12th and 88th percentiles) based on the empirical cumulative distribution of observed precipitation for the 1902–2002 calibration period were used to delineate dry years and wet years of the long-term reconstruction. The longest reconstructed drought by this classification in the 232-year reconstruction is 2 years, which occurred in the 19th century. Analysis of 500 mb height data for the period 1948–2002 suggests reconstructed extreme dry and wet events can provide information on past atmospheric circulation anomalies over a broad region including the Mediterranean, Europe and eastern Asia.

A network of 54 ring-width chronologies is used to estimate changes in summer climate within the Winnipeg River basin, Canada, since AD 1783. The basin drains parts of northwestern Ontario, northern Minnesota and southeastern Manitoba, and is a key area for hydroelectric power production. Most chronologies were developed from Pinus resinosa and P. strobus, with a limited number of Thuja occidentalis, Picea glauca and Pinus banksiana. The dominant pattern of regional tree growth can be recovered using only the nine longest chronologies, and is not affected by the method used to remove variability related to age or stand dynamics from individual trees. Tree growth is significantly, but weakly, correlated with both temperature (negatively) and precipitation (positively) during summer. Simulated ring-width chronologies produced by a process model of tree-ring growth exhibit similar relationships with summer climate. High and low growth across the region is associated with cool/wet and warm/dry summers, respectively; this relationship is supported by comparisons with archival records from early 19th century fur-trading posts. The tree-ring record indicates that summer droughts were more persistent in the 19th and late 18th century, but there is no evidence that drought was more extreme prior to the onset of direct monitoring.

The authors develop a model framework that includes a set of hydrologic modules as a water resources management and planning tool for the upper Santa Cruz River near the Mexican border, Southern Arizona. The modules consist of: (1) stochastic generation of hourly precipitation scenarios that maintain the characteristics and variability of a 45-year hourly precipitation record from a nearby rain gauge; (2) conceptual transformation of generated precipitation into daily streamflow using varied infiltration rates and estimates of the basin antecedent moisture conditions; and (3) surface-water to ground-water interaction for four downstream microbasins that accounts for alluvial ground-water recharge, and ET and pumping losses. To maintain the large inter-annual variability of streamflow as prevails in Southern Arizona, the model framework is constructed to produce three types of seasonal winter and summer categories of streamflow (i.e., wet, medium, or dry). Long-term (i.e., 100 years) realizations (ensembles) are generated by the above described model framework that reflects two different regimes of inter annual variability. The first regime is that of the historic streamflow gauge record. The second regime is that of the tree ring reconstructed precipitation, which was derived for the study location. Generated flow ensembles for these two regimes are used to evaluate the risk that the regional four ground-water microbasins decline below a preset storage threshold under different operational water utilization scenarios.

Two reconstructions of spring (May–June) precipitation have been developed for southwestern Turkey. The first reconstruction (1776–1998) was developed from principal components of nine chronologies of Cedrus libani, Juniperus excelsa, Pinus brutia ,a ndPinus nigra. The second reconstruction (1339–1998) was derived from principal components of three J. excelsa chronologies. Calibration and verification statistics of both reconstructions indicate reasonably accurate reconstruction of spring precipitation for southwestern Turkey, and show clear evidence of multi-year to decadal variations in spring precipitation. The longest period of reconstructed spring drought, defined as consecutive years with less than 80% of normal May–June precipitation, was 4 years (1476–79). Only one drought event of this duration has occurred during the last six centuries. Monte Carlo analysis indicates a less than 33% probability that southwestern Turkey has experienced spring drought longer than 5 years in the past 660 years. Apart from the 1476–79 extended dry period, spring droughts of 3 years in length have only occurred from 1700 to the present. The longest reconstructed wet period, defined as consecutive years with more than 120% of normal May–June precipitation, was 4 years (1532–35). The absence of extended spring drought during the 16th and 17th centuries and the occurrence of extended wet spring periods during these centuries suggest a possible regime shift in climate. Preliminary analysis of links between large-scale climatic variation and these climate reconstructions shows that there is a relationship between extremes in spring precipitation and anomalous atmospheric circulation in the region. Copyright  2003 Royal Meteorological Society.

The variability of the North American Monsoon System (NAMS) is important to the precipitation climatology of Mexico and the southwestern United States. Tree-ring studies have been widely applied to climatic reconstruction in western North America, but as yet, have not addressed the NAMS. One reason is the need for highly resolved seasonal dendroclimatic information. Latewood-width, the portion of the annual tree ring laid down late in the growing season, can potentially yield such information. This paper describes a pilot study of the regional summer precipitation signal in latewood-width from a network of five Pseudotsuga menziesii chronologies developed in the heart of the region of NAMS influence in Arizona, USA. Exploratory data analysis reveals that the summer precipitation signal in latewood is strong, but not equally so over the full range of summer precipitation. Scatter in the relationship increases toward higher levels of precipitation. Adjustment for removal of inter-correlation with earlywood-width appears to strengthen the summer precipitation signal in latewood-width. To demonstrate a possible application to climatic reconstruction, the latewood precipitation signal is modelled using a nonlinear model—a binary recursive classification tree (CT) that attempts to classify summers as dry or not dry from threshold values of latewood-width. The model identifies narrow latewood-width as an effective predictor of a dry summer. Of 14 summers classified dry by latewood-width for the period 1868–1992, 13 are actually dry by the instrumental precipitation record. The results suggest that geographical expansion of coverage by latewood-width chronologies and further development of statistical methods may lead to successful reconstruction of variability of the NAMS on century time scales. Copyright © 2001 Royal Meteorological Society

Water resources are the lifeblood of the Near East region. Careful planning and management of water resources in dry land regions requires information on the likelihood of extreme events, especially prolonged drought. It is essential to understand the variability of climate on time scales of decades to centuries to assign reasonable probabilities to such events. Tree-ring analysis is one way to increase our knowledge of the climate variability beyond the short period covered by the instrumental data. In this paper, we reconstruct October-May precipitation from a Juniperus phoenicia tree-ring chronology in southern Jordan to gain a long-term (A.D. 1600–1995) perspective on runs of dry years and on time series fluctuations in precipitation averaged over several years. The reconstruction equation derived by regression of log-transformed precipitation on tree-ring indices explains 44 percent of the variance of observed precipitation. The longest reconstructed drought, as defined by consecutive years below a threshold of 217.4 mm, was four years, compared with three years for the 1946–95 instrumental data. A Monte Carlo analysis designed to account for uncertainty in the reconstruction indicates a lower than 50 percent chance that the region has experienced drought longer than five years in the past 400 years.

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 ¯

https://doi.org/10.15447/sfews.2011v9iss1art4 Blue oak tree-ring chronologies correlate highly with winter–spring precipitation totals over California, with Sacramento and San Joaquin river stream flow, and with seasonal variations in the salinity gradient in San Francisco Bay. The convergence of fresh and saline currents can influence turbidity, sediment accumulation, and biological productivity in the estuary. Three selected blue oak chronologies were used to develop a 625-year-long reconstruction of the seasonal salinity gradient, or low salinity zone (LSZ), which provides a unique perspective on the interannual-to-decadal variability of this important estuarine habitat indicator. The reconstruction was calibrated with instrumental LSZ data for the winter–spring season, and explains 73% of the variance in the February–June position of the LSZ from 1956 to 2003. Because this calibration period post-dates the sweeping changes that have occurred to land cover, channel morphology, and natural streamflow regimes in California, the reconstruction provides an idealized estimate for how the LSZ might have fluctuated under the seasonal precipitation variations of the past 625 years, given the modern geometry and bathymetry of the estuary and land cover across the drainage basin. The February–June season integrates precipitation and runoff variability during the cool season, and does not extend into the late-summer dry season when LSZ extremes can negatively affect Sacramento–San Joaquin Delta (Delta) agriculture and some aquatic organisms. However, there is such strong inter-seasonal persistence in the instrumental LSZ data that precipitation totals during the cool season can strongly pre-condition LSZ position in late summer. The 625-year-long reconstruction indicates strong interannual and decadal variability, the frequent recurrence of consecutive 2-year LSZ maxima and minima, large-scale ocean atmospheric forcing, and an interesting asymmetrical influence of warm El Niño–Southern Oscillation (ENSO) events.

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 Steinbeck's 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].

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 1930's 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 1930's drought was equaled or surpassedin severity by droughts in the 1750's, 1820's and 1860's. 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...

Long-term streamflow series in the western United States were examined for evidence of secular changes related to climate. Streamflow series contained appreciable low-frequency variation related to the combined influence of temperature and precipitation. Evidence of nonstationarity was found in selected records for the Pacific Northwest and the Upper Colorado Basins: mean annual streamflow increased significantly (0.05 level) from the first to last half of the 1914–80 period in the Pacific Northwest, and decreased significantly over the same period in the Upper Colorado region. Correlation analyses and examination of drought years revealed a strong tendency for anomalies of opposite sign in the Pacific Northwest and the Southwest. Drought in the Upper Colorado Basin was statistically independent of drought in the Pacific Northwest. Under exceptional meteorological conditions (e.g., water-year 1976–77), however, low flows occurred over a vast area from the Northwest coast to the mountains of centr...

Previous tree-ring studies indicated that the total area affected by drought in the western United States has rhythmically expanded and contracted over the past 300 years, with a period near the 18.6-year lunar nodal and 22-year double-sunspot cycles. Recently collected tree-ring data from the U.S. Corn Belt for the years 1680 to 1980 were examined for evidence of either of these cycles on a regional scale. Spectral analysis indicated no periodicity in the eastern part of the Corn Belt, but a significant 18.33-year period in the western part. The period length changed from 17.60 to 20.95 years between the first 150 years and the last 151. High-resolution frequency analysis showed that the structure of the 18.33-year spectral peak was complex, with contributions from several frequencies near both the lunar nodal and double-sunspot periods. A t-test of difference of means in reconstructed annual precipitation weakly corroborated a previous finding of an association between drought area and the phase of the double-sunspot cycle. Both the high-resolution frequency analysis and the t-test results indicate that the periodic component of drought near 20 years is too weak and irregular to be of use in drought forecasting for the Corn Belt.