Palmer Drought Severity Index Reconstruction Research Articles

PDSI Reconstruction from Tree Rings in Central Siberia (Russia)

This report describes an international summer course, “Tree Rings, Climate, Natural Resources, and Human Interaction”, held in Cheriomushki, Russia, in the summer of 2018. The course was attended by 12 participants from six countries (Belgium, India, Lebanon, Poland, Russia, and South Africa) and instructors from the USA and included basic training in dendrochronology skills and dendroclimatology and dendroecology projects. This report focuses on a nested May–July Palmer Drought Severity Index (PDSI) reconstruction from annual rings of Pinus sylvestris trees growing in the region and also explores false-ring (FR) occurrence in samples collected during the course. Four chronologies, one developed during the course from Maina District, Khakassia, Russia, are used in the reconstruction, which was based on principal component (PC) regression. The nested reconstruction demonstrated a strong statistical relationship between PDSI and tree-ring growth and allowed for an assessment of climate variability on both interannual and interdecadal time scales. FR occurrence in tree cores collected along an elevational transect from a site along the Yenisei River north of Cheriomushki was found to differ depending on the position of trees on the slope. The frequency of FRs and the location of the FR within the annual ring also appear to be related to seasonal precipitation anomalies.

A tree-ring-based drought reconstruction from 1466 to 2013 CE for the Aksu area, western China

The Aksu area is located in western Xinjiang, China, where an oasis is the main space for human production and life in this region. The stability of the oasis is crucial for societal and economic development and can be severely impacted by continued drought. The Palmer Drought Severity Index (PDSI), a widely applied index for dryness, has been reconstructed in many areas by tree rings. Here we develop a new regional long tree-ring-width chronology of Schrenk’s spruce (Picea schrenkiana Fisch. et Mey.) from the Aksu area. Based on the regional PDSI from the previous September to current March (PDSIp9–c3), regional drought variability is reconstructed in the past 548 years (1466–2013 CE). The regional PDSI reconstruction captures distinct wet and dry periods and also some low-frequency climate signals. A decadal analysis reveals 12 wet and 12 dry periods in the past 548 years, of which 1514–1560 CE was the longest dry period. The prominent mega-drought during the mid-sixteenth century is particularly pronounced. The PDSI in the Aksu area records a notable sudden onset and long period of wet conditions from the 1980s until the present day. Mechanism analysis indicates that the regional drought has been mainly affected by the North Atlantic Oscillation (NAO) over the last centuries through an anti-phase relationship.

Drought reconstruction based on tree-ring earlywood of Picea obovata Ledeb. for the southern Altay Mountains

ABSTRACT In this study, a 177-year April–June Palmer Drought Severity Index (PDSI) for the southern slopes of the Altay Mountains (China) was reconstructed based on the earlywood width of 96 Siberian spruce (Picea obovata Ledeb.) samples located at eight different sites. The reconstructed PDSI tracked the observed data well (r = 0.723, p < .0001, n = 52). This PDSI reconstruction captures some drought years and periods noted in historical documents, and it agrees well with other hydroclimatic reconstructions for the surrounding area. The 22-, 11-, 2.3-, and 2.1-year cycles of the reconstruction and the spatial correlation with sea surface temperature revealed that the variability of the PDSI for the study area might be influenced by both the Schwabe cycle of solar activity and the interannual oscillation of land–atmosphere–ocean circulation systems. Spatial correlation also showed the reconstruction contained climatic signals, being representative of a larger area extending from northern Xinjiang to northeastern Kazakhstan and a part of southern Russia. Synoptic climatology analysis revealed a relationship between anomalous atmospheric circulation and wet/dry years on the southern slopes of the Altay Mountains in China and its surrounding area.

Seasonal Palmer drought severity index reconstruction using tree-ring widths from multiple sites over the central-western Da Hinggan Mountains, China since 1825 AD

Many evidences indicate that the drought in Northern China has progressively intensified, threatening people’s lives and properties. Drought has become one of the most severe environmental problems in Northern China, and thus has serious impacts on the regional social and economic development. The present study investigated the seasonal Palmer drought severity index (PDSI) over the central-western Da Hinggan Mountains (CW-DHM), northeastern China. 283 cores of Mongolian pine trees from 6 locations were used to generate a regional ring-width chronology. The chronology in CW-DHM was significantly correlated with the May–July PDSI, with an explained variance of 49% (r = 0.700, 1951–2013, p < 0.0001). The regional May–July PDSI (PDSI5−7) from the CW-DHM was reconstructed from 1825 to 2013 AD. The ensemble empirical mode decomposition method (EEMD) and multi-taper method (MTM) spectral analyses revealed that the cycles in the reconstructed PDSI5−7 were close to those of the ENSO and solar activity. This suggests that both the ENSO and solar activity have strong influence on the PDSI5−7 variation in the CW-DHM region. In addition, EEMD also revealed that the Pacific decadal oscillation and the Atlantic multi-decadal oscillation influenced the drought variation in this region. The PDSI5−7 reconstruction showed a long-term declining (dry) trend during the period of the 1950s–2010s. This drying trend was also detected in the PDSI data of other parts of China after the 1950s. We believe that these phenomena may be related to a large extent with the weakening of the East Asian summer monsoon.

Indigenous impacts on North American Great Plains fire regimes of the past millennium

Fire use has played an important role in human evolution and subsequent dispersals across the globe, yet the relative importance of human activity and climate on fire regimes is controversial. This is particularly true for historical fire regimes of the Americas, where indigenous groups used fire for myriad reasons but paleofire records indicate strong climate-fire relationships. In North American grasslands, decadal-scale wet periods facilitated widespread fire activity because of the abundance of fuel promoted by pluvial episodes. In these settings, human impacts on fire regimes are assumed to be independent of climate, thereby diminishing the strength of climate-fire relationships. We used an offsite geoarchaeological approach to link terrestrial records of prairie fire activity with spatially related archaeological features (driveline complexes) used for intensive, communal bison hunting in north-central Montana. Radiocarbon-dated charcoal layers from alluvial and colluvial deposits associated with driveline complexes indicate that peak fire activity over the past millennium occurred coincident with the use of these features (ca. 1100-1650 CE). However, comparison of dated fire deposits with Palmer Drought Severity Index reconstructions reveal strong climate-fire linkages. More than half of all charcoal layers coincide with modest pluvial episodes, suggesting that fire use by indigenous hunters enhanced the effects of climate variability on prairie fire regimes. These results indicate that relatively small, mobile human populations can impact natural fire regimes, even in pyrogeographic settings in which climate exerts strong, top-down controls on fuels.

Interannual variability of PDSI from tree-ring widths for the past 278 years in Baotou, China

This paper provides the first interannual variability of PDSI series in Central Inner Mongolia, China. A Pinus tabulaeformis tree-ring-width chronology in Baotou, China, was developed. Based on the chronology, we reconstructed the local interannual variability of the Palmer Drought Severity Index (PDSI) from the previous September to current July (PDSIp9–7) for the past 278 years. The reconstruction explained 41.3% of the variance (40.3% after adjusting the degrees of freedom) of the actual interannual variability of the PDSI during the calibration period (1951–2012 AD). The spectral analyses revealed 3.5-, 2.6-, 2.3-, 2.1- and 2-year quasi-cycles at a 95% confidence level and 7.9-, 7.1- and 5.2-year quasi-cycles at a 90% confidence level. The PDSI reconstruction series was consistent with several reconstructed climatic series based on the tree-ring chronology in neighboring regions. The PDSIp9–7 exhibits excellent regional representativeness, which indicates the consistency of the drought variation of the central part of the monsoon margin area in north China, whereas PDSIp9–7 can represent the drought history of the central part of the north monsoon margin area. In addition, significant relationships were found between the PDSIp9–7 series and the Arctic Oscillation, North Atlantic Oscillation, El Nino-Southern Oscillation and Pacific Decadal Oscillation series, which indicates that the PDSI variation in the study area was influenced by a combination of these factors.

Past changes of spring drought in the inner Tianshan Mountains, China, as recorded by tree rings

Drought is a common natural disaster in arid Central Asia, and the severity and frequency of drought and other hydroclimatic events are critical to the region's ecology and its residents’ livelihoods. In particular, water resource management requires reliable knowledge of climatic variability and annual and centennial drought patterns. In this paper, we establish a 587‐year tree ring chronology using 215 Schrenk spruce (Picea schrenkiana Fisch. et Mey) samples and reconstruct a spring Palmer Drought Severity Index (PDSI) for the inner Tianshan Mountains spanning the 1645–2008 period. The reconstruction explains 40.3% of the variance during the sub‐period for which we also have reliable observational data (1938–2005). Cross‐validation during this common period also supports our reconstruction model. Spatial correlation analyses with gridded PDSI data show that our PDSI reconstruction reveals a strong regional drought signal for the central and western Tianshan Mountains.

Increased tree-ring network density reveals more precise estimations of sub-regional hydroclimate variability and climate dynamics in the Midwest, USA

Understanding the historic variability in the hydroclimate provides important information on possible extreme dry or wet periods that in turn inform water management plans. Tree rings have long provided historical context of hydroclimate variability of the U.S. However, the tree-ring network used to create these countrywide gridded reconstructions is sparse in certain locations, such as the Midwest. Here, we increase (n = 20) the spatial resolution of the tree-ring network in southern Indiana and compare a summer (June–August) Palmer Drought Severity Index (PDSI) reconstruction to existing gridded reconstructions of PDSI for this region. We find both droughts and pluvials that were previously unknown that rival the most intense PDSI values during the instrumental period. Additionally, historical drought occurred in Indiana that eclipsed instrumental conditions with regard to severity and duration. During the period 1962–2004 CE, we find that teleconnections of drought conditions through the Atlantic Meridional Overturning Circulation have a strong influence (r = −0.60, p < 0.01) on secondary tree growth in this region for the late spring-early summer season. These findings highlight the importance of continuing to increase the spatial resolution of the tree-ring network used to infer past climate dynamics to capture the sub-regional spatial variability. Increasing the spatial resolution of the tree-ring network for a given region can better identify sub-regional variability, improve the accuracy of regional tree-ring PDSI reconstructions, and provide better information for climatic teleconnections.

Tree-ring-based reconstruction of drought variability (1792–2011) in the middle reaches of the Fen River, North China

We developed a tree-ring chronology based on 52 ring-width series from 25 Pinus tabulaeformis trees at Tianlong Mountain (TLM) using the signal-free method. TLM is located in the middle reaches of the Fen River, North China, and is influenced by the East Asian monsoon system. Tree growth was highly correlated (0.789) with the Palmer Drought Severity Index (PDSI) from May to July and indicated a drought-stress growth pattern. Therefore, we developed a robust May-July PDSI reconstruction for 1792–2011 that explained 62.3% of the instrumental variance for 1951–2005. Severe drought years determined by the reconstruction are consistent with conditions reported in historical documents. The TLM PDSI reconstruction was consistent with other tree-ring-based hydroclimate reconstructions in North China; thus, it may accurately represent dry/wet changes that occur over a large area. Cyclical spectral peaks at 2–8 years in the reconstructed PDSI may indicate ENSO activity, as suggested by the positive correlation with the western Pacific sea-surface temperatures (SSTs) and the negative correlation with the eastern Pacific SSTs on the inter-annual scale.

Tree-ring-width-based PDSI reconstruction for central Inner Mongolia, China over the past 333 years

A tree-ring-width chronology was developed from Pinus tabulaeformis aged up to 333 years from central Inner Mongolia, China. The chronology was significantly correlated with the local Palmer Drought Severity Index (PDSI). We therefore reconstructed the first PDSI reconstruction from March to June based on the local tree ring data from 1680 to 2012 AD. The reconstruction explained 40.7 % of the variance (39.7 % after adjusted the degrees of freedom) of the actual PDSI during the calibration period (1951–2012 AD). The reconstructed PDSI series captured the severe drought event of the late 1920s, which occurred extensively in northern China. Running variance analyses indicated that the variability of drought increased sharply after 1960, indicating more drought years, which may imply anthropogenic related global warming effects in the region. In the entire reconstruction, there were five dry periods: 1730–1814 AD, 1849–1869 AD, 1886–1942 AD (including severe drought in late 1920s), 1963–1978 AD and 2004–2007 AD; and five wet periods: 1685–1729 AD, 1815–1848 AD, 1870–1885 AD, 1943–1962 AD and 1979–2003 AD. Conditions turned dry after 2003 AD, and the PDSI from March to June (PDSI36) captured many interannual extreme drought events since then, such as 2005–2008 AD. The reconstruction is comparable to other tree-ring-width-based PDSI series from the neighboring regions, indicating that our reconstruction has good regional representativeness. Significant relationships were found between our PDSI reconstruction and the solar radiation cycle and the sun spot cycle, North Atlantic Oscillation, the El Nino-Southern Oscillation, as well as the Pacific Decadal Oscillation. Power spectral analyses detected 147.0-, 128.2-, 46.5-, 6.5-, 6.3-, 2.6-, 2.2- and 2.0-year quasi-cycles in the reconstructed series.

Bridging Past and Future Climate across Paleoclimatic Reconstructions, Observations, and Models: A Hydroclimate Case Study

AbstractPotential 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.

Causes of interannual to decadal variability of Gila River streamflow over the past century

Abstract Study region The Gila River, New Mexico, is characterized by two peaks in streamflow: one in the winter–spring (December–May), and summer (August–September). The region is influenced both by Pacific SST variability as well as the North American Monsoon. Study focus The mechanisms responsible for the variability of the winter–spring and summer streamflow peaks are investigated by correlation of streamflow with precipitation and sea surface temperature for 1928–2012. Decadal variability in the flow record is examined for a longer term perspective on Gila River streamflow using tree ring-based reconstructions of the Palmer Drought Severity Index (PDSI) and the Standardized Precipitation Index (SPI). New hydrological insights for the region Results indicate a strong influence of winter–spring precipitation and Pacific SST anomalies on the winter–spring streamflow, with El Nino conditions in the Pacific causing increased precipitation and streamflow. Decadal Pacific variability helps explain the transition from high winter flow in the late 20th century to lower flows in the most recent decade. The summer streamflow has a somewhat weaker correlation with precipitation and Pacific SST than the winter–spring streamflow. Its variability is more likely influenced by local North American Monsoon precipitation variability. PDSI and SPI reconstructions indicate much more severe and extended periods of droughts and pluvials in past centuries as well as periods of concurrent winter and summer drought.

Reconstruction of drought variability in North China and its association with sea surface temperature in the joining area of Asia and Indian–Pacific Ocean

Using tree-ring data from the northernmost marginal area of the East Asian summer monsoon (EASM) in North China, May–July mean Palmer drought severity index (PDSI) was reconstructed back to 1767AD. The reconstruction captured 52.8% of the variance over the calibration period from 1945 to 2005AD and showed pronounced pluvial periods during 1850–1905, 1803–1811 and 1940–1961 and dry periods during 1814–1844, 1916–1932 and 1984–2012. These anomalous periods have previously been reported in other parts of North China. Spatial correlation analyses and comparisons with other hydroclimatic indices in North China indicated that our new PDSI reconstruction could represent spatial and temporal drought variability in this region well. Our work also suggested that the drying tendency currently observed in the northern part of North China (including the study area) is consistent with the weakening of the EASM. Meanwhile the drying trend was seemingly restrained at present in the southern part of North China. Spatial correlation patterns with global sea surface temperature (SST) indicated that the regional hydroclimatic variability in North China was tightly linked to SST over the joining area of Asia and Indian–Pacific Ocean (AIPO), especially over the tropical western Pacific. When SST from prior November to current July (NJ-SST) in the AIPO area was anomalously high (low), the thermal contrast between Asian land and ocean was weakened (strengthened), and the EASM was correspondingly weakened (strengthened), thereby causing droughts (pluvials) in North China. The results of this study do not only provide useful information for assessing the long-term climate change in North China, but also suggest that abnormal variability in NJ-SST over the AIPO area could be used to forecast hydroclimatic conditions in North China.

A tree-ring based drought reconstruction (AD 1760–2010) for the Loess Plateau and its possible driving mechanisms

We have developed a 272-year ring-width chronology of Chinese pine (Pinus tabulaeformis) growing in the Huanglong Mountains, North China. Climatic response analyses revealed that mean January–July Palmer drought severity index (PDSI) has positive effect on the radial growth of pine trees. Based on the relationships, the mean January–July PDSI was reconstructed for the period from 1760 to 2010. The percentage of variance in the data explained by the reconstruction was 41% during the calibration period of 1950–2010. Spatial correlation analyses between the PDSI reconstruction and gridded PDSI data show that the PDSI reconstruction captures regional drought variations over the environmentally-sensitive area linked to the East Asian summer monsoon. Relatively wet periods are identified for AD 1766–1781, 1795–1804, 1811–1821, 1838–1859, 1884–1889, 1909–1914, 1937–1977 and 2003–2008. Dry conditions prevailed during AD 1760–1765, 1782–1794, 1805–1810, 1822–1837, 1860–1883, 1890–1908, 1915–1936, 1978–2002 and 2009–now. There is a reasonable agreement with dry/wet periods previously estimated from tree-ring data of the Kongtong Mountains of the Loess Plateau. Spatial correlation analyses with sea surface temperature in the Pacific Ocean and tropical Indian Ocean indicated that the Asian summer monsoon circulations play a role in modulating drought variations in the study area whereas the effects of the Pacific Decadal Oscillation are relatively strong.

Reconstruction of the March–August PDSI since 1703 AD based on tree rings of Chinese pine (&amp;lt;i&amp;gt;Pinus tabulaeformis&amp;lt;/i&amp;gt; Carr.) in the Lingkong Mountain, southeast Chinese loess Plateau

Abstract. We utilised tree-ring cores, collected from three sites at Lingkong Mountain located in the southeast part of the Chinese Loess Plateau (CLP), to develope a regional ring-width chronology. Significant positive correlations between the tree-ring index and the monthly Palmer drought severity index (PDSI) were identified, indicating that the radial growth of trees in this region was moisture-limited. The March–August mean PDSI was quantitatively reconstructed from 1703 to 2008 with an explained variance of 46.4%. Seven dry periods during 1719–1726, 1742–1748, 1771–1778, 1807–1818, 1832–1848, 1867–1932 and 1993–2008 and six wet periods during 1727–1741, 1751–1757, 1779–1787, 1797–1805, 1853–1864 and 1934–1957 were revealed in our reconstruction. Among them, 1867–1932 and 1934–1957 were identified as the longest dry and wet periods, respectively. On the centennial scale, the 19th century was recognised as the driest century. The drying tendency since 1960s was evident. However, recent drought in 1993–2008 was still within the frame of natural climate variability based on the 306 yr PDSI reconstruction. The dry and wet phases of Lingkong Mountain were in accordance with changes in the summer Asian-Pacific oscillation (IAPO) and sunspot numbers, they also showed strong similarity to other tree-ring based moisture indexes in large areas in and around the CLP, indicating the moisture variability in the CLP was almost synchronous and closely related with large-scale land–ocean–atmospheric circulation and solar activity. Spatial correlation analysis suggested that this PDSI reconstruction could represent the moisture variations for most parts of the CLP, and even larger area of northern China and east Mongolia. Multi-taper spectral analysis revealed significant cycles at the inter-annual (2–7 yr), inter-decadal (37.9 yr) and centennial (102 yr) scales. Results of this study are very helpful for us to improve the knowledge of past climate change in the CLP and enable us to prevent and manage future natural disasters.

Intensification of North American Megadroughts through Surface and Dust Aerosol Forcing*

Abstract Tree-ring-based reconstructions of the Palmer drought severity index (PDSI) indicate that, during the Medieval Climate Anomaly (MCA), the central plains of North America experienced recurrent periods of drought spanning decades or longer. These megadroughts had exceptional persistence compared to more recent events, but the causes remain uncertain. The authors conducted a suite of general circulation model experiments to test the impact of sea surface temperature (SST) and land surface forcing on the MCA megadroughts over the central plains. The land surface forcing is represented as a set of dune mobilization boundary conditions, derived from available geomorphological evidence and modeled as increased bare soil area and a dust aerosol source (32°–44°N, 105°–95°W). In the experiments, cold tropical Pacific SST forcing suppresses precipitation over the central plains but cannot reproduce the overall drying or persistence seen in the PDSI reconstruction. Droughts in the scenario with dust aerosols, however, are amplified and have significantly longer persistence than in other model experiments, more closely matching the reconstructed PDSI. This additional drying occurs because the dust increases the shortwave planetary albedo, reducing energy inputs to the surface and boundary layer. The energy deficit increases atmospheric stability, inhibiting convection and reducing cloud cover and precipitation over the central plains. Results from this study provide the first model-based evidence that dust aerosol forcing and land surface changes could have contributed to the intensity and persistence of the central plains megadroughts, although uncertainties remain in the formulation of the boundary conditions and the future importance of these feedbacks.

A 426-year drought history for Western Tian Shan, Central Asia, inferred from tree rings and linkages to the North Atlantic and Indo–West Pacific Oceans

We have reconstructed the Palmer Drought Severity Index (PDSI) from 1580 to 2005 for Western Tian Shan, Central Asia, using tree rings of Picea schrenkiana. The reconstruction explains 35% of the actual January–May PDSI variance during the common period 1925–2005. Split-sample validation supports our use of the reconstruction model based on the full period of reliable observational data (1925–2005). Spatial climate correlation analyses with gridded PDSI data revealed that our PDSI reconstruction represents a strong regional drought signal for Western Tian Shan. Dry periods occurred during ad 1614–1628, 1700–1722, 1758–1790, 1806–1833, 1873–1898, 1908–1936, 1943–1951, 1960–1966 and 1973–1988; while the periods of ad 1580–1613, 1629–1699, 1723–1757, 1791–1805, 1834–1872, 1899–1907, 1937–1942, 1952–1959, 1967–1972 and 1989–present were relatively wet. Our reconstruction agrees reasonably well with the dry and wet periods previously estimated from tree rings in Tian Shan. Significant spectral peaks are identified at 2–5, 60.2 and 213 years. Our reconstructed drought is significantly correlated with sea surface temperature in the North Atlantic and Indo–West Pacific Oceans. The linkages to the North Atlantic and Indo–West Pacific Oceans suggest the connection of moisture variations of Central Asia to the westerly circulation and tropical ocean–atmosphere systems.

Climatic response of Chinese pine and PDSI variability in the middle Taihang Mountains, north China since 1873

We reconstructed the first Palmer Drought Severity Index (PDSI) in the Taihang Mountains, north China from 1873 to 2008, based on ring width of the Chinese Pine (Pinus tabulaeformis Carr). The reconstruction can explain 43.2 % of the PDSI from instrument record during the calibration period (1955–2005). The PDSI variability was relatively stable before the twentieth century. However, it became more variable and persistent during the twentieth and early twenty-first century. Persistent long dry intervals were found in 1920–1931, 1973–1982 and 1992–2001, and the wet intervals were identified in 1944–1965 and 2004–2008. Comparing with the 1920s, the dryness of the 1970s and 1990s were more intense and the 1990s ranked the most severe drought for the whole reconstructed period in the Taihang Mountains. After a decreasing trend since the 1950s, the PDSI shows increasing tendency after 2000, demonstrating a better moisture condition in recent years in the studied area. Comparisons of our reconstruction with nearby dry/wet index and other tree-ring-based PDSI reconstruction showed high consistency in dry (wet) periods, showing similar dry–wet regimes around the sampled site, which was also further proved by the spatial correlation analysis. This reconstruction is also highly correlated with the East Asian Summer Monsoon (EASM) index, suggesting the reconstructed PDSI could be used to indicate the strength of EASM. Spectral analysis revealed significant peaks at 2.05, 2.07, 2.65, 3.75, 10 and 60 years, implying the influence of larger circulation system (such as ENSO, PDO and NAO) and solar activity on local climate.

Forcing of hydroclimatic variability in the northwestern Great Plains since AD 1406

The 20th century hydroclimatology of northwestern North America has been linked to naturally recurring large-scale climate patterns such as the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO). Few observed hydroclimatic records from this region exceed in length the ∼60-year periodicity of the lower frequency climate oscillations; however, tree-ring proxy data from semi-arid western North America document natural hydroclimate variation over centennial to millennial scales. We reconstructed the summer Palmer Drought Severity Index (PDSI) over the western Canadian Prairies, providing a record of drought for the past 600 years. This long reconstruction was examined for information about the severity, intensity, and duration of positive (wet) and negative (dry) summer moisture anomalies during the different phases of the PDO and the ENSO, as reconstructed by other researchers from independent proxy datasets. As well, by comparing our PDSI reconstruction to other researchers' independent regional summer temperature reconstruction, we were able to identify warm versus cold droughts, and warm versus cold wet periods. The reconstruction was also compared to 500 hPa geopotential heights. These results suggest that summer moisture conditions are associated with the North Pacific Ocean and tropical Pacific Ocean variability. Summer droughts generally have occurred during the positive PDO phase; however, ENSO conditions have varied between El Niño and La Niña. The most severe droughts, such as those of AD 1858–1872 and AD 1930–1941, were commonly associated with higher summer temperatures, the positive phase of the PDO, and increased ENSO variability.

Recent tree-growth reduction in north central China as a combined result of a weakened monsoon and atmospheric oscillations

Tree-ring records are a valuable source of information for understanding long-term, regional-scale drought changes. In this study, a tree ring width chronology spanning the last 330 years (A.D. 1681–2010) is developed for the northern fringe of the Asian summer monsoon in north central China based on tree ring widths of the Chinese pine (Pinus tabulaeformis) at three sites in the Hasi Mountain (HSM). An annual (running from the previous August to the present July) Palmer Drought Severity Index (PDSI) series is reconstructed for the period A.D. 1698 to 2010 using a linear regression model. This reconstruction accounts for 49 % of the actual PDSI variance during the calibration period (A.D.1951–2005). During the last past 330 years, the year 1759 drought was the most severe and the 1926–1932 drought was the most long-lasting. These drought episodes resulted in huge economic losses and severe famine. Similar periods of drought are also found in the Great Bend of the Yellow River region, northeastern Tibetan Plateau and northern China. Our drought reconstruction is consistent with the dry-wet index derived from historical documents for the Great Bend of the Yellow River region for the last three centuries, revealing that our annual PDSI reconstruction reflects broad-scale climate anomalies and represents drought variations in the northern fringe of the Asian summer monsoon. The PDSI reconstruction correlates significantly with sea surface temperature (SST) in the eastern equatorial Pacific Ocean and northern Indian Ocean at an annual timescale, implying that El Nino-Southern Oscillation and the Indian monsoon might be influencing drought variability in the study area. Some extremely dry years of 1707, 1764, 1837, 1854, 1878, 1884, 1926 and 1932 coincided with major El Nino events in historical times. The decadal-scale variability is linked to Pacific Decadal Oscillation (PDO) and SST variations in the Atlantic Ocean. The observed recent tree growth reduction is unusual when viewed from a long-term perspective.