Qiufang Cai

Tree-ring hydrologic reconstructions for the Heihe River watershed, western China since AD 1430

Based on the tree-ring-width analysis, the total precipitation from previous July to current June of the Qilian Mountains from 1634 to 2000 AD and the average runoff from previous September to current June in the middle section of the Heihe River from 1430 to 2007 have been reconstructed. This allowed detailed examination of the hydrologic history of the watershed of the Heihe River in western China. Precipitation, runoff and groundwater level were found to be significantly correlated with each other on the decadal scale. The three curves display quite synchronous trends of natural variation before AD 1940 to present before the onset of man-made disturbances. A remarkable period is AD 1925-1940 when the precipitation is low in the upper section, the runoff decreases in the middle section, and the groundwater level declines in the downstream section. After 1940, the groundwater level shows a lag effect, which may be a result of high water consumption in the middle and downstream sections. All three tree-ring based hydrologic indices commonly display the most significant periodicities around 80 (78-82), 50 (49-58) and 2 year. These cycles correspond to large-scale oscillation found in the climate system and appear mainly related to ocean-atmosphere interaction.

Reconstruction of May–July precipitation in the north Helan Mountain, Inner Mongolia since A.D. 1726 from tree-ring late-wood widths

By analyzing statistical characteristics of five tree-ring standard chronologies, early-wood ring width (EWW), late-wood ring width (LWW), total ring width (TRW), minimum early-wood density (MinD), maximum late-wood density (MaxD) and, their climatic response respectively, we reconstructed the May to July precipitation using late-wood ring width (LWW) over the north Helan Mountain since A.D. 1726. The explained variance is 42% (R2adj = 41%,F= 31.46,p < 0.000001). After 11-a moving average, the explained variance reaches 82% (F= 156.9,p < 0.05). On the decadal scale, the rainfall reconstruction of the northern Helan Mountain displays a quite similar variation pattern with that of the April to early July precipitation in Baiyinaobao, east of Inner Mongolia for the last 150 years. It may reflect the intensity variation of the East Asia Summer Monsoon front to a certain extent. Spectrum analysis shows 11-a and 22-a periodicities in the May to July precipitation reconstruction at the north Helan Mountain.

Tree-ring precipitation records from Baiyinaobao, Inner Mongolia since A.D. 1838

Correlation function analysis shows that total precipitation from April to early July (to July 10) in growing season limits the ring width ofPicea Koraiensis significantly in Baiyinaobao, Xilinhot, Inner Mongolia. Thus a transfer function is designed to reconstruct the total precipitation from April to early July over the region for the last 160 years. The explained variance of the reconstruction is 49.3% (and 45.7% when adjusted for loss of degrees of freedom,N = 31,r = 0.702,F = 13.608,p < 0.0001). There are three relatively wet periods in the reconstruction which are: 1869–1875, 1886–1921 and 1943–4968, and four relatively dry periods: 1851–1868, 1876–1885, 1922–1943 and 1969–1999. Among them, the wet periods of 1886–1921 and 1943–1968, and the dry period of 1922–1943 appeared almost at the same time with those in Mt. Qilian region. Meanwhile, the dry period of 1922–1943 corresponds to high temperature and low rainfall during the 1920s and thereafter, and wetness of 1943–1968 corresponds very well to low temperature and more precipitation after the 1940s in Northwest, Northern China and the Changjiang River drainage basin. Calculation shows that the reconstruction series is significantly correlated with local dry-wet index (r = −0.234,p < 0.007,N = 143). Wavelet analysis indicates that there is a 22-a cycle in the precipitation of April to early July during 1838–1920, and an 11-a cycle during 1920–1952. No more cycles that reach 95% confidence limits are identified after 1953. The precipitation from April to early July quickly dropped into low during the 1920s. After 1950, precipitation shows a declining trend, but the reconstruction shows a tendency of increase in the late of 1990s.

Tree-Ring-Derived Precipitation Records from Inner Mongolia, China, Since A.D. 1627

Two Chinese pine (Pinus tabulaeformis) tree-ring width chronologies up to 375 years long were used to reconstruct rainfall from February to early July for the Wu Dangzhao region and from February to mid-July for the La Madong region, Inner Mongolia, China. The predictor variables account for 44.3% and 42.7% of the variance in precipitation, respectively. Both historical records and two other tree-ring based precipitation reconstructions from the environmentally sensitive zone (the northern Helan Mountain range and Baiyinaobao) confirm our results. After applying a 10-year moving average, the trends of four tree-ring based precipitation reconstructions vary synchronously. Periods with below-normal precipitation occurred during the 1720s–1730s, 1740s–1750s, 1790s, early 1810s, late 1830s–1860s, 1880s–1910s, late 1920s–1930s and after the late 1960s–early 1970s. Periods with above-normal precipitation occurred in the 1760s to early 1770s, 1820s to early 1830s, 1870s–1880s, early 1920s, 1940s to early 1960s, and 1990s. The late 1920s period was the most severe drought over a broad area in north China in the last 375 years. In contrast, the wettest period was in the late 1990s.

Recent enhancement of central Pacific El Nino variability relative to last eight centuries

The far-reaching impacts of central Pacific El Niño events on global climate differ appreciably from those associated with eastern Pacific El Niño events. Central Pacific El Niño events may become more frequent in coming decades as atmospheric greenhouse gas concentrations rise, but the instrumental record of central Pacific sea-surface temperatures is too short to detect potential trends. Here we present an annually resolved reconstruction of NIÑO4 sea-surface temperature, located in the central equatorial Pacific, based on oxygen isotopic time series from Taiwan tree cellulose that span from 1190 AD to 2007 AD. Our reconstruction indicates that relatively warm Niño4 sea-surface temperature values over the late twentieth century are accompanied by higher levels of interannual variability than observed in other intervals of the 818-year-long reconstruction. Our results imply that anthropogenic greenhouse forcing may be driving an increase in central Pacific El Niño-Southern Oscillation variability and/or its hydrological impacts, consistent with recent modelling studies.

A Picea crassifolia Tree-Ring Width-Based Temperature Reconstruction for the Mt. Dongda Region, Northwest China, and Its Relationship to Large-Scale Climate Forcing.

The historical May–October mean temperature since 1831 was reconstructed based on tree-ring width of Qinghai spruce (Picea crassifolia Kom.) collected on Mt. Dongda, North of the Hexi Corridor in Northwest China. The regression model explained 46.6% of the variance of the instrumentally observed temperature. The cold periods in the reconstruction were 1831–1889, 1894–1901, 1908–1934 and 1950–1952, and the warm periods were 1890–1893, 1902–1907, 1935–1949 and 1953–2011. During the instrumental period (1951–2011), an obvious warming trend appeared in the last twenty years. The reconstruction displayed similar patterns to a temperature reconstruction from the east-central Tibetan Plateau at the inter-decadal timescale, indicating that the temperature reconstruction in this study was a reliable proxy for Northwest China. It was also found that the reconstruction series had good consistency with the Northern Hemisphere temperature at a decadal timescale. Multi-taper method spectral analysis detected some low- and high-frequency cycles (2.3–2.4-year, 2.8-year, 3.4–3.6-year, 5.0-year, 9.9-year and 27.0-year). Combining these cycles, the relationship of the low-frequency change with the Pacific Decadal Oscillation (PDO), North Atlantic Oscillation (NAO) and Southern Oscillation (SO) suggested that the reconstructed temperature variations may be related to large-scale atmospheric-oceanic variations. Major volcanic eruptions were partly reflected in the reconstructed temperatures after high-pass filtering; these events promoted anomalous cooling in this region. The results of this study not only provide new information for assessing the long-term temperature changes in the Hexi Corridor of Northwest China, but also further demonstrate the effects of large-scale atmospheric-oceanic circulation on climate change in Northwest China.

The forecast of seasonal precipitation trend at the north Helan Mountain and Baiyinaobao regions, Inner Mongolia for the next 20 years

By using Caterpillar-SSA analysis method, through the process of embedding, singular value decomposition, grouping and diagonal averaging, the seasonal precipitation trend at north Helan Mountain and Baiyinaobo regions, Inner Mongolia for the next 20 years is forecasted. The results show an increasing precipitation trend from 1992 to 2004. In the subsequent decade the precipitation should reduce quickly, and it will reach a minimum near 2012 to 2014 in both regions. The drought caused by the decrease of the precipitation from May to July in the north Helan Mountain area during the period of 2013–2014 is probably quite similar to that around 1929. Further, the period of precipitation gradual increase follows in the researched regions.

Monsoonal precipitation variation in the East Asia since A.D. 1840

Three tree-ring rainfall reconstructions from China and Korea are used in this paper to investigate the East Asian summer monsoon-related precipitation variation over the past 160 years. Statistically, there is no linear correlation on a year-by-year basis between Chinese and Korean monsoon rainfall, but region-wide synchronous variation on a decadal-scale was observed. More rainfall intervals were 1860–1890, 1910–1925, and 1940–1960, and dry or even drought periods were 1890–1910, 1925–1940, and 1960–present. Reconstructions also display that the East Asian summer monsoon precipitation suddenly changed from more into less around mid-1920. These tree-ring precipitation records were also confirmed by Chinese historical dryness/wetness index and Korean historical rain gauge data.

Tree-ring stable carbon isotope-based April–June relative humidity reconstruction since ad 1648 in Mt. Tianmu, China

Based on accurate dating, we have determined the stable carbon isotope ratios (δ13C) of five Cryptomeria fortunei specimens from Mt. Tianmu, a subtropical area in southern China. The five δ13C time series records are combined into a single representative δ13C time series using a “numerical mix method.” These are normalized to remove temporal variations of δ13 C in atmospheric CO2 to obtain a carbon isotopic discrimination (Δ13C) time series, in which we observe a distinct correlation between Δ13C and local April to June mean relative humidity (RHAMJ) (n = 64, r = 0.858, p < 0.0001). We use this relationship to reconstruct RHAMJ variations from ad 1648 to 2014 at Mt. Tianmu. The reconstructed sequence show that over the past 367 years, Mt. Tianmu area was relatively wet, but in the latter part of the twentieth century, under the influence of increasing global warming, it has experienced a sharp reduction in relative humidity. Spatial correlation analysis reveals a significant negative correlation between RHAMJ at Mt. Tianmu and Sea Surface Temperature (SSTs) in the western equatorial Pacific and Indian Ocean. In other words, there is a positive correlation between tree-ring δ13C in Mt. Tianmu and SSTs. Both observed and reconstructed RHAMJ show significant positive correlations with East Asian and South Asian monsoons from 1951 to 2014, which indicate that RHAMJ from Mt. Tianmu reflects the variability of the Asian summer monsoon intensity to a great extent. The summer monsoon has weakened since 1960. However, an increase in relative humidity since 2003 implies a recent enhancement in the summer monsoon.

TREE-RING BASED JUNE-JULY RUNOFF RECONSTRUCTION FOR THE DATONG RIVER WATERSHED, WESTERN CHINA SINCE AD1525: TREE-RING BASED JUNE-JULY RUNOFF RECONSTRUCTION FOR THE DATONG RIVER WATERSHED, WESTERN CHINA SINCE AD1525

A tree-ring chronology has been built up based on the tree-ring samples from the Qilian Mountains.Response analysis showed that there is significant relationship between the tree-ring width and the June to July runoff of the Datong River watershed.Based on the analysis,a transfer function was designed to reconstruct the total runoff from June to July of the Datong River during the period 1525—2009.There are 5 wet and 5 dry periods in the reconstructed runoff history.The wet periods are 1530—1555,1576—1586,1733—1760,1776—1786,1855—1912,and the dry periods 670—1680,1691—1730,1814—1836,1926—1942,1964—1978 respetively.The reconstructed series also displayed that the standard deviations in the dry periods are higher than the wet periods,and the extreme dry and wet periods have the highest standard deviation.The reconstructed June-July runoff series shows significant periodicities around 2(2.04~2.3),50(50~57),100,133 and 200years.