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Incursion of the Indian Monsoon flow onto the Qinghai–Tibetan Plateau is an important regional moisture-delivery mechanism, but the causes of intra-seasonal and longer-term variations are poorly understood. We observed a clear intra-seasonal cycle in the oxygen isotope composition ( δ 18 O) of Smith fir ( Abies forrestii var. georgei) growing at two different altitudes (high-elevation site: 29°39′N, 94°42′E, 4200 m a.s.l.; low-elevation site: 29°39′N, 94°43′E, 3800 m a.s.l.) on a SE-facing slope in the Sygera Mountains, southeastern Qinghai–Tibetan Plateau. Mechanistic modeling suggests that δ 18 O of precipitation, which in turn depends on relative humidity and precipitation amount, is important in controlling the intra-seasonal tree-ring isotopic cycle, and we speculate that the same controls also operate on interannual and longer time scales. If so, multi-century δ 18 O records with intra-seasonal resolution may enable us to infer long-term variations in the incursion of the Indian Monsoon onto the Qinghai–Tibetan Plateau. [ABSTRACT FROM AUTHOR]

Abstract: We developed a new tree-ring cellulose δ18O chronology for the southeastern Tibetan Plateau from Balfour spruce (Picea likiangensis var. balfouriana [Rehd. et Wils.]) that covered the period from 1600 to 2008, and compared the results with a previous study to explore climatic variations in the Nyingchi-Bomi area. Our tree-ring δ18O chronology correlated significantly with the previous study (Shi et al., 2012; Climate of the Past 8, 205–213) during the common period from 1781 to 2005, and provided new insights into long-term regional hydroclimatic variations. Besides the significant positive correlations between tree-ring δ18O and the temperature and sunshine duration during the growing season, tree-ring δ18O was strongly negatively correlated with regional cloud cover, relative humidity, and precipitation in July and August. The correlations with cloud cover data were stronger than in previous research, but the correlations with precipitation and relative humidity in July and August were weaker. When Indian summer monsoon conditions prevail, regional hydroclimate variations (and especially cloud cover) have the dominant influence on tree-ring δ18O in the study area. Based on the regional data, δ18O in tree rings can be an effective proxy to infer the temporal variations in regional hydroclimatic conditions and the strength of the Indian Summer Monsoon. Our results reveal that the Indian Summer Monsoon weakened from 1600 to 1650, followed by continuous strengthening until 1740 and a slight weakening from 1740 to present. The temporal variations in the cellulose δ18O chronology generally corresponded well to the δ18O and glacier snow accumulation records found in ice cores from the middle Himalaya. [Copyright &y& Elsevier]

Annual tree ring δO and δC chronologies from 1790 to 2008 were established using Tianshan spruce ( Picea schrenkiana) in the central Tianshan Mountains of northwestern China. Temperature has a positive effect on tree ring δO and δC in the study area, while precipitation and relative humidity have negative effects. The standardized precipitation-evapotranspiration index (SPEI) considered all of these effects and was significantly negatively correlated with tree ring δO and δC. We combined the tree ring δO and δC series to reconstruct the past 192 years of SPEI, which accounted for about 46 % of the total variance of SPEI from 1950 to 2006. The reconstruction showed good spatial agreement with gridded data in Palmer Drought Severity Index and precipitation and an inverse relationship with temperature. Our SPEI reconstruction reveals several wet and dry periods over the past 192 years and has good agreement with other drought records. Wavelet analysis showed quasi-periodic 10-, 20-, 30-, and 70-year fluctuations in the reconstruction. The 10-, 20-, and 30-year periodicities may reflect the potential influence of North Atlantic Oscillation (NAO). Overall, this study indicates that the SPEI is a potential drought index, and the winter NAO affects regional moisture conditions in the long term. [ABSTRACT FROM AUTHOR]