Wenling An

Relative humidity history on the Batang–Litang Plateau of western China since 1755 reconstructed from tree-ring δ18O and δD

We measured the annual variation in the stable isotopes of oxygen (δ18O) and hydrogen (δD) in tree rings of Abies georgei on the Batang–Litang Plateau of western China. Although correlations between tree-ring δ18O and δD are relatively weak in semi-arid regions, we found a strong correlation between the δ18O and δD time series from 1755 to 2009 under the wetter environment. Tree-ring δ18O and δD time series are both significantly and negatively correlated with moisture conditions from June to August, including relative humidity and total precipitation, respectively, from 1960 to 2009. Considering the difference in low-frequency domain between the two isotopes, the relative humidity histories from June to August, reconstructed separately from the tree-ring δ18O and δD data with instrumental climate data, reveal a persistent drying trend since 1850s, especially since the early 1970s. There is an obvious offset of reconstructed relative humidity from tree-ring δ18O and δD in the period 1755–1820, despite the strong similarity in their 21-year moving averages. The decreased relative humidity since the 1850s may be associated with the thermal contrast between the sea surface temperature of the Indian Ocean and the Qinghai-Tibetan Plateau, which determines the strength of moisture transfer via the Indian summer monsoon.

Specific climatic signals recorded in earlywood and latewood δ 18 O of tree rings in southwestern China

ABSTRACT Earlywood and latewood form during different parts of the growing season and therefore capture climate of distinct time intervals. Here we present a comparison of earlywood and latewood δ18O in tree rings from the Yulong Snowy Mountains of southwestern China, covering the period from 1902 to 2005. Earlywood and latewood δ18O exhibit different long-term behaviour obviously during the past century. Climate–response analysis indicates that the dominant parameters for earlywood δ18O are temperature and relative humidity during the early part of the monsoon season (May to July); however, for latewood, it is the moisture condition (precipitation and relative humidity) from August to October. Sea-surface temperatures over the Indian Ocean and the Pacific Ocean have imprinted their different influences on the earlywood and latewood δ18O. The δ18O of source water were reconstructed from the earlywood and latewood δ18O. We found that the source of the water synthesised into earlywood was mainly contributed by current precipitation, while for latewood it is more complicated. The signals from the Indian Summer Monsoon and the East Asian Summer Monsoon are temporally superimposed (though differently) on the source water of earlywood and latewood, as well as the El Niño–Southern Oscillation events.

Pooled versus separate tree-ring δD measurements, and implications for reconstruction of the Arctic Oscillation in northwestern China

Stable hydrogen isotope ratios (δD) in tree rings are an attractive but still rarely explored terrestrial archive of past climatic information. Because the preparation of the cellulose nitrate for δD measurements requires more wood and a longer preparation time than preparation techniques for other isotopes in cellulose (δ18O or δ13C), it is challenging to obtain high-resolution records, especially for slow-growing trees at high elevations and in boreal regions. Here, we tested whether annually pooled samples of Qinghai spruce (Picea crassifolia Kom.) trees from northwestern China provided results similar to those derived as the mean of individual measurements of the same trees and whether the resulting chronologies recorded useful climate information. Inter-tree variability of δD was higher than that of measured ring width for the same trees. We found higher and significant coherence between pooled and mean isotope chronologies than that among the individual series. It showed a logarithmic relationship between ring mass and δD; however, accounting for the influence of ring mass on δD values only slightly improved the strength of climatic signals in the pooled records. Tree-ring δD was significantly positively correlated with the mean, maximum, and minimum temperatures during the previous winter and with maximum temperature during the current August, and significantly negatively correlated with precipitation in the previous November to January and the current July. The winter climate signal seems to dominate tree-ring δD through the influence of large-scale atmospheric circulation patterns, i.e. the Arctic Oscillation. These results will facilitate reconstruction of winter atmospheric circulation patterns over northwestern China based on a regional tree-ring δD networks.

Unstable relationships between tree ring δ18O and climate variables over southwestern China: possible impacts from increasing central Pacific SSTs

In this study, we investigated the potential influence of central and eastern Pacific sea surface temperatures (SSTs) on the unstable relationship between earlywood δ18O and climatic factors in the southwestern China from 1902 to 2005. The results show that the strength of the climate signals recorded in the earlywood δ18O series has declined since the late 1970s. This reduction in signal strength may have been caused by the changes in the local hydroclimate, which is associated with the increasing SSTs in the central Pacific Ocean over recent decades. Alongside these increasing SSTs in the central Pacific, southwestern China has experienced more droughts, as well as more severe droughts through the late spring and early summer during the central Pacific (CP) El Niño years than during the eastern Pacific (EP) El Niño years in recent decades. This increased drought frequency may have weakened the response of earlywood δ18O to climate variables.