Tree-ring-based moisture variability in western Tianshan Mountains since A.D. 1882 and its possible driving mechanism

The western Tianshan Mountains are a very important source of water. The area is dominated by westerly winds and is important in global change research. In this paper, we combine two tree-ring width chronologies, meteorological data from the Climatic Research Unit (CRU), a standardized precipitation-evapotranspiration index (SPEI) and Palmer Drought Severity Index (PDSI) to analyze radial growth response to climate. Studies show moisture from the previous July to current June was consistently the dominant climatic factor accounting for interannual variation of tree-ring width in the western Tianshan Mountains. Then, we reconstruct the precipitation, SPEI and PDSI, and the three series are verified with mathematical methods and other observation data. Power spectral and wavelet analyses reveal significant 2- to 6-year high-frequency cycles and a 17-year low-frequency cycle. Moisture has varied as four wetter periods and four drier periods over the past century. We show that over all northern slopes of the Tianshan Mountains, moisture change has been rarely consistent. From the 1980s to present, the climate has had the longest, most significant and rapid wetting period in the past century. The moisture variability pertains to most areas of the western Tianshan Mountains, especially the northern slope of the mountains in Kyrgyzstan. Our results indicate a strong negative relationship between the Summer North Atlantic Oscillation (SNAO) and moisture in the western Tianshan Mountains. The moisture variability may have been controlled by the SNAO in the past century, and may be associated with large-scale oscillation in the climate system.