Abstract: The study of relative humidity is important to understand better the past climatic variations.However,there have been few long-term humidity reconstructions using tree-ring widths worldwide.In this paper,we established a reconstruction of mean relative humidity from May to September from AD 1639 to 2013 using Sabina tibetica Kom from the southern Qinghai Plateau region in the northeastern Tibetan Plateau,China.In total,there were 56 tree cores from 28 trees collected at Angsai Township of Zaduo County of Qinghai Province (AS,95°37.765'E,32°42.856'N).All samples were surfaced,cross-dated and measured according to standard dendrochronology techniques.The reconstructed equation was stable and reliable and its variance interpretation quantity reached 42.3%.In the past 375 years,there were five wet periods (1694-1710,1753-1778,1830-1847,1892-1908,and 1978-1989) and eight dry periods (1646-1673,1682-1693,1711-1731,1735-1752,1796-1809,1817-1829,1848-1861 and 1873-1886) in the reconstructed humidity sequence.The longest wet period is 1753-1778 (26 years),and the longest dry period is 1646-1673 (28 years).There were 12 extremely dry years:1689,1700,1724,1727,1739,1749,1872,1910,1942,1953,1995 and 1998;and 5 extremely wet years:1640,1699,1703,1704 and 1873.The multi-taper method (MTM) spectral analysis indicates that there are 3 periodic changes of 28-30 a,6-9 a,2-5 a.The 2-5 a cycle may be related to the Quasi-biennial and Southern Oscillations influenced by the constant change between east wind and west wind in the equatorial stratosphere in a cycle of 26-30 months.The 6-9 a cycle may be related to ENSO.By comparing the reconstructed sequence from this study with a few typical reconstructed series which can reflect the status of dry and wet in the Southern Qinghai Plateau,it is found that there were four curves which shared the same wet periods (1710s,1830s-1840s and 1890s-1900s) and drought periods (1660s,1680s-1690s,1730s-1740s,1820s and 1950s) at an inter-annual scale.The result shows that these series presented good consistency on the change of low frequency.Moreover,our relative humidity reconstruction was correlated well with the corresponding grids MADA dataset in the public section during the common period.All of these have proved the accuracy of the reconstruction sequence in this paper.

Key words: southern Qinghai Plateau, tree-ring residual chronology, relative humidity, reconstruction