Summer moisture variations in northwestern China during the past 700 years

Abstract:

The northwestern China is one of the most arid regions in the middle latitudes, where water is scarce year round. This study investigated the variations of summer moisture in this region using Palmer Drought Severity Index (PDSI) derived from instrumental data and tree-ring records. Based on rotational Empirical Orthogonal Function (REOF) analysis, the observed summer moisture variations in northwestern China can be classified into three major patterns: the south Xinjing, north Xinjing and eastern parts of the northwestern China. The areal-averaged PDSI in south Xinjing is steading increasing while the PDSI in the eastern parts of the northwestern China is steading decreasing since 1951. The PDSI in north Xinjing, however, was decreasing before middle 1970s followed by abrupt increasing in recent three decades. The tree-ring reconstructed PDSI (MADA) replicates the three major moisture patterns, and has been used to search possible paleo-analogs during the past 700 years. The observed multi-year droughts in south Xinjing occurred 10 times since 1300AD. The spatial distribution of those droughts show extended dry conditions in northwestern China and stretched southeastward to cover the middle and lower Youngtze River Valley. The droughts associated with north Xinjing pattern are more local, mostly confined in north Xinjiang and neighboring regions. The droughts that affected the eastern parts of the northwestern China also affected majority of the eastern China. Overall, there are no noticeable changes in the drought intensity and duration in south and north Xinjing during the past 700 years. However, the modern droughts in the east parts of the northwestern China are much stronger than that during the historical time. The recent drought during 1995-2005 in this region was the most severe during the past 700 years.