Summer Dry/wet Variations In Northwestern China In The Last 700 Years And Their Mechanisms

The northwestern China, far away from the ocean and lack of water resources, is one of the most arid regions in the Northern Hemisphere mid-latitudes. This dissertation investigated the spatial and temporal variation of summer moisture, the relationship between the variation and general circulation, and the primary factors and mechanism impact on the variation in this region using Palmer Drought Severity Index (PDSI) derived from instrumental data and tree-ring records. Conclusions are as follows:1. Based on rotational Empirical Orthogonal Function (REOF) analysis from 1951 to 2005, the observed summer moisture variation in northwestern China can be classified into three major patterns:the south Xinjiang, north Xinjiang and eastern parts of the northwestern China. The area-averaged PDSI shows strong spatial variation of summer moisture. The area-averaged PDSI in south Xinjiang is steadying increasing while the PDSI in the eastern parts of the northwestern China is steadying decreasing since 1951. The area-averaged PDSI in north Xinjiang 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. There are no noticeable changes in the drought intensity and duration in south and north Xinjiang during the past 700 years. However, the modern droughts in the eastern 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. The spatial distribution of observed multi-year droughts in south Xinjiang shows northwest-southeast oriented transect with extended dry conditions in northwestern China and stretched southeastward to cover the middle and lower Yangtze River Valley. The droughts associated with north Xinjiang 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.2. The variation of summer moisture in northwestern China is significantly influenced by the general circulation. The moisture in the Indian Ocean is conductively transported to south Xinjiang when the geopotential height field of Indian Ocean-Central Asia-Western Siberia is in the positive-negative-positive phase. The westerlies conductively transport the moisture to north Xinjiang when the geopotential height field of Southern Europe-Central Asia is in the positive-negative phase. The mid-latitudes quasi-stationary wave benefits the moisture of the eastern parts of the northwestern China when Southern Europe - Central Asia - Northern China - Southeastern China is in the positive-negative-positive-negative phase. From the perspective of moisture transport, the moisture impact on south Xinjiang is mainly from the Indian Ocean while the variation in north Xinjiang is influenced by the moisture predominantly transported from the Caspian Sea, Aral Sea and Lake Balkhash. The moisture in the eastern parts of the northwestern China is mainly from the Bay of Bengal, South China Sea, and Eastern Pacific.3. The primary factors impact on the moisture variations in northwestern China are the Indian Ocean SST (IOSST) and North Atlantic Oscillation (NAO). The former affects the zonal circulation and summer monsoon, while the latter affects the mid-latitudes quasi-stationary wave in the Earth system. The moisture in the eastern parts of the northwestern China decreases (increases) while it increases (decreases) in the western parts of the northwestern China, when IOSST increases (decreases). The moisture in the eastern parts of the northwestern China increases (decreases) while it decreases in north Xinjiang, when NAO raises (decreases).