Studies On Climate Change With Aridity Or Wetness And Its Causes In Four Provinces Of Northwest China

The Spatial-Temporal variations of temperature and precipitation in Northwestern China during the period of1961-2008are studied, using the monthly and annual precipitation data of113surface stations. Variation of aridity-wetness in four provinces of Northwest China is studied according to De Martonne index. The time of aridity index is divided into strong or weak period, and the circulation background is analyzed combining with high, humidity, wind of the NCEP/NCAR reanalysis data. The five circulation indexes are selected and partial correlation of five indexes and aridity index is found, and then the possible causes of the change are analyzed. The main conclusions are as follows:1. The precipitation and temperature in northwest China exists evident inter-decadal and inter-annual variations. Average temperature presents a rising trend, warming significantly, and the annual precipitation has declined. Temperatures in spring, summer, autumn and winter are increased, especially in winter, while precipitation in spring and autumn has reduced, and it increases slightly in summer and winter.2. The time of northwest can be divided into wet (1960s)-drought (1970s)-wet (1980s)-drought (since1991). The distribution of aridity index decreases from the south to the north. Partial dry area is located in the Qaidam basin, north of Gansu province, as well as in Ningxia north of the Yellow River. Partial wet area is located in the south of Qilian mountain area, south of Shaanxi and Qinghai mainly. Partial dry area is located in the Qaidam basin, north of Gansu province, as well as in Ningxia north of the Yellow River; Partial wet area in Qinghai located mainly in the south Qilian mountain area, south Shaanxi; Other areas in partial dry and partial wet between. There are3years oscillation period in the whole area. There are also four to five years oscillation period in south of Shaanxi, Gansu and Ningxia and west of Gansu, and8years oscillation period only exists in central and southern of Shaanxi, Gansu and Ningxia.3. Atmospheric circulation of wet and dry period is abnormal. From wet period to dry period, in the lower troposphere, height in most of the Eurasia areas changes from negative to positive, the positive center in Mongolian replaces negative center between Barr Lake and Baikal kasha. The lower troposphere southerly anomaly to the northerly wind. Upper tropospheric jet axis changes from45°N to30°N. The anticyclonic divergence circulation is formed easily in the transition zone between positive and negative, and it is easy to form precipitation in the troposphere and low level convergence flow. There is no obvious abnormality from the ground to the300hPa water vapor flux divergence. Divergence (convergence) in high flow is in favor of convergence(divergence) in the lower layer, abnormal divergence (convergence) in high level effects of precipitation, resulting in northwest drought wet change.4. There is significant partial correlation between AO, PNA, NAO and aridity index in the inter-decadal time scale, and significant partial correlation also exists between Nino3.4and aridity index in the inter-annual time scale, and weaker relationships between WP and aridity index. The Arctic Oscillation influences most of northwest area in the inter-decadal time scale. The relationship of NAO and aridity-wetness change is mainly reflected in t he inter-decadal time scale, especially in South of the northwest region and south of Qilian Mountains. Opposite phase relationships between PNA, WP and aridity index exists in the inter-decadal time scale, and no significant effect in the inter-annual time scale, while positive phase relations between ENSO and aridity index exists in the inter-decadal time scale, and no significant effect in the inter-annual time scale.