Variation Characteristics of Autumn Precipitation in Shaanxi Province

Based on monthly rainfall data of 74 stations and NCEP/NCAR reanalysis dataset from 1960 to 2009, the variation characteristics of autumn rainfall over Shaanxi Province and the main factors influence on autumn rainfall are analyzed by the methods of Mann Kendall and REOF with a horizontal resolution of 2.5°×2.5°. It shows that the rainfall of September has decisive effect on the total autumn rainfall over Shaanxi Province. The September rainfall occupies 57.3% of the total autumn rainfall from 1960 to 2009, and from 2000 to 2009 it occupies 61.5%. The September rainfall can be divided into 2 types by spatial distribution: One is fewer in the north and more in the south, the other is more in the north and fewer in the south. The first type shows no obvious decadal variations, but the second type varies obviously in these decades as it occurs a lot from 1960 to 1985, less frequently from 1986 to 2000, and then more frequently again from 2001 to 2009. Statistics on the variation of atmospheric circulation and the second type rainfall distribution indicates that when subtropical high over western Pacific (SHWP) is more northwest (northeast and southwest) to its regular position, there are 12 years (25 years) that the September rainfall over the northern Shaanxi Province is more (less) in the past 50 years. When the zonal circulation index (ZCI) is weaker (stronger), there are 14 years (16 years) that the September rainfall over the northern Shaanxi Province is more (less) in the past 50 years. The correlations between the rainfall, the SHWP and ZCI are more obvious since the 1980s. Meanwhile the sea level pressure is low in the west and high in the east in the years when September rainfall over the northern Shaanxi Province is more, because there are more low weather systems from the northwest Xinjiang. So the east part of northwest China is controlled by the subtropical high over west Pacific and the southeast winds provides sufficient water vapor.