| Based on the daily data of97stations in Southwest China (SWC) form1960to2010, the dry-wet index which considers precipitation and temperature is calculated. The spatio-temporal variation characteristics of autumn, winter and spring dry-wet index is studied. Using similarity method, a composite similar index is introduced. According to the composite similar index, the distribution of dry-wet can be divided into7types:the consistent dry type (A-), the consistent wet type (A+), eastern wet western dry type (B-), eastern dry western wet type (B+), southern wet northern dry type (C-), southern dry northern wet type (C+) and atypical type (D). The atmospheric circulation characteristics and the factors of influence in anomalous years are discussed with NCEP/NCAR Reanalysis data. The results show that:1. The trend of drought of autumn appears in the late1980s and the trend is evident in the early21st century. Due to the rising temperature, the drought trend of winter appears in the mid1980s. From the early1970s to the mid1990s, there is a slightly wet tendency in spring and the drying trend appears in early21st century due to warming.There are three main modes of autumn and spring dry-wet variation in SWC:consistent across the whole region, a dipole in the latitudinal direction and a dipole in the longitudinal direction. Winter dry-wet variation only has the first two main modes.2. A type appears most (about50%); B type takes second place (about25%); C type appears less (about15%); D type occurs few times. From autumn to winter, A-type has a good persistent. That is to say, to a certain extent, autumn dry-wet condition can affect the winter dry-wet condition. From Oct. to Nov., eastern wet areas have a good chance to expand to the whole area in the next month.3. In the consistent dry anomalous years, the East Asia major trough is weaken or positioned by east and cannot go deep into the SWC. The southward invasion of cold air is difficult. The Western Pacific subtropical high is strengthened and moves westward. The South Asia high is enhanced and positioned by east. The Western Pacific subtropical high and South Asia high is overlapped. SWC is controlled by high pressure for a long time and the weather is sunny. There is an obvious anomalous cyclonic circulation over the South China Sea. SWC is located to the northwest of the anomalous cyclone circulation. The water vapor which delivers to the SWC is reduced. Such abnormal circulation situation maintains, making the region continued dry. The atmospheric circulation characteristics in the consistent wet anomalous years are basically opposite. The anomaly of eastern wet (dry) western dry (wet) type is correlated with the vertical movement anomalies in western/eastern SWC and the wind anomalies in eastern SWC. The autumn dry-wet condition can be affected by the anomalies of previous surface heat field in Tibet Plateau, the SST near East Indian Ocean and Western Pacific Warm Pool through the atmospheric circulation situation. |
