| The frequency and intensity of global drought had a increasing tendency under the climatic changing. China had always been one of the worst-hit countries, drought occurred more frequently in recent years, and drought area had a tendency to increase constantly, at the same time, the drought started to develop from arid and semi-arid regions to humid regions. Southwest of China, which had sumptuous rainfalls and humid climate in the history, repeatedly occurred severe drought disasters in recently years. Especially a once-in-60yeas extremely severe drought from autumn of2009to spring of2010took place in southwest and caused unquantifiable losses, and it had attracted great attention of more and more meteorologists at home and abroad. In this paper, firstly, the latest advances of drought study in southwest China were summarized. Then the drought intensity’s and range’s spatial-temporal distribution characteristics in southwest from1960to2012were analyzed based on the SPI drought index by means of objective statistics; period signature of southwestern drought was discussed and analyzed by means of the wavelet transform. Based on the analyses of spatial-temporal characteristics of drought, by picking rainy year and drought year, atmospheric circulation exceptional characteristics of different precipitation years were comparatively analyzed and the direct atmospheric circulation anomaly characteristics which caused continuous drought from2009to2012,especially the extreme drought event from2009to2010were revealed.According to our research, southwestern precipitation showed a type of "southwest-northeast" in space, which decreased from southwest to northeast; and the precipitation of most part of southwest tended to decrease in time and decreased most obviously after2000especially after2009; the precipitation decreased apparently both in summer monsoon season and in winter monsoon season in recent10years,Based on the SPI drought index,the drought of most part of southwest had a increasing tendency and was relatively concentrated in the middle of southwest, and the drought center position was in the east part of Yunnan from2009to2012, after entering the21st century, southwestern drought range and intensity both reached maximum since1960, especially the autumn-winter-spring continuous drought from2009to2010was the most severe since record began. Southwestern drought had a certain periodicity and mainly existed25-year,15-year and8-year periods. Complicated atmospheric circulation anomaly caused the precipitation anomaly of southwest, then the drought appeared. Based on the analysis on the main atmospheric circulation systems which directly influenced southwestern drought we could find that while west pacific subtropical high’s area was bigger and intensity was higher, with lying to the north or west of the normal position, southwest was under the control of abnormal down draft, the northern air significantly strengthened, and moisture transport decreased, and moisture content in middle-lower troposphere was abnormally less, which made precipitation become abnormally less and drought occur. The abnormal characteristics were very obvious during the continuous drought from2009to2012, especially the extreme drought event from2009to2010. |
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