Drying Trend Over China And The Possible Causes Of Extreme Drought In Southwest China

The drought, which caused great loss in China, was one of the worst natural disasters. Recently, extremely arid events occurred frequently, especially for nearly three years in Southwest China, which influenced the life and production of people seriously. However, the current drought events were mostly based on some historical statistics, lack of objectivity, and we mainly used relatively single index to make analysis of the drought characteristics, especially the reason research of the winter and spring extreme drought in the Southwest China was few. Using CI and the objective statistics, this study has given drought events in typical arid areas in recent nearly60years, and analyzed the drying trend characteristics in China and Southwest using temperature, precipitation, CI and PDSI, and discussed the reasons and atmospheric circulation anomalies of the autumn winter and spring drought events happened in Southwest China in2009/2010, and through selecting typical dry and wet years of winter and spring drought in Southwest China, systematically analyzed the characteristics and contributing factors, and revealed the key factor of the winter and spring extreme drought.The results showed that, there was a overall consistent warming and drying trend in China, and the drought lasting days and coverage areas were obviously increased, especially, the drying trend of the Sichuan-Chongqing area was significant. For the four typical arid areas, the extreme drought events were most in Southwest China, where drought mainly occurred in winter and spring, showed heating, less rainfall and drying trend. The autumn winter and spring drought was the most serious in Southwest China in2009/2010. The characteristics of atmospheric circulation anomalies were as follows, firstly, the intensity, area and ridge of the Western Pacific Subtropical High (WPSH) was stronger, greater and more wester than usual; secondly, the convection was weaker; thirdly, the airflow was divergence; finally, the moisture flux was convergent at upper levels and divergent at lower levels. The drought was mainly affected by the high SSTA in Bengal, which caused the abnormal sinking airflow in Southwest China. The characteristics of atmospheric circulation anomalies of winter and spring drought in Southwest China also showd stronger, greater and more wester of WPSH, weaker convection, and the reduced zonal moistures transport, which caused the overall moisture loss. The SSTA of Arabian sea, Bengal and the northwest Pacific were the key factors of the winter and spring drought in Southwest China. When drought occurred, the SSTA of the three regions was higher than usual, causing the abnormal anti-cyclone, influencing the drought in Southwest China by zonal vertical circulation. Since the1990s, the SSTA of the three key regions was consistent higher than usual, resulting significant drying trend in Southwest China, owing to lack of effective water vapor transportation.