Analysis of Autumn Rainfall Characteristics and Its Causes in West China in 2011

Since the autumn of 2011, there has been obvious precipitation in northern areas of West China, Huang-Huai and southern areas of North China, which is characterized by high local intensity, long duration and serious disasters. The formation of this precipitation is mainly due to the SST of the middle and east Pacific Ocean abnormally lower, which was effected by the La Niña events and results in anomaly cyclone and anticyclone gyre circulation pattern in the Northwest Pacific and offshore region. Under such conditions, southwest airflow which moves northward along the Bay of Bengal, keeps strong and joins southeast airflow from the Western Pacific, thus warm and moist air from the ocean is continuously transported northwards. The eastern water vapor transport from the western Pacific is significantly obvious and mainly contributes to the significantly more pricipitation. In the meantine, abnormal trough maintains to the East of Lake Baikal, which guides the cold air frequently to the north of West China and its northern areas. The main subtropical high, located abnormally north and moving little, results in the cold and warm air converge and maintains from the northern part of the West China to the Huang-Huai area, causing the continuous precipitation.