Abstract:In summer 2015, the mean precipitation over China was 297.6 mm, which is less than normal by 8.5%. The spatial distribution of precipitation anomalies showed the north less and south more pattern, and precipitation over the middle and lower reaches of Yangtze River and the Yangtze Huaihe River Valley was significantly more than normal. The Meiyu rainy season lasted longer with more rainfall than normal. Further study indicated that positive Indian Ocean basin wide mode (IOBW) has been enhanced since May 2015, triggering a local zonal circulation with upward flow in the west and downward motion in the east. This circulation is favorable for the establishment of a strong and westward Northwest Pacific subtropical high, resulting in high frequency of strong southwest low level jet in the southeast of China. The low level jet transported water vapor from the South China Sea to the Yangze Huai River Valley and released unstable energy, and such combined condition is useful for the development of convection and the occurrence of precipitation, extending the Meiyu season and increasing the amount of rainfall.