The characteristics of droughts and floods in China during the summers(May–August)of 2016 and 1998 were compared in great detail,together with the associated atmospheric circulations and external-forcing factors.Following results are obtained.(1)The precipitation was mostly above normal in China in summer 2016,with two main rainfall belts located in the Yangtze River valley(YRV)and North China.Compared with 1998,a similar rainfall belt was located over the YRV,with precipitation 100%and more above normal.However,the seasonal processes of Meiyu were different.A typical"Secondary Meiyu"occurred in 1998,whereas dry conditions dominated the YRV in2016.(2)During May–July 2016,the Ural high was weaker than normal,but it was stronger than normal in 1998.This difference resulted from fairly different distributions of sea surface temperature anomalies(SSTAs)over the North Atlantic Ocean during the preceding winter and spring of the two years.(3)Nonetheless,tropical and subtropical circulation systems were much more similar in May–July of 2016 and 1998.The circulation systems in both years were characterized by a stronger than normal and more westward-extending western Pacific subtropical high(WPSH),a weaker than normal East Asian summer monsoon(EASM),and anomalous convergence of moisture flux in the mid and lower reaches of the YRV.These similar circulation anomalies were attributed to the similar tropical SSTA pattern in the preceding seasons,i.e.,the super El Ni?o and strong warming in the tropical Indian Ocean.(4)Significant differences in the circulation pattern were observed in August between the two years.The WPSH broke up in August 2016,with its western part being combined with the continental high and persistently dominating eastern China.The EASM suddenly became stronger,and dry conditions prevailed in the YRV.On the contrary,the EASM was weaker in August 1998 and the"Secondary Meiyu"took place in the YRV.The Madden–Julian Oscillation(MJO)was extremely active in August 2016 and stayed in western Pacific f
A heavy rainstorm named Beijing "7.21"heavy rainstorm hit Beijing on 21 to 22 July 2012, which is recorded as the most severe rainstorm since 1951. The daily precipitation amount in many stations in Beijing has broken the history record. Based on the NCAR/NCEP reanalysis data and precipitation observation,the large-scale conditions which caused the "7.21"heavy rainstorm are investigated, with the emphasis on the relationship between it and an equatorial convergence zone, Asian summer monsoon as well as the tropical cyclone over the ocean from the Philippines to the South China Sea(SCS). The results indicated that a great deal of southerly warm and wet moisture carried by northward migrating Asian summer monsoon provided plenty of moisture supplying for the "7.21"heavy rainstorm. When the warm and wet moisture met with the strong cold temperature advection induced by cold troughs or vortexes, an obviously unstable stratification formed, thus leading to the occurrence of heavy precipitation. Without this kind of intense moisture transport, the rainstorm only relying on the role of the cold air from mid-and higher-latitudes could not reach the record-breaking intensity. Further research suggested that the northward movement of an Asian monsoonal warm and wet moisture transport conveyor(MWWTC) was closely related with the active phase of a 30-60 day intra-seasonal oscillation of the Asian summer monsoon. During this time, the monsoon surge triggered and maintained the northward movement of the MWWTC. In addition, compared with another heavy rainstorm named"63.8"heavy rainstorm, which occurred over the Huaihe River Basin in the mid-August 1963 and seriously affected North China, a similar MWWTC was also observed. It was just the intense interaction of the MWWTC with strong cold air from the north that caused this severe rainstorm.
