The flood disasters caused a great loss of life and property in Yangtze River Basin during June-July of 2016. Meiyu became one of the main causes of flood disasters due to its high frequency, long duration and high intensity. In this study, chronology of extreme Meiyu events since 1736 was derived from Chinese historical documents and the observational data. Furthermore, and the long-term variation characteristics of extreme Meiyu events and relationships between extreme Meiyu and El Niño events were analyzed. The general circulation patterns were diagnosed to explain how El Niño influenced extreme Meiyu. The results show that there were 44 extreme Meiyu events (including 21 mega-Meiyu events) from 1736 to 2016. The most frequent occurrences of extreme Meiyu events were detected during 1901-1920 and 1991-2000. There were 21 mega-Meiyu events over the past 300 years, whose intensities were similar to that in 2016. Closely corresponding relationships was found between extreme Meiyu events and El Niño in this period. For example, 37 of total 44 extreme Meiyu events occurred in the El Niño episodes or their following years, and 16 of total 21 mega-Meiyu events occurred in the El Niño episodes. In the El Niño years, the meridional circulations were intensified over the mid-latitudes. The water vapor transported westerly and northerly to the mainland of China. A great amount of water vapor from south intersected with the cold air from high latitudes in the mid-lower reaches of the Yangtze River, which led to continuous precipitation. Moreover, in the El Niño following years, there was a steady subtropical high system at low latitudes. The transportation path of vapor was northerly, and then the vapor fluxes increased in Yangtze River Basin. It tends to cause extreme Meiyu events under this circulation background.

Based on the simulation of 1 000-year control experiment by the Community Earth System Model (CESM) with constant pre-industrial external forcing, 212 El Niño events and 226 La Niña events were identified according to the monthly SST (Sea Surface Temperature) of Nino3.4 region. Furthermore, the spatial pattern of May to September precipitation in eastern China in the El Niño (La Niña) episodes and their following years were illustrated. The relationship between the ENSO episodes and the floods/droughts of four regions including North China, Jianghuai, Jiangnan and South China in eastern China were analyzed. The results show that during the El Niño episodes, precipitation decreases by 2%-10% (with respect to the mean value of the whole 1 000 years) in North China and South China from May to September, and precipitation slightly increases by 0-2% in the Yangtze River Basin. In the following year of El Niño, precipitation in Jiangnan region turns to increase by 2%-10%, but precipitation in North China is still in decreasing conditions. During the La Niña episodes, precipitation in North China increases by 2%-10%. In the following year of La Niña, precipitation in Jianghuai decreases obviously by 2%-5%. The stronger El Niño (La Niña) events lead to more intense precipitation changing, and the percentages of precipitation anomalies increase. In the year when El Niño hasn’t vanished and La Niña starts to develop, the probability of flooding in the Jiangnan region might be more than twice as that in normal years. This finding provides background of abnormal climate for understanding the 2016 extreme flood in Yangtze River, and thoroughly reveals the influence of inter-annual internal variability in climate system on the changes of precipitation’s spatial patterns and regional flood/drought.

The differences of oceanic and atmospheric circulation anomalies between 2015 and 2016 are analyzed and their impacts on summer precipitation in China are investigated in this paper by using the monthly precipitation data of 160 stations in China and NCEP/NCAR monthly data. The results show that the distributions of summer precipitation anomalies are obviously different in 2015 and 2016. The summer precipitation anomalies are less over North China, but more over South China in 2015. Compared with 2015, the summer precipitations in 2016 increase obviously, especially in northern China, forming a longitudinal rain belt. The abnormal sea surface temperature (SST) in 2015 is corresponding to the development year of El Niño, warm phase of basin-wide SSTA variation in the tropical Indian Ocean (IOBW) and positive phase of Tropical Indian Ocean Dipole (TIOD), while the SST in 2016 shows the ending year of El Niño, warm phase of IOBW and negative phase of TIOD. In 2015, the western Pacific subtropical high is stronger and more southward than normal under the effect of SST anomalies in the tropical Indian-Pacific Ocean. It combined with active cold airs and southward westerly jet axis and thus resulted in the less precipitation over northern China and more precipitation over southern China. In 2016, El Niño decay and negative TIOD led the subtropical high northward, and the battery charge caused by warm IOBW led subtropical high stronger. The northward and stronger subtropical high met with weak cold air and northward westerly jet axis, which caused more precipitation in the middle and lower reaches of Yangtze River and the region north to it.