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The middle and lower reaches of the Yangtze River basin (MLRY) suffered a once-in-a-50-year drought during the spring of 2011. The abnormality of the atmospheric circulation in the spring is characterized by the deeper-than-normal trough over East Asia and the northwest Pacific while stronger-than-normal high-pressure system over the west, which results in a stronger-than-normal meridional circulation. Meanwhile, the western Pacific subtropical high is weaker than normal and retreats to the east, so the spring monsoonal moist air from lower latitudes is relatively weak. The anomalous northerly wind in higher latitudes suppresses the northward motion of the moist air and brings dry air to the MLRY. The northerly wind also suppresses the northward motion of warm air and brings cold air to the MLRY. So finally, the air over the MLRY is drier, but colder than normal, which is different from many other droughts. The coldness of the air plays a negative role to the drought. Results from comparing the C q and C T , the measures of the changes in moisture and temperature between this spring and the normal spring defined based on the tight precipitation-relative humidity relation, indicate that the dryness of the air is much stronger than the coldness, and the air over the MLRY thus maintains a lower-than-normal relative humidity. The animation of daily C q and C T shows that dry and cold air moves from the northwest to the MLRY in a manner of low-frequency oscillation.

Using observed precipitation and the National Centers for Environmental Prediction-National Center for Atmospheric Research reanalysis, the changes in the metrics of the summer precipitation in China, the dominance of frequency and intensity of daily extreme precipitation, and the linkage with changes in moisture and air temperature are explored. Results show that over the recent 50 years, the total summer rainfall increased over the southeast and the west and decreased over the northeast. The changes in the frequency, identified with the 95% threshold and Poisson regression, and rainfall extremes show similar spatial patterns. The relative importance of the changes in frequency and intensity in the variability and changes in extreme precipitation are estimated. It is shown that, while the interannual variability of the rainfall amount is dominated by the frequency change in almost all stations, the long-term change of rainfall amount can be dominated by both frequency and intensity, depending on the station. The change in the rainfall total is linked to changes in atmospheric moisture and temperature. The results show that the variability and change of the rainfall total can be dominated by changes in both moisture and air temperature, and the relative importance depends on the region.

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