The fourth-generation atmospheric general circulation model, developed by the Institute of Atmospheric Physics, Chinese Academy of Sciences, was used to simulate decadal variations in the East Asian summer monsoon (EASM) and in summer rainfall in eastern China during the late 1970s. It was shown that the model simulated well the decadal weakening of the EASM, a northerly anomaly along the East Asian coast, and changes in the western Pacific subtropical high. The model also reproduced the anomalous summer rainfall pattern in eastern China (i.e., an increase in the Yangtze River basin and a decrease in North China and South China), except that the modeled pattern was located somewhat south of the observed pattern. Further analysis, using singular value decomposition, indicated that the decadal weakening in the EASM was mainly driven by warming in the tropical ocean, which is related to the Pacific decadal oscillation (PDO) phase transition in the late 1970s. The model also simulated the cooling trend in the Yangtze River basin, which led to a decrease in the thermal contrast between the continent and the ocean, and a weakened EASM.