A new analog error correction(AEC) scheme based on the moving North Pacific index(MNPI) is designed in this study. This scheme shows obvious improvement in the prediction skill of the operational coupled general circulation model(CGCM) of the National Climate Center of China for the rainy season rainfall(RSR) anomaly pattern correlation coefficient(ACC) over the mid-to-lower reaches of the Yangtze River(MLRYR). A comparative analysis indicates that the effectiveness of the new scheme using the MNPI is better than the system error correction scheme using the North Pacific index(NPI). A Euclidean distanceweighted mean rather than a traditional arithmetic mean, is applied to the integration of the analog year's prediction error fields. By using the MNPI AEC scheme, independent sample hindcasts of RSR during the period 2003–2009 are then evaluated. The results show that the new scheme exhibited a higher forecast skill during 2003–2009, with an average ACC of 0.47; while the ACC for the NPI case was only 0.19. Furthermore,the forecast skill of the RSR over the MLRYR is examined. In the MNPI case, empirical orthogonal function(EOF) was used in the degree compression of the prediction error fields from the CGCM, whereas the AEC scheme was applied only to its first several EOF components for which the accumulative explained variance accounted for 80% of the total variance. This further improved the ACC of the independent sample hindcasts to 0.55 during the 7-yr period.
The two northward jumps of summer West Pacific Subtropical High(WPSH) are defined based on the pentad-scale ridge data of the WPSH ridge in 1951 to 2012. The times of the northward jumps are found to have obvious inter-annual and decadal characteristics, i.e., the occurrence of the first northward jump of WPSH shows a "consistently early–consistently late" decadal pattern, with the transition around 1980; the occurrence of the second northward jump of WPSH shows a"consistently late–consistently early–consistently late" decadal pattern, with the transitions about 1955 and 1978, respectively, which is consistent with global warming. In the meantime, the times of the two northward jumps not only have a good correspondence to the beginning and ending dates of the rainy season, but also greatly influence the position of the main rain belt in Eastern China. When the first northward jump occurs early, the main rain belt is located from just north of30?N to the south of North China, while the opposite situation appears when the first jump occurs late. When the second jump occurs early, more rain falls over North China and South China, but less falls in the Yangtze River region, while the opposite situation appears when the second jump occurs late. In the four cases when abnormalities occur in the same year as early or late northward jumps, the position of the main rain belt can be considered as a superposition of isolated abnormal effects of the two northward jumps. Moreover, the prophase and synchronous forces of the sea surface temperature in the Pacific has great influence on the times of the northward jumps, and the driving forces of the two jumps differ.
