Interdecadal change in the seasonality of rainfall variation in South China
The interdecadal change in seasonality of rainfall around 1992/1993 in South China and its possible reasons were investigated, using the station data and NCEP/NCAR reanalysis. The bimodality pattern in rainfall is a salient climatological feature in South China. The present analysis reveals that the typical bimodality feature becomes unconspicuous with postponement and enhancement in the first rainfall peak after 1992/1993. The first rainfall peak occurred in pentad 30 (P30) before 1992/1993, while the rainfall peak postponed to P32 ~ P33 after 1992/1993. This postponement leads to the increase in precipitation in summer, and the decrease in spring. The changes in heavy and torrential rain amount have major contribution to the change in the total rainfall.
The postponement of first rainfall peak is associated with the change in South China Sea summer monsoon (SCSSM) onset and subsequent northward propagation. Before 1992/1993, the SCSSM onset occurs over the central South China Sea in P30 (26–30 May), and the southwesterly wind over South China is intensified concurrently. During the second epoch, the onset occurs in P27/28, and the maximum southwesterly wind regime stagnates for two pentads, then extends northward gradually to South China from P30 to P33, corresponding to the rainfall peak occurrence in P33. The interdecadal climatological intraseasonal oscillation (CISO) shift is superimposed on the seasonal evolution of monsoon from spring to summer. The first CISO wet phase propagation manifests the advance in SCSSM onset and coincides with the postponement of first rainfall peak. During the period when rainfall is enhanced, there is a significant decrease in the Asian subtropical jet intensity, and the westerly jet over East Asia displaces southward. Additionally, the baroclinicity in lower atmosphere before P32 is weaker during the second epoch, which could give rise to less front activity, and lead to the decrease of precipitation in spring.
The authors are grateful for the insightful comments and suggestions of the reviewer and editor, which improved the manuscript greatly. This work was jointly supported by the National Basic Research Program of China (973 Program, grant 2012CB955901), the National Natural Science Foundation of China (grant 41130963), and the Jiangsu Collaborative Innovation Center for Climate Change.
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