Decadal Variation Characteristics of South China Pre-flood Season Persistent Rainstorm and Its Mechanism

Based on the daily precipitation data of 243 stations in Fujian, Guangdong, Guangxi during 1961-2012, indices (intensity of daily rainstorm I_RD, intensity of persistent rainstorm I_RS, intensity of South China pre-flood season persistent rainstorm I_RZ) are calculated and persistent rainstorm processes are selected. Indices reveal decadal variation characteristics of pre-flood season persistent rainstorm by kinds of mutation test and percentile method. It indicates that the decadal variation of pre-flood season persistent rainstorm can be divided into three sections, multiple (1961-1972)-fewer (1973-1991)-multiple (1992-2012). The present stage is in multiple sections with stronger intensity and longer duration; pre-flood season precipitation exhibits a significant low-frequency oscillation characteristic. The decadal variation of duration and intensity of persistent rainstorm during pre-flood season are closely related to that of configuration and intensity of low-frequency periodic signal. There is a positive correlation between the intensity of pre-flood season persistent rainstorm and the low-frequency intensity of 10-60 days with the intensity strongest in 10-20 days, the second in 30-60 days' and the third in 20-30 days'. The low-frequency signal of Guangdong and Fujian is more obvious than Guangxi in all sections. Frequent, strong and long persistent rainstorm is more likely to occur in multiple sections as the precipitation intensity of low-frequency signal oscillating acutely, and vice versa. The decadal meridional propagation features of tropical intraseasonal oscillation and mechanism and its possible impact on the decadal variation of pre-flood season persistent rainstorm are also studied by using East Asia-western NorthPacific ISO index (EAWNP ISO). The position of the meridional propagation of the East Asia-Pacific Northwest Pacific with different phases can be seen according to different phases' reconstruction of 850 hPa wind speed and OLR anomaly field. The decadal variation of tropical low-frequency northward movement signal cycle and intensity may probably be one of the main causes that lead to the decadal variation of pre-flood season persistent rainstorm. Since tropical low-frequency signal has various cycles, it may cause low-frequency variation of circulation system advantage to the precipitation of pre-flood season, which corresponds to the low-frequency variation characteristics of precipitation there and leading to changes of the intensity and duration of persistent rainstorm when low-frequency convection propagates northward to southeast coast of China. The longer (shorter) cycle and stronger (weaker) intensity of tropical low-frequency signal northward to southeast coast of China is, the longer (shorter) duration and stronger (weaker) intensity of pre-flood season persistent rainstorm may be.