| Based on the daily precipitation of 58 observational stations in South China and the daily NCAR/NCEP reanalysis dataset, the features of different periods during the pre-flood season in South China and the differences of moisture transport and atmospheric circulation between drought and flood years are compared. At the mean time, the relationship between precipitation of the pre-flood season in South China and South China Sea summer monsoon is investigated. The main results are as follows:(1)The pre-flood season in South China is composed of the frontal precipitation period and the summer monsoon precipitation period. The former mainly appears in April accompanied with typical transient circulation pattern from winter to summer, strong subtropical westerly jet streams prevailing in the high level and steady stratified atmosphere while the moisture is mainly originated from the westerly transport over Arabian Sea and the easterly transport at the south side of WPSH; Before the onset of SCSSM, subtropical high prevails over SCS , associated with steady stratified atmosphere and the first jump of the westerly jet streams; After the onset of SCSSM, the subtropical high withdraws outside SCS, accompanied with convection developing within South China; The peak period of summer monsoon precipitation is in June when South Asia High prevails over Tibetan Plateau and South China is at the east side of South Asia High and the right side of the high-level westerly jet streams, associated with unsteady stratified atmosphere, remarkable convection developing and powerful cross-equatorial moisture transport toward South China from the southern hemisphere, through BOB and SCS.(2) In the mid-upper layer of troposphere, the high meridional temperature gradients lies in the southern latitude while the low in the northern part during the frontal precipitation period . During the summer monsoon precipitation period, vice verse. The onset of South China Sea summer monsoon (the fourth pentad of May) is consistent with the reversion of meridional temperature gradients at its longitude.(3)The most obvious feature of the first leading term of EOF applied to precipitation during the pre-flood season in South China shows that there is a good consistency for the variations of total precipitation during the pre-flood season in South China. Generally speaking, the precipitation have obvious interannual changes with quasi-3years-,5years-,7years-, 9years- periods. By decomposing the vertically integrated water vapor transport vector into the sum of its nondivergent (rotational) and divergent (irrotational) components in terms of the stream function and potential, the water vapor background of the precipitation of the pre-flood season in South China during the flood or drought years is investigated, it is indicated that the main differences of water vapor between the flood and drought years is the easterly transport at the south side of WPSH. The precipitation anomaly in South China causes seemingly by the anomalous water vapor transport from SCS to South China, in the final analysis it results from the anomalous water vapor transport of the easterlies at the south side of WPSH to SCS, however. The cold Continental Anticyclone, the East Asian Trough and the intensity of WPSH at 500hPa in the flood years is stronger than those in the drought years.(4) The result s showed that the onset date of summer monsoon is later (or earlier) when the amount precipitation and it's latent heat flux of the pre-flood season in South China becomes more (or less) . The relationship between the rainfall anomaly of the pre-flood season in South China and the intensity of summer monsoon is not significant, however , intensity of summer monsoon can be considered as a prediction factor to the next year precipitation. In the years of strong (weak) summer monsoon in South China Sea, whereas rainfall is flood (normal and drought) during the following pre-flood season in South China. |
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