April-May persistent drought events in South China and the related general circulation background

The relationships of interannual variations of the monthly precipitation in springy in South China (SC) and the corresponding pattern of the general circulation to the persistent droughts in April to May (AM)in SC arc investigated by using the station-observed rainfall data and the NCEP-DOE rcanalysis II data. The linkage between the persistent AM droughts in South China and the preceding winter and spring sea surface temperatures(SSTs) and the possible echanism of impact of tropical SSTs on the persistent droughts in SC arc also discussed. The region-wide inphase pattern is the dominant anomalous mode of spring rainfall in SC. There exists a decreasing trend of the spring rainfall in SC in the recent thirty years.The year to year variations of rainfall in April and May arc highly synchronous.The spring drought events mainly result from the notable decrease of rainfall in April and May in SC. In the persistent AM drought cases in SC, there exist anomalous anti-cyclones at the lower and upper levels near SC, and these anticyclones result in anomalous descending motions over SC. Furthermore.the southern part of East Asian trough weakens and is not in favour of the southward invasion of cold air and rainfall in SC. The positive sea surface temperature anomalies (SSTA)in the central equatorial Pacific associated with the persistent AM drought events in SC may appear from the preceding winter till spring. The possible mechanism for the spring drought in SC is as follows:the positivc SSTA in the central equatorial Pacific forces anomalous ascending motion over there and forms cyclonic circulation anomalies at the low-level over the two sides of the equator in the western tropical Pacific due to the Rossby-wave response process,accompanied with the anomalous ascending motion over the western North Pacific cast of the Philippines forced.and then causes anomalous descending motion over SC.resulting in drought there. On the other hand.the northerly anomalies at the lowlevel over the northern South China Sea and SC might weaken the water vapor transport to SC from the South China Sca and the western tropical Pacific.which is in favour of the formation of drought in SC. In contrast to tropical circulation anomalies due to the SSTA in the tropical Pacific.the circulation anomalies over the mid-and high-latitudes of the Eurasian continent show an apparent wave pattern oriented from northwest to southeast in years of persistent drought from April to May, which makes a remarkable impact on the persistent AM droughts in SC.