Monsoon circulation background of ‘0506' continuous rainstorm in South China

The indexes are advanced to decide the summer monsoon extent of East Asia and identify the activie region of the tropical and subtropical summer monsoon.By using the NCEP/NCAR troposphere data of wind,geopotential height,humidity,temperature,the satellite-derived OLR and TBB,the activity and variability characteristics of the monsoon circulation during ‘0506' continuous rainstorm are diagnosed from monthly,pentad and the process time scales.The results indicate that if the subtropical anticyclone is stronger,the west-point of subtropical anticyclone ridge is more farther westward and southward than the climatological average,and the cross-equatorial stream in tropical western Pacific(130-140°E) is stronger.South China is located in a region of the anomalous cyclonic depression.These characteristics are exhibited in either monthly or rainstorm process time scale.Besides the Bay of Bengal and South China sea,the water vapor of the rainstorm process is mainly from the abnormal water vapor flux over the South China sea and the tropical western Pacific.The water vapor in the tropical western Pacific transports to South China following the stream at the edge of the subtropical anticyclone via the South China sea.Consequently,plentiful water vapor is provided during the rainstorm process.The front part of tropical summer monsoon stagnates for two pentads longer than the climatological average.This characteristic is a valuable signal for forecasting South China rainstorms.The mean monsoon strength over the South China sea in J une is stronger,which is showed by the stronger meridional wind.But the rainstorm process begins in the period of inactive summer monsoon over the South China sea,which is related to the northward spread of the tropical intraseasonal oscillation.The activity and variability characteristics of monsoon mentioned above act as favorable dynamic condition and the abundant water vapor sources.