Characteristics And Causes Of The South China Sea Monsoon From The Onset To Withdrawal Before And After 1993/94

Studying the changes and causes of the evolution of the South China Sea(SCS)summer monsoon(SCSSM)from onset to retreat in recent decades is critical for SCSSM extended-range prediction to improve the accuracy.The characteristics and relationship with eastern Asia rainfall of the South China Sea summer monsoon evolution from the onset and persistence to withdrawal before and after 1993/94 are investigated by using ERA–Interim,CPC rainfall,and OLR data.During late–onset period in 1979–1993,the SCSSM is characterized by a stronger onset intensity and a more gradual withdrawal,which result in a continuous strong pre–flood season in Southern China and slower rain belt retreat from north to south China in September.Rain belt in the Yangtze River basin persisted much longer in summer.During the monsoon persistence period,the low–level zonal winds appear with two–peak–pattern in summer.Most moisture are delivered to the Northwest Pacific Ocean.Therefore,the monsoon meridional circulation is southward,with greater intensity,and the downdraft anomaly southward led to the subtropical high ridge southward and water vapor converges in the middle and lower reaches of the Yangtze River,so the rain belt in the middle and lower reaches of the Yangtze River maintains for a long time.During early–onset period in 1994–2016,the SCSSM is associated with a weaker onset intensity and relatively faster retreat.The advanced pre–flood season lasted intermittently during May and until October rain–belt retreats rapidly,which makes rain belt in Southern China persisted for a longer time.During the monsoon persistence period,the zonal wind is abnormally weak,and more moisture is transported to South China.Therefore,the mon soon meridional circulation is northward and the intensity is weak,and the downdraft center is northward,which is beneficial for the subtropical high ridge northward,while the water vapor is mostly transported to South China,resulting in the rain belt staying for a long time in Southern China.Further analysis indicates that there are two active intraseasonal oscillations(ISO)over the SCS in summer during late–onset period but three in early–onset period.The wet ISO in the Northwest Pacific propagating northwestward into SCS and enhanced SCSSM ISO activity may contribute to the early onset and faster withdrawal after 1993/94.The warm western Pacific SST effect on the SCSSM evolution is also discussed.