Influence Of The Cross-equatorial Flows And Australian High On South China Sea Summer Monsoon Onset

The seasonal and interannual relationship of the onset of SCS summer monsoon (SCSSM),cross-equatorial flows(CEFs) in the Eastern Hemisphere and Australian High(AH) are investigated by using the NCEP/NCAR reanalysis data and the monthly mean sea-surface temperature data from UK/GISST of Hadley Center of England.The main results are:(1)In the lower and upper level of the troposphere in the Eastern Hemisphere,the CEFs centers are located in 925hPa and in 150hPa,respectively.The Somali CEF,CEF in 85°E and the South China Sea play crucial roles in the onset of SCSSM.The key period of time that influence the onset of SCSSM is from the 25th to the 29th pentad.The SCSSM onset tends to occur earlier when the three CEIls in the lower level are stronger between the 25th to the 29th pentad,and vice versa.Besides,if the CEF in the SCS region is earlier,the onset of SCSSM is earlier as well, while the other two CEFs are not affected much.Different CEFs act different roles in the onset of SCSSM.(2)The meridional circulation pattern has already changed from summer to winter right before the onset of SCSSM.The pattern change of meridional circulation plays a crucial role in the onset of SCSSM as well.The SCSSM onset tends to occur earlier when the meridional circulation changes earlier,and vice versa. The pattern change of meridional circulation is always before the onset of SCSSM,which indicates that change of meridional circulation has some ability to reflect and forecast the onset of SCSSM.(3)An index is developed to reflect the establishment and strength of AH, AH establishes at the 18th pentad averagely.The CEFs in 85°E and the South China Sea between the 25th to 29th pentad are stronger when AH establishes earlier,and vice versa. The interannual relationship between AH establish dates and SCSSM onset dates is evident.When the AH establish date is earlier (later),the SCSSM onset date is earlier (later).(4)Temperature difference between Australia and the Ocean on its east,AOI and nino4 index in March play a crucial role in the establishment of AH.The AH establishment tends to occur earlier (later) when the temperature difference between sea and land transform from negative to positive earlier (later),AOI in March is weaker (stronger) or Nino4 index is stronger (weaker).