The Interannual Variability Of The Onset Of SCS And Indian Summer Monsoon

Based on the NCEP/NCAR reanalysis data,the onset dates of the South China Sea summer monsoon(SCSSM)and the Indian summer monsoon is investigated.They both have obvious interannual variations.There are great differences between early and late onset years in atmosphere characteristic,the heat source and SST in the preceding winter.In the early onset years,the anomaly of the heat source is the same as that of the SST in the preceding winter.It's warm in west-Pacific Ocean and cold in east-Pacific Ocean and west-Indian Ocean,which is like the La Ni(?)a pattern.So the Walker circulation is strong.In the late onset years,it's warm in west-Indian Ocean and cold in east-Indian Ocean,and it's like the El ni(?)o pattern in Pacific Ocean.So the Walker circulation is weak.The great and small-difference years were defined by using the d-value of the onset date of the summer monsoon.The results show that the difference also has obvious interannual variations.In small-difference years,when the South China Sea summer monsoon(SCSSM)breaks out in the 31^stpentad,the prevailing wind in India is from west and the Indian summer monsoon breaks out simultaneously.But in great-difference years,seven pentads posterior to the onset of SCSSM,there prevails west wind and the summer monsoon breaks out much later in India.The atmospheric circulations are different obviously.Ulterior,the onset of Indian summer monsoon is mainly rested with the turning of meridional temperature while the onset of SCSSM lies on the turning of zonal temperature.The onset of the Asian tropical summer monsoon is independent.The turning of zonal temperature is more important to the Asian tropical summer monsoon difference.When the temperature turns depends on the appearance of the intensive belt of the temperature differences and the intensity of heat source in the key areas(one is from 98°E to107°E,23°N to 30°N and the other is from 66°E to 78°E,8°N to 18°N).In the preceding winter and spring,when the SST of China seas and mid-east-Pacific Ocean is higher,and the SST of west-Pacific Ocean is lower,the difference of the onset of the Asian tropical summer monsoon is greater. It's like the El ni(?)o pattern.