The annual variation of thermal and circulational peculiarities are analyzed, while the article high lighted that South Asia is a sensitive region in seasonal change. For better representation of seasonal change, an index of south branch of westerlies, as well as a new area index of subtropical high for north and south regions were proposed abed calculated. Statistical results show that there is prominent significance of these index with Meiyu and rainy season of North China. A series of index earlier to serious drought and flood were also sought out for reference to long range weather forecast.