Abstract: The seasonal variation of the circulation on 100 mb level in low latitudes is much more pronounced in the eastern than in the western hemisphere.In winter,the circumpolar westeries prevail in the eastern hemisphere in low latitudes,while in summer there shows strong easterlies and a quasi-stationary sub-tropical high in the upper troposphere over the Asia-African continent.The variation of circulation is related to the transitions of season over eastern Asia.This phenomenon may be called "Gross Monsoon".The wave with wave number 1 along 300N may be considered as an index of the "Gross Monsoon".