東亞夏季季風具有隨季節移動的階段性發展之氣候特徵，而南海夏季季風肇始常被視為東亞夏季季風發展之起源。但季風環流系統的多重尺度特徵，造成肇始定義的界定相當困難。本文分析1979∼1995年間南海夏季季風肇始期間的環流演變，並比較數種不同的季風肇始定義和其季風肇始日期，以討論南海地區的局地季風環流型態和季風肇始定義之間的關係。此17年間夏季季風肇始時期環流型態顯示，在南海地區局地季風環流發展過程中，中緯度天氣系統的角色相當重要。本文並且提出一組半客觀的南海夏季季風肇始定義，期能定義出較確切的局地季風環流肇始時間，以利用未來對中緯度天氣系統和南海局地夏季季風環流發展之相互關係的討論。

　　　　　The climatological feature of the East Asian summer monsoon is characterized by gradational development as the season migrates, and the South China Sea is regarded as the root of the East Asian summer monsoon. Due to multi-scale variations of the monsoon circulation, it is difficult to objectively define the onset of the South China Sea summer monsoon. By the use of 1979-1995 observational data, based on the evolution of monsoon development and the comparison of monsoon onset dates determined by different onset definitions, this study discusses in details the local circulation pattern in the South China Sea area during the onset period and its association with the onset definition. The The monsoon circulation patterns as revealed in this 17-year analysis indicate that the mid-latitude weather system plays an important role in the development of South China Sea summer monsoon. This study also suggests a semi-objective definition of the South China Sea summer monsoon onset. Hope this onset definition will be beneficial to the study of the interrelationship between the mid-latitude weather system and the South China Sea summer monsoon development.