The climatological mean summer monsoon onset in the South China Sea (SCS) is a remarkably abrupt event. However, defining onset dates for individual years is noticeably controversial. The controversies and complications arise primarily from a number of factors: the intermittent southward intrusion of cold fronts into the northern SCS, the bogus onset in late April before the establishment of tropical monsoons over Indochina, and active intraseasonal oscillation. In this paper, a simple yet effective index, U SCS , the 850-hPa zonal winds averaged over the central SCS (5Њ-15ЊN and 110Њ-120ЊE), is proposed for objectively defining the SCS monsoon onset. This onset index depicts not only the sudden establishment of the tropical southwesterly monsoon over the SCS but also the outbreak of the rainy season in the central-northern SCS.In this paper the East Asian summer monsoon (EASM) is defined as the broadscale summer monsoon over East Asia and the western North Pacific region (0Њ-40ЊN, 100Њ-140ЊE). It is shown that the seasonal transition of EASM can be objectively determined by the principal component of the dominant empirical orthogonal mode of the 850-hPa zonal winds, U EOF1 . It is found that the local index U SCS represents U EOF1 extremely well; thus, it can be used to determine both the SCS onset and the commencement of the broadscale EASM. The result suggests that the SCS summer monsoon onset indeed signifies the beginning of the summer monsoon over East Asia and the adjacent western Pacific Ocean. Evidence is also provided to show the linkage between the two salient phases of EASM: the local onset of the SCS monsoon and the local onset of the mei-yu (the rainy season in the Yangtze River and Huai River basin and southern Japan).

Abstract. Monsoon has earned increasing attention from the climate community since the last century, yet only recently have regional monsoons been recognized as a global system. It remains a debated issue, however, as to what extent and at which timescales the global monsoon can be viewed as a major mode of climate variability. For this purpose, a PAGES (Past Global Changes) working group (WG) was set up to investigate the concept of the global monsoon and its future research directions. The WG's synthesis is presented here. On the basis of observation and proxy data, the WG found that the regional monsoons can vary coherently, although not perfectly, at various timescales, varying between interannual, interdecadal, centennial, millennial, orbital and tectonic timescales, conforming to the global monsoon concept across timescales. Within the global monsoon system, each subsystem has its own features, depending on its geographic and topographic conditions. Discrimination between global and regional components in the monsoon system is a key to revealing the driving factors in monsoon variations; hence, the global monsoon concept helps to enhance our understanding and to improve future projections of the regional monsoons. This paper starts with a historical review of the global monsoon concept in both modern and paleoclimatology, and an assessment of monsoon proxies used in regional and global scales. The main body of the paper is devoted to a summary of observation data at various timescales, providing evidence of the coherent global monsoon system. The paper concludes with a projection of future monsoon shifts in a warming world. The synthesis will be followed by a companion paper addressing driving mechanisms and outstanding issues in global monsoon studies.

Abstract. Monsoon has earned increasing attention from the climate community since the last century, yet only recently regional monsoons have been recognized as a global system. It remains a debated issue, however, as to what extent and at which time scales the global monsoon can be viewed as a major mode of climate variability. For this purpose a PAGES Working Group (WG) was set up to investigate the concept of the global monsoon and its future research directions. The WG's synthesis is presented here. On the basis of observation and proxy data, the WG found that the regional monsoons can vary coherently, although not perfectly, at various time scales, ranging from interannual, interdecadal, centennial and millennial, up to orbital and tectonics time scales, conforming the global monsoon concept across time scales. Within the global monsoon system each subsystem has its own features depending on its geographic and topographic conditions. Discrimination of global and regional components in the monsoon system is a key to reveal the driving factors of monsoon variations, hence the global monsoon concept helps to enhance our understanding and to improve future projection of the regional monsoons. This paper starts with a historical review of the global monsoon concept in both modern and paleo-climatology, and an assessment of monsoon proxies used in regional and global scales. The main body of the paper is devoted to a summary of observation data at various time scales, providing evidence for the coherent global monsoon system. The paper concludes with a projection of future monsoon shifts into a warming world. The synthesis will be followed by a companying paper to discuss driving mechanisms and outstanding issues in the global monsoon studies.