A negative correlation between δ 18 O in mon-soon precipitation and f, the ratio of precipitable water in monsoon region to that in water source area, is hypothesized. Using the Rayleigh model, a new method for identifying origin of summer monsoon rainfall is developed based on the hypothesis. In order to validate the method, the isotopic data at New Delhi, a typical station in the southwest monsoon region, and Hong Kong, a typical station in the southeast monsoon region, were collected and analyzed for case studies. The case studies indicate that the water source areas of the monsoon rainfall at the two stations identified by the method are accordant with the general atmosphere circulation patterns. The method developed in this paper is significantly important for tracing the origin of summer monsoon precipitation.Identifying origin of precipitation is always a hotspot in water cycle research. It is valuable for hydrologists to further understand the processes of water cycle and its structure. Some researchers simulated the water source regions by the general circulation models (GCMs) [1][2][3] . However, the simulated results are mainly dependent on spatial and temporal resolutions of the GCMs and the validity of the models' parameterization. The stable isotopes, 18 O and 2 H, are two natural tracers in hydrological studies. Waters with different origins have distinctive isotopic ratios due to the equilibrium and non-equilibrium fractionation of stable isotopes when water phase changes, which makes it possible to study water cycle processes by stable isotopic variations in waters. A worldwide network of isotopic investigation in precipitation established by the International Atomic Energy Agency (IAEA) and the World Meteorological Organization (WMO), with about 550 IAEA/ WMO stations in different parts of the world, has provided precious isotopic data for studying global and local hydrological cycles. At present, it is an international study hotspot to identify origin of precipitation by the stable isotope method. Deuterium excess in precipitation, d (d = δ D−8δ 18 O [4] ), is mainly dependent on meteorological conditions in water source region, such as relative humidity, sea surface temperature and wind speed [5,6] . It is a useful parameter for tracing source area of precipitated water. Therefore many scientists identified origin of precipitation by the parameter [7][8][9][10][11][12] . Chinese scientists made a significant contribution to origin of precipitation in Tibetan Plateau using the stable isotope method [13][14][15][16][17] . They concluded that the distinctive origins of precipitated water result in the different isotopic patterns in the northern and southern Tibetan Plateau.It is difficult to trace origin of summer monsoon precipitation accurately and effectively by either the GCMs simulation or deuterium excess tracer because of the complexity of monsoon hydrological cycle system and its variability. There is a need to develop a new method for identifying origin of monsoon precipitation. It is ho...

The Indian Ocean Dipole (IOD) shows a significant negative correlation with the extent of Eurasian snow cover in spring. The anomalies in the land-sea temperature contrast, which are induced by the anomalies of Eurasian snow cover in spring, reverse the patterns of convection activity anomalies over the western and eastern tropical Indian Ocean in summer. Moreover, for heavier and lighter than normal Eurasian snow cover in spring, anomalies in the vertical zonal circulation over the tropical Indian Ocean (Walker circulation) and in the vertical meridional circulation between the Indian Ocean and the Eurasian continent (Hadley circulation) occur in summer. The Walker and Hadley circulation anomalies probably play an important role in the occurrence and duration of IOD events. Eurasian snow cover in spring is presumably one of the factors that trigger IOD events. These results provide a basis for further investigation of the mechanisms linking snow cover, the Indian monsoon, atmospheric circulation, and sea surface temperature.

Analyzed results of the atmospheric wind speed and stable isotopic data (6 18 0) in summer precipitation at Bangkok, Bombay, New Delhi, Kunming and Lhasa, the IAEAlWMO stations, indicate that 6 18 0 in monsoon precipitation correlate positively to wind speed and that there exists a monsoonal vapor layer over these monsooncontrolled areas during monsoon seasons. The isotopic exchange happens between monsoon vapors and falling raindrops in the layer, resulting in this correlation between 6 18 0 and wind speed. This suggests that wind speed is probably one of key factors affecting the 6 18 0 variation besides air temperature and rainfall in the southwest monsoon domain.