The Asian summer monsoon accompanies two systems, the Indian monsoon and the East Asian monsoon. A major difference between the Asian and the Indian monsoon is that beginning in early May, the Asian monsoon takes about two months, to migrate over an extensive area form southern to northern China. At the same time, the associated heavy rainfall also moves from south to north. The monsoon rain-band associated with the Indian monsoon takes only one to two weeks to migrate from Southern to Northern India. The onset of the Southeast Asian southwest monsoon initiates the East Asian monsoon (EASM).

Based on the NMC global spectral model, with significant modifications to virtually all of the physical parameters, we have developed a general circulation atmospheric model i.e. NTUGCM. The NTUGCM was used to perform an AMIP integration from 1January1979 to 31December1988. For the JJA(June,July,August) climatology, the simulated stream function at 850 hPa captures most of the major features in the observed climatologi- , cal-mean fields. e.g., the subtropical high over the ocean, the cyclonic circulation over South Asia and the eddy straddling the equator in the Indian Ocean, as well as the Tibetan high, simulated in the 200 hPa stream function. In general, the NTUGCM gives a better simulation at 850 hPa than at 200 hPa and a better simulation around a large forcing region, e.g. South Asia, than around a weak forcing region, e.g. the Eastern Pacific and South American coast. The best simulation for precipitation is found in the Asian monsoon region where maximum precipitation occurs.

The EASM rainfall distribution is the result of complex interactions between the atmosphere, the earth's surface and the tropical and extra tropical systems. The EASM rainfall distribution involves a wide range of spatial and temporal scales from the mesoscale to the planetary scale. The NTUGCM is able to simulate the large scale features of the EASM and the sudden change of the monsoon rainfall which is associated with abrupt changes in large scale atmospheric circulation. This study also analyzes the onset characteristics of the EASM simulated by the model. The AMIP simulation of the NTUGCM gives the onset date of the EASM as 13 May.

The present results are encouraging, however, there are still many obvious issues that remain to be studied, such as the linkage between the monsoon's variation and surface boundaries forcing, the cause of the seasonal northward jump of the subtropical high, the dynamic and thermodynamic effects of the Tibetan Plateau and an improved model representing the physical parameters.