Simulation of the Onset and Intraseasonal Variability ofTwo Contrasting Summer Monsoons

To simulate the onset and intraseasonal variability of summer monsoons, the National Centers for Environmental Prediction Eta Model (80 km, L38) is nested in the Center for Ocean–Land–Atmosphere Studies GCM (R40, L18). The region of the Eta Model is (30°S–50°N and 30°–140°E), which includes the Indian, Chinese, and Southeast Asian monsoons. The summer monsoons of 1987 and 1988 are simulated by integrating the nested model from mid-April to the end of September, prescribing the seasonal variations of SST of the respective years. The summer monsoons of 1987 and 1988 were extreme. In 1987, an El Niño year, the Indian monsoon rainfall was far below normal but over southeast China the rainfall exceeded normal. In contrast, in 1988, a La Niña year, Indian monsoon rainfall was far above normal but the rainfall over southeast China was below normal. The Eta Model was able to simulate the typical observed features of the monsoon onset, that is, an abrupt increase in the precipitation rate as well as in the strength of the circulation. The simulated onset dates for 1987 and 1988 were in good agreement with observations. The Eta Model was also able to simulate the observed circulation features of the break and active periods during these two years. To investigate the contrasting characteristics of the Indian and the Chinese monsoons, for these two years the following hypothesis, largely based on observational evidence, is verified. There are two preferred locations of ITCZ: one over the warm waters of the equatorial Indian Ocean and the other over the heated continent in the vicinity of the seasonal monsoon trough. There is a northward migration of the convective precipitation bands from the equatorial ITCZ to the continental ITCZ with the timescale of a few weeks. There exists an inverse relationship between the strength of the two ITCZs. During an El Niño year, sea level pressure over the Indian subcontinent and over the Maritime Continent increases. Consequently, the ITCZ over the Indian subcontinent and over the Maritime Continent weakens and the ITCZ over the equatorial Indian Ocean, Southeast Asia, and southeast China strengthens. The Eta Model simulated circulations are in support of the hypothesis. The simulations also show that there is a northward migration of convective precipitation bands from the equatorial ITCZ to the continental ITCZ.