A Simulation of the Tropospheric General Circulation with the MRI Atmospheric General Circulation Model Part III: The Asian Summer Monsoon

Abstract

Asian summer monsoon simulated by a 5-layer tropospheric general circulation model (MRI•GCM-I) is described. Gross features of the July circulations are well simulated. Although the detailed rainfall distribution in the monsoon region, e.g. a maximum over northeastern India, is not well reproduced, the time evolution of the total diabatic heating distribution over the entire monsoon region was reasonably simulated. One month period from mid-April is a transition period in the model from the northern winter regime to the summer regime. A reverse transition occurs between September and October. It is pointed out that both the transitions do not occur smoothly, but rather rapidly, and that circulation changes associated with the monsoon onset are not confined to the monsoon region but extend globally.
The present simplified treatment of the land surface processes made the model overestimate the latent heat flux over the Tibetan Plateau compared to the observed one (e.g., Luo and Yanai, 1985). It is found out that reduction of the field capacity of the model ground water over the Tibetan Plateau tends to improve surface energy balances there.
Quasi-periodic oscillations are found in the model monsoon system. Time evolution of them is quite similar to the one described by Krishnamurti and Bhalme (1976), although the simulated oscillation period is not quasi-biweekly but 8-9 days. The oscillation period seems to be influenced by dominant quasi-10 day period of Kelvin waves found in the model tropics. As a result, the timing of the monsoon onset coincides with one of the favorable timings for the intensification of the upper level easterlies over the equatorial Indian Ocean provided by the Kelvin wave.
Preceding the monsoon onset, a long-lasting westward moving disturbance was formed in the model lower troposphere in the south of the equator over the Indian Ocean. Further studies are needed to confirm the reality of this disturbance and the role of it on the monsoon onset.