Roles of the Tibetan Plateau (TP) upon the transition of precipitation in South Asian summer monsoon are investigated by a simplified regional climate model. By use of NCEP/NCAR reanalysis data, sinking motion at middle troposphere, which can be considered as a suppressor against precipitation, prevails over northern India before onset of the south Asian monsoon. The subsidence motion gradually weakens and retreats from this region before July corresponding to the northwestward migration of the monsoon rainfall. A series of numerical experiments are conducted to test a hypothesis that the dynamical and thermal effects of TP cause the middle tropospheric subsidence.
While mechanical effect of TP generates descending flow over northern India during winter when zonal westerly is relatively strong, the effect becomes negligibly weak after April. Thermal effect of TP, contrastingly, enhances the descending flow in pre-monsoonal season. The descending flow formed by the thermal effect of TP has a seasonal cycle because global-scale upper level westerly changes the energy propagation of the thermal forcing. The subsidence formed by mechanical and thermal effects of TP disappears over northern India after the strong westerly shifts to north of the plateau, seasonal change of which is in good agreement with that found in reanalysis data. The retreat of descending motion can be regarded as the withdrawal of the pre-monsoon season. After that, the deep convection, indicating the onset of Indian summer monsoon, can develop over India in relation to the ocean-atmosphere and land-atmosphere interaction processes. We, therefore, conclude that the seasonal variation of precipitation over South Asia is directly connected with the thermal forcing of TP through changing the middle tropospheric subsidence as well as mechanical forcing.