Impact of the Tibetan Plateau on the Asian monsoon. II: Seasonal evolution of the South Asian monsoon

While in Part I we examine the impact of the Tibetan Plateau on the East Asian monsoon, here we focus on the South Asian monsoon. During the pre-monsoon season and monsoon onset (April-May-June), when westerly winds over the Southern Tibetan Plateau are still significant, the Tibetan Plateau triggers early monsoonal rainfall downstream, particularly over South China. The downstream moist convection is accompanied by strong monsoonal low-level winds, and subsidence upstream of the Plateau. In experiments where the Tibetan Plateau is removed, monsoon onset occurs about a month later, but the monsoon circulation becomes progressively stronger and reaches comparable strength during the mature phase. During the mature and decaying phase of the monsoon (Jul-Aug-Sep), when westerlies winds over the southern Tibetan Plateau almost disappear, the monsoon circulation strength is not affected significantly by the presence of the Plateau. We further explore the Tibetan Plateau-induced downstream convergence, by performing idealized experiments with a dry dynamical core in which east-west oriented mountains interact with background zonal westerlies. The dry dynamical core consistently simulates convergence downstream of the mountains. In a moist atmosphere, the mechanically-driven downstream convergence is expected to be associated with significant moisture convergence. We speculate that the mechanically-driven downstream convergence in the presence of the Tibetan Plateau is responsible for the zonally asymmetric monsoon onset, particularly over the Bay of Bengal and South China.