The Role of Land--sea Distribution and Orography in the Asian Monsoon. Part II: Orography

Abstract: The role of various mountains in the Asian monsoon system is investigated by AGCM simulations with different mountains. The comparison of the simulation with Asian mountains (MAsia run) with the simulation without mountains (NM run) reveals that the presence of the Asian mountains results in a stronger South Asian summer monsoon (SASM), characterized by enhanced lower-tropospheric westerly winds, upper-tropospheric easterly winds, and stronger water vapor convergence. In East Asia, the southerly winds and water vapor convergence are significantly strengthened in association with the intensified zonal pressure gradient between the East Asian continent and the Pacific Ocean. Both the dynamical and thermodynamic forcing of the Tibetan Plateau play important role in strengthening the Asian summer monsoon. In winter, the presence of Asian mountains significantly strengthens the continental high, which leads to a stronger Asian winter monsoon. The presence of African--Arabian mountains helps to intensify the exchange of mass between the Southern Hemisphere and Northern Hemisphere by strengthening the cross equatorial flows in the lower and upper troposphere over East Africa. Asian mountains also play a crucial role in the seasonal evolution of Asian monsoons. In comparison with the NM run, the earlier onset and later withdrawal of lower-tropospheric westerly winds can be found over South Asia in the MAsia run, indicating a longer SASM period. The African--Arabian mountains also moderately contribute to the seasonal variation of the South Asian monsoon. In East Asia, the clear south-to-north march of the southerly winds and subtropical rainfall starts to occur in early summer when the effects of Asian mountains are considered.

Keywords:
• mountains,
• Asian monsoon,
• rainfall,
• seasonal transition