Impacts of orography on large-scale atmospheric circulation: application of a regional climate model

Orography has considerable impacts on the large-scale atmospheric circulation, emphasizing the necessity of adequate representation of the impacts of orography in numerical models. The regional climate model version 4 (RegCM4) is used to investigate the impacts of orography on the large-scale atmospheric circulation. Three numerical experiments in four different years for winter and summer have been conducted over a large geographical area, covering Eurasia, Africa and Oceania. These experiments include control simulations using the real orography, simulations with the removed orography of the whole domain and simulations with the removed orography of the whole domain except the Tibetan Plateau. In winter, the Tibetan Plateau prevents the development of the sea-level high pressure in South Asia and contributes to the intensification of the Siberian high through blocking cold air advection from Siberia toward India. The Tibetan Plateau is also responsible for the southward displacement of low-level easterly flows in the North Indian Ocean, such that elimination of this Plateau is associated with more zonal orientation and intensification of easterly winds, as well as an increase of moisture flux over India and the Arabian Sea. Descending motions associated with lee waves of the Western Ghat Mountains contribute to a decrease of precipitation over the Arabian Sea. In summer, the Tibetan Plateau reinforces the South Asian low-pressure system and pushes the South Asian monsoon to South Asia. Both the tropical easterly jet stream over the southern Tibetan Plateau and the subtropical westerly jet stream over the Tibetan Plateau are weakened when the whole orography is removed. Removal of the whole orography is associated with a considerable equatorward displacement of the intertropical convergence zone over South Asia. In austral winter, low-level subtropical anticyclones in Southern Africa and Australia are intensified when the whole orography is removed.