Effects of the Tibetan Plateau on the Asian summer monsoon: a numerical case study using a regional climate model

To understand impacts of the Tibetan Plateau (TP) upon the Asian summer monsoon (ASM), including both the Indian summer monsoons (ISM) and East Asian summer monsoons (EASM), a series of numerical experiments using the National Centers for Environmental Prediction (NCEP) regional spectral model (RSM) are conducted with various TP heights ranging from the flat surface (1.2 km) to 140% of the actual height. It was found that an increase in the TP height leads to an increase in the simulated ISM precipitation over northern India and conversely a decrease in the height leads to a decrease in precipitation. This sensitivity is associated with both the thermal and dynamical effects of the TP: however, although the ISM precipitation over India is affected by the changes in atmospheric circulation, it is not directly affected by the thermal effect. The thermal effect of surface heating plays a role in developing positive vorticity with a consequent increase in monsoon precipitation over northern India. The width of the plateau also seems to be associated with the intensity of the sensitivity for the ISM region. For the EASM region, the orographic effect caused by changes in the lower-atmospheric circulation and its link with upper-atmospheric circulation are crucial to the monsoon circulation and precipitation. With increased TP height, the monsoon precipitation moves inland in a northwestward direction, which qualitatively follows the previous findings based on general circulation models (GCMs). but with a detailed dynamical mechanism in linkage between the lower- and upper-atmospheric circulation in the regional climate modelling (RCM) framework in this study. Copyright (C) 2009 Royal Meteorological Society