The Role of Mountains in the South Asian Monsoon Circulation.

An 11-level numerical model of the atmospheric circulation which has a prescribed seasonal variation of insulation and sea surface temperatures is integrated with respect to time for approximately three model years. The model is global in domain and incorporates a smoothed mountain topography. In order to investigate the role that mountains play in the south Asian monsoon circulation, a second numerical experiment, exactly the same as the first except that all mountains are removed, is integrated with respect to time from 25 March through July.Analysis of the model with mountains reveals that the large-scale circulation associated with the south Asian monsoon is well simulated. However, the onset of the monsoon is approximately 10-15 days later than normal, and the atmosphere over the western Pacific seems to he dynamically too active, while the atmosphere over the northern reaches of the Bay of Bengal and northern India is relatively inactive.Comparison of the simulation with mountains with the simulation without mountains reveals that the presence of mountains is instrumental in maintaining the south Asian low pressure system as the continental low forms far to the north and east in the simulation without mountain topography. In the model with mountains, much higher temperatures are maintained in the middle and upper troposphere over the Tibetan Plateau, a region where upward motion and latent heating dominate. Without mountains, downward motion and sensible heating by the earth's surface dominate in this region. In the simulation with mountains, high temperatures over Tibet produce a low pressure envelope over these mountains which extends southward over the plains of south Asia. The low pressure belt being located farther south than in the simulation without mountains produces a stronger north-south pressure gradient which enables moist southerly flow at the surface to penetrate farther northward into Asia. Many of the features of the monsoon break persist in the model without mountains as copious precipitation extends northward only to south India. Clearly, mountain effects help to extend a monsoon climate farther north onto the Asian continent.The evolution of the south Asian monsoon is also influenced by the effects of mountains. Near the time of onset in the model with mountains, the subtropical jet abruptly jumps northward from a latitude just south of Tibet, 25°N, to a mean summertime position along 45°N. In the model without mountains, the subtropical jet gradually moves northward over a period of about two months, finally reaching a summertime position approximately 10° farther south than in the model with mountains. At the time of onset in the model with mountains, humid southerly flow near the earth's surface suddenly extends northward from equatorial latitudes to the south Asian low pressure belt centered at 30°N. In the model without mountains, humid southerly flow extends northward from equatorial regions, but it doesn't extend as far northward as northern and central India. These differences are attributed to mechanical and thermodynamical effects of the Tibetan Plateau.