Monsoon and Tibetan Plateau
Monsoon refers to those seasonal winds which changes their direction twice in a year with the change in season. The monsoon system of the Indian sub-continent differs considerably from that of the rest Asia. The centers of action, air masses involves, and the mechanism of precipitation of Indian monsoon are altogether different from other monsoon systems. Although monsoonal tendencies develop over other parts of the world, it is only around the Indian Ocean that monsoonal circulation in the true sense is observed. Here, the monsoons, appears as truly massive interruptions and reversal of the normal global atmospheric circulation.
During the past few years, the upper layers of the atmosphere have been studied comprehensively and meteorologists have forward some
new theories of the origins of monsoons. The thermally induced surface low and high pressure centers alone cannot produce the monsoon circulation. Recent theories have laid greater emphasis on the influence of Tibetan plateau and the jet stream on the origin of monsoonal circulation over the Indian subcontinent and its adjoining areas.
According to one theory, Tibetan highland plays a crucial role in initiating the monsoon circulation over the Indian subcontinent. Tibetan plateau is an enormous block of high ground acting as a formidable barrier. It is also one the most geographical controls on general circulation. Tibetan plateau affects the atmosphere in two ways, acting separately or in combination; as a mechanical barrier, or as highland heat source. At the beginning of the June, the subtropical jet stream disappears from the southern side of the plateau. In other words, the jet stream disappear completely over northern India. It is believed that there is a correspondence between the shifting of the jet stream and the slowing down of the westerlies over the whole of Eurasia. In addition, the plateau accemuates the northward displacement of jet stream. On the contrary, the plateau proves to be the most important factor in causing the advance of the jet stream far to the south in the middle of October. This abrupt onset of summer monsoon at the beginning of June is promoted by the hydrodynamic effect the Himalayas and not by the thermally induced low pressure centre over northwest India.
Recently it has been emphasized that the summer time heating of the extensive Tibetan Plateau makes it a high level heat source. A thermal anticyclone appears over Tibet during the southwest monsoon. This warm core high developed in mid troposphere at 500 mb-level is the result of a process called dynamic anti-cyclogensis. On the south side of the anticyclone the tropical jet stream is produced.
Part of the energy for tropical easterly jet stream comes from the intense heating of middle and upper troposphere above the Tibetan plateau. At this point, it may be mention that there is a sensible heat transfer from the elevated surfaces of the Himalayas and Tibet to the atmosphere. Besides this, the large amounts of latent heat released by monsoon rains over India are also added to the upper troposphere anticyclone. Thus the presence of Tibet Highland is very important even if there is no significant barrier effect in the flow of air.
It may be pointed out that the tropical easterly jet stream first develops in longitudes lying east of India, and then extents westwards across India and the Arabian Sea to eastern Africa. These upper level easterly jet stream create flow of air on the southern side of Tibetan Plateau that reaches down to low levels over northern most India. During summer, the insolation heating of air above Tibet Plateau weakens the western subtropical jet stream south of the Himalayas with the resultant reversal of pressure gradient and wind flow over northern India.
Courtesy: Anupam Rastogi