In 1973, the Monsoon Expedition (Monex) was organised under the joint auspices of the former Soviet Union and India in which 4 Russian and 2 Indian ships equipped with modern scientific instruments collected weather data from the Indian Ocean and the Arabian Sea between May to July, 1973. By the analysis of the data so obtained the Soviet meteorologists came to the conclusion that Tibet Highland plays a dominant role in the origin of Monsoon circulation over the Indian sub-continent.
It is worth mentioning here that in 1958 itself Dr. P. Koteswaram, Director General of Indian Observatories while participating in an international symposium on “the Monsoons of the World” opined that summertime heating of Tibet Plateau was the most important factor in the causation and maintenance of monsoonal circulation over India.
The Tibet Plateau is 600 km wide in the west and 1000 km wide in the east. It has average length of 2000 km and height between 4000-5000 m. Thus it is an enormous block of highland acting as a formidable barrier. Due to its protruded height it receives 2-3°C more insulation than the neighbouring areas.
The plateau affects the atmosphere in two ways: (a) as a mechanical barrier, and (b) as a high- level heat sources. According to Yin the Tibet Plateau acts as a mechanical barrier. At the beginning of June the subtropical jet stream is completely withdrawn from India and occupies a position along 40° N. Maung Tun Yin has searched out a correlation between the shifting of the Jet and the slowing down of the westerly’s over the Eurasia.
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In fact the plateau accentuates the northward displacement of the jet stream. Hence the burst of monsoon in June is prompted by the hydrodynamic effect of the Himalayas and not by the thermally induced low pressure cell. In the middle of October the plateau proves to be the most important factor in causing the advance of the jet south of the Himalayas or bifurcating it into two parts.
The summer-time heating of the Tibetan Plateau makes it a high-level heat source which produces a thermal anticyclone over this region. This high in mid troposphere at 500 my level caused by dynamic anticyclogenesis weakens the western subtropical jet stream south of the Himalayas but produces tropical easterly jet on the southern side of the anticyclone. This tropical easterly jet stream first develops in longitudes east of India and then extends westwards across India and the Arabian Sea to eastern Africa. Blowing along Kolkata-Bangalore axis the air under this jet descends over the Indian Ocean and intensifies its high pressure cell so as to finally move as south-west monsoon.
The data collected under Molex support that higher the intensity of the tropical easterly jet greater would be potency of the high pressure cell over the Indian Ocean and stronger would be the impact of south-west monsoon Fig. 5.5 exhibits the meridional cross-section of the westerly and easterly jet streams and their relationship with the Tibetan Plateau. R. Frost does not agree with Koteswaram.
According to him the onset of monsoon precedes the displacement of jet stream rather than succeeds it. He is convinced that the breakdown of the lower tropopause boundary brought about by intense innovational heating of the atmosphere below the 200 my level and the evectional or dynamic cooling of the air above it are mainly responsible for the onset of the monsoon over the Indian sub-continent.
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In October the conditions are reversed. The middle and upper troposphere anticyclone over Tibet disintegrates and the tropical easterly jet becomes non-existent. The subtropical westerly jet stream is re-established over the northern India and the north-east monsoon sets in.