During the past few years the upper layers of the atmosphere have been studied on a comprehensive scale as a result of which meteorologists have raised certain doubts about the validity of classical concept of the origin of Indian monsoon.
The thermally induced surface low and high pressure centers alone, it is now believed, cannot produce the monsoon circulation. More recent theories have laid greater emphasis on the circulation over the Indian sub-continent and its adjoining areas.
Indian Monsoon and the Tibet Plateau: In 1973, the Monsoon Expedition (Monex) was organized under the joint auspices of the Soviet Union and India. Under this expedition, four Russian and two Indian ships equipped with modern scientific instruments were pressed into service on the Indian Ocean and the Arabian Sea to investigate into the phenomenon of Indian monsoon.
The period of investigation extended from the month of May to July, 1973. On the basis of the data obtained the Soviet meteorologists arrived at the conclusion that Tibet Highland plays a crucial role in initiating the monsoon circulation over the Indian sub-continent.
In this connection it may be recalled that in 1958, an international symposium was held on “the Monsoons of the World”.
Dr. P. Koteswaram, the then Director General of Indian Observatories, while participating in the deliberations, had expressed his views that the summertime heating of the plateau of Tibet was the most important factor in the causation and maintenance of monsoonal circulation. The Indian as well as the Soviet scientists were unanimous in their views on this score.
The Tibet Plateau is 600 kilometers wide in the west and 1,000 kilometers wide in the east. Its length is about 2,000 km. The average height of the plateau is about 4,000 meters. Thus, it is an enormous block of high ground acting as a formidable barrier. It is also one of the most important geographical controls on general circulation.
Tibet Plateau affects the atmosphere in two ways, acting separately or in combination: as a mechanical barrier, or as a high-level heat source.
According to Yin, the Tibet Plateau acts as a mechanical barrier. At the beginning of June, the subtropical jet stream disappears from the southern side of the plateau. In other words, the jet disappears completely over northern India.
At this time, the jet stream shifts to the north of Himalayas and Tibet and takes up a position at about 40°N. Maung Tun Yin considers that there is a correspondence between the shifting of the jet and the slowing down of the westerlies over the whole of Eurasia.
In addition, the plateau accentuates 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 far to the south in the middle of October.
Thus, he is of the opinion that the abrupt onset of summer monsoon at the beginning of June is prompted by the hydrodynamic effect of the Himalayas and not by the thermally induced low pressure centre over northwest India.
In recent years it has been argued that the summer-time heating of Tibetan Plateau makes it a heat source. A warm-core anticyclone is formed over this plateau during the summer monsoon period. The formation of this anticyclone takes place in the middle part of the troposphere at 500 mb level.
It is the result of a process called anticyclogenesis. However, on the southern side of this upper-air anticyclone, the direction of air flow is from east to west. In fact, these easterly winds blowing in the mid-troposphere are known as the tropical easterly jet.
The tropical easterly jet extends far to the south of Tibet and the air flow is roughly along the Calcutta-Bangalore axis. These upper-air easterlies descend in the permanent high pressure area formed over the south Indian Ocean.
This naturally intensifies the ‘High’ already present there, is from this high pressure cell that the onshore winds start blowing towards the thermally induced low pressure area developed in the northwestern part of the Indian sub-continent.
After crossing the equator such winds become southwesterly and are known as the southwesterly summer monsoon. These surface winds have vast potentiality for precipitation.
It is, therefore, clear that the strength of the easterly jet is directly related to the intensification of the permanent ‘high’ formed over the south Indian Ocean. Since this higher pressure makes the pressure gradient steeper, so it is the main causative factor for determining the vigour of the summer monsoon.
Besides the easterly jet, the El Nino effect on summer monsoon is noteworthy. Whenever this unusually warm ocean current is produced near the Peruvian coast in South America, the amount of precipitation in the coastal regions is unusually high.
It has also been observed that appearance of the E1 Nino makes the easterly jet weaker. So the pressure in the high over the oceanic area does not rise to the desired level with the result that the strength of the summer monsoon diminishes.
Part of the energy for tropical easterly jet steam comes from the intense heating of middle and upper troposphere above the Tibetan Plateau. At this point it may be worthwhile to 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 tropospheric anticyclone. However, the interactions between Indian summer monsoon regime and Tibet Plateau are far from being simple.
Thus, the presence of Tibet Highland is very important even if there is no significant barrier effect on the flow of air. It shows the meridional cross-section of the Indian summer monsoon and its relationship with the Tibetan- Himalayan massif.
It may be pointed out that the tropical easterly jet stream first develops in longitudes east of India, and then extends westwards across India and the Arabian Sea to eastern Africa.
This upper-level easterly jet creates flow of air on the southern side of Tibetan Plateau that reaches down to low levels over northernmost India. During summer the insolational heating of air over 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.
All these conditions set the stage for the burst of southwest monsoon over the Indian sub-continent. It is at this time of the year that the Intertropical Convergence is displaced to its northernmost limit over the subcontinent at about 25°N.
The deep and unstable current of warm and moist southwesterly air drawn towards the Intertropical Convergence is known as the southwest summer monsoon. According to Trewartha, the southwest monsoon is, in fact, only the equatorial westerlies displaced to the north.
In October the conditions are reversed. The middle and upper tropospheric anticyclone over Tibetan Plateau disintegrates. The tropical easterly jet stream becomes non-existent. On the contrary, the subtropical westerly jet stream re-establishes itself over northern India with the result that the summer monsoon retreats towards the south.
According to Koteswaram the development of an anticyclone over Tibet Plateau is closely related to the burst of Indian summer monsoon. However, R. Frost does not agree with Koteswaram regarding the explanation of the burst of the monsoon.
He is convinced that the breakdown of the lower tropopause boundary brought about by intense insolational heating of the atmosphere below the 200 mb level, and the advectional or dynamic cooling of the air above it are mainly responsible for the onset of the monsoon over the Indian sub-continent.
Frost holds that the advent of the monsoon does not follow the displacement of the jet stream north of Tibet. It rather precedes the same.
Flohn, while fully appreciating the significant role played by Tibetan Highland in causing sudden eruption of the surface equatorial westerly flow at the beginning of June, considers the periodical change of wind direction on the surface and aloft to be the result or normal seasonal migration of the planetary circulation zones.
In his opinion, the so-called winter monsoon over India is simply the re-establishment of the northeast trades prevailing in these latitudes. Undoubtedly, the southward displacement of the thermal equator as well as the NITC (Northern Intertropical Convergence) brings back the north-easterly trade winds during winter.
On the other hand, he considers the southwest monsoon to be a northward expansion of the equatorial corresponding to the westerly equatorial winds. According to Flohn’s hypothesis, the Indian summer monsoon is not the northward prolongation of southern trade winds.
On the contrary, it should be deemed to represent northward shift of the belt of equatorial westerlies.
The origin of winter monsoon from the thermal high-pressure system developed over northwestern India due to the intense cooling of the land areas, according to this hypothesis, appears to be doubtful. It has now been firmly established that these highs are too shallow to cause a reversal of the prevailing wind direction.