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Comprehensive studies of the upper layers of atmosphere have brought into sharp focus the debate over the validity of the classical concept of the monsoons.
This concept explains the basically Indian sub-continental phenomenon in terms of differential seasonal heating of land and sea which induces low and high pressure centres in successive seasons. The recent theories, on the other hand, have highlighted the role of two important factors in the origin of the Indian monsoon. These factors are—(i) influence of Tibet plateau; (ii) Jet streams.
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Dr P. Koteswaram had, in an international seminar on ‘The Monsoons of the World’, held the summer time heating of the Tibetan plateau to be the most important factor in the causation and maintenance of monsoonal circulation. Again, in 1973, when a joint Indo-Soviet monsoon expedition (Monex) was organised, the Indian and Soviet meteorologists arrived at the conclusion that the Tibetan highland plays a crucial role in initiating the monsoon circulation over the Indian subcontinent.
The Tibet plateau is 600 km wide in the west and 1,000 km wide in the east, with a length of about 2,000 km. The average height of the plateau is 4,000 m. Thus, the plateau is capable of acting as a massive physical barrier. It is also one of the most important geographical controls on general circulation.
According to Maung Tun Yin, the abrupt onset of the summer monsoon at the beginning of June is related to the sudden northward shift of the Tibet plateau'(or northern plains) to a position along 40°N. According to Yin, the plateau accentuates the northward displacement of the jet stream. Similarly, during October the plateau plays an important role in pushing the jet far to the south (Fig. 13.23).
Yin, thus, gives more importance to the hydrodynamic effect of the Himalayas than to the thermally induced low pressure centre over northwestern India in monsoonal wind’ circulation.
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Another process called dynamic anti- cyclogenesis has been emphasised recently. By this process, a warm-core high pressure thermal anti-cyclone appears in mid-troposphere during the south-west monsoon. On the southern side of this anti-cyclone, the tropical easterly jet is produced. The energy for the tropical easterly jet stream comes from three sources—(i) intense heating of middle and upper troposphere above the Tibetan plateau; (ii) large amounts of latent heat released by the south-west monsoon over the Indian sub continent; and (iii) heat transfer from elevated surfaces of the Himalayas and-Tibet to the upper atmospheric anti-cyclone. This tropical easterly jet stream actually originates in longitudes east of India and then travels across India and the Arabian Sea towards eastern Africa.
These upper level easterly jets create an air flow on the southern side of the Tibetan plateau reaching down to low levels over northernmost India. This phenomenon, along with the weakening of western subtropical jet south of the Himalayas and the apparent shift of the ITCZ northwards at about 25°N, has the effect of drawing the southwest monsoon into the Indian subcontinent.
In October, the conditions are reversed. The middle and upper tropospheric anti-cyclone over the Tibetan plateau disintegrates. The tropical easterly jet stream becomes non-existent. On the contrary, the subtropical westerly jet stream reestablishes itself over northern India with the result that the summer monsoon retreats towards the south.
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Thus, the presence of the Tibetan highland is very important, even if there is no significant barrier effect on the flow of air.
R. Frost does not agree with Koteswaram’s assertion that the development of an anti-cyclone over Tibet is closely related to the burst of the Indian summer monsoon. He attributes the onset of monsoon over the Indian subcontinent to intense insolational heating of the atmosphere leading to the breakdown of the lower troposphere boundary and the advection or dynamic cooling of the air above it. Frost also argues that the advent of monsoon does not follow the northward displacement of the jet stream, rather it follows the latter.
Significance of Monsoon Rains in India’s Economy:
India’s economy has often been referred to as a gamble in the monsoons, and with some justification.
Nearly 80 per cent of rains in India are caused by the south-west monsoons during June-September. These rains feed about 70 per cent (or 99 million hectares) of the total net sown area of 141 million hectares. This is the rain-fed cropped area. Also, the monsoon rains account for a substantial part of the water for irrigated areas through canals, ground water, tanks and reservoirs.
In a country where more than 50 per cent of the total population is directly dependent on agriculture, a failure or inadequacy of the monsoon rains can play havoc with the economy. Rainfall is also essential for fodder crops and grass which sustain the large livestock population in India.
The livestock act as the crucial buffer for the vulnerable sections of society—especially during the lean season. Late onset and early withdrawals result in loss of valuable investments which the farmers may have made in the form of costly fertilisers, improved seeds and modern machinery.
Besides agriculture, the all-important energy sector is also heavily dependent on the monsoon rains. This is because hydel power, which accounts for about 15 per cent (as of 2008-09) of total power generation, derives the desired buffer of water resources from the monsoon rains. Inadequate water levels in the reservoirs hydel power plants during deficient monsoon situation lead to below capacity power generation. This affects the efficiency of secondary (industrial) and tertiary (services) sectors of the economy.
Both excessively high and low amounts of rainfall result in hazardous situations of drought and flood which are common in India. To protect the vulnerable sections against these adversities, the government spends large amounts from its already constrained resources. This has a dampening effect on economic growth.
Flohn attributes the seasonal reversal of wind direction on the surface to seasonal migration of planetary circulation zones. According to him, the south-west monsoon represents the northward shift of equatorial westerly low belt and the winter monsoon is simply the re-establishment of the north-east trade winds prevailing in these latitudes.
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According to this hypothesis, the origin of winter monsoon from the thermal high pressure system developed over northern India due to intense cooling of landmasses, appears to be doubtful. It has now been firmly established that these high pressure systems are too shallow to cause a reversal of the prevailing wind direction.
According to another theory, just about the time the monsoon is about to lash the Kerala coast with a sudden burst of torrential rain, changes become apparent in the different layers of the atmosphere. This has led some scientists to suggest that the monsoon’s onset is related to a sudden acceleration of air from the southern hemisphere towards India.
They say that a broad belt of high pressure develops around the Mascarene Islands near Mauritius in the Indian Ocean and this generates the cross-equatorial flow known as the Somali Jet which brings heavy rain to India’s west coast. A strong, low level jet usually means a strong monsoon over peninsular India.
Scientists now know that monsoon vagaries on a 10- to 15-day time scale are related to the behaviour of the monsoon trough. When positioned normally over the Gangetic plains, it controls moisture convergence and rainfall and areas within upto 500 kin on either side of the trough get moderate, but well-distributed rain. However, the trough is not stationary and scientists explain that it sometimes moves northwards and closer to the Himalayan foothills and this can interrupt rainfall in the northern plains.
Peculiar Features of the Indian Monsoon:
The Indian monsoon is characterised by great variability. Although the annual average precipitation in India is 98 cm, it may show a deficit of 20 cm (as in 1899) and a surplus of 30 cm (as in 1907). For moderate rainfall regions, even small variability can cause severe damage to the crop. These are the regions of recurrent droughts and famines.
A delay in onset usually results in an early retreat. Such a situation is harmful to both rabi and ‘kharif crops.
Sometimes, monsoon rains have a tendency to persist in one area or be subject to long breaks during July-August. This happens just when the summer crops are growing and need plenty of moisture. Winter crops are also affected as they rely on the residue moisture from summer crops. Sometimes, a situation occurs where drought and floods affect different parts of the same state.
The monsoon precipitation over India is characterised by great spatial inequality. While certain areas like the Western Ghats and the north-east receive upto 400 cm of rain in a year, western Rajasthan receives less than 10 cm rainfall. Most of India, though, receives between 60 cm and 100 cm of rainfall.
The monsoon precipitation is concentrated only in a few months, 80 per cent of it occurring in the June-September period while the rest is distributed as—13 per cent during October- December, 3 per cent during January-March and 10 per cent during March-May (‘Mango showers’ in south India).
The Indian monsoon rainfall is characterised by an erratic nature—at times the rainfall is slight and at times, it occurs in the form of heavy downpour and torrents causing soil erosion.
There is rainfall in every month of the year in some part of the country:
i. January, February—over northern India;
ii. March—thunderstorms and heavy rains in Bengal, Assam;
iii. April-May—Mango showers over South India;
iv. July-September—south-west monsoon;
v. October-December—retreating monsoon over east coast.