Recent concepts of Monsoon are based on the findings of researches conducted after 1950 using meteorological data from the upper atmosphere and their computer based analysis. These concepts mainly rely on the role of jet streams, Tibet Plateau and ocean bodies in explaining the origin of Indian Monsoon.

(a) Jet Streams

Jet streams, discovered during the Second World War, are high altitude (9000-12000 meters) westerly winds blowing between middle latitudes (summer 35°N-45°N; winter 20°N-35° N in northern hemisphere) with high speed (300-500 km/hr) in a meandering course. Recent observations have shown that these winds exert considerable impact on sur­face weather conditions.

During winter season the upper air westerly jet streams are positioned in Asia at the height of 12 km in the troposphere. These jet streams are bifurcated in two branches due to ob­struction caused by the Himalayas and the Tibetan Plateau. The northern branch blows from west to east in actuate shape to the north of the

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Himalayas and the Tibetan Plateau, while the south­ern branch to the south of the mighty mountains. This southern branch follows a path which inscribes an anticyclone (clockwise) arc across Afghanistan followed by a cyclonic (anticlockwise) arc along the southern flank of the Himalayas.

A ‘high’ pressure system is formed south of the jet-stream over Afghanistan and north-west Pakistan from which air tends to subside over India leading to atmos­pheric stability and dry conditions and causing the flow of north-east winter monsoons. The jet stream also helps periodical disturbances in the north-west of the sub-continent which tend to follow paths immediately beneath the jet-stream.

These distur­bances are found to move along the eastern Mediter­ranean and into north-western Pakistan-India ap­pearing here as perturbations or waves rather than as well developed frontal cyclones.

They occur as troughs of low pressure with strongly convergent air in the westerly stream at above 2000 meters, but overlie gentle easterly (trade wind) at the surface. Sharp cold rainstorms form towering clouds and a drop in temperature accompany the passage of such temperate cyclones. Their average frequency is 4 to 8 storms per month between October and April limiting their impact up to Patna in the east.

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During summer season as sun falls vertically over the Tropic of Cancer the polar surface high pressure is weakened and upper-air circum-polar whirl shifts northward as a result of which the upper- air westerly jet streams are also withdrawn from southern slopes of the Himalayas and shift north­ward. By 6-10 June the southern branch of the jet-streams disappears from the southern flank of the Himalayas making room for the equatorial westerly’s to occupy the Indian sub-continent.

The removal of the jet-stream to north of the Tibetan Plateau (Fig. 5.4.B) leads to a reversal of the curva­ture of flow of free air to the north and north-west of the sub-continent. Over northern Iran and Afghani­stan the trajectory of free air takes on a cyclonic curve (anticlockwise), leading to a dynamic depres­sion aloft where previously there was an anticy­clone. Here, then to the north-west of Indo-Pakistan there develops a dynamic depression overlying the thermal depression already established at the sur­face, and it appears that this event may well be the trigger that sets off the ‘burst’ of the monsoon, allowing the vigorous inflow of equatorial air deep into India.

So long as the position of upper-air jet-steams is maintained above the surface low pressure (to the south of the Himalayas), the dynamic cyclonic con­ditions persist over Afghanistan, north-west Paki­stan and north-west India. The winds descending from the upper air high pressure obstruct the ascent of winds from the surface low pressure areas; with the result the weather remains warm and dry. This is why the months of April and May are drying inspired of high temperature and evaporation. Contrary to it, upper air low pressure is formed to the east of the Himalayas as a result of which the winds coming from southern Myanmar are forced to ascend and produce rainfall in Myanmar, Assam and Bangla­desh.

It may be remembered that during northern summer there is winter season in the southern hemi­sphere. Hence, the southern polar whirl (jet) is more developed and is stretched up to the equator. This pushes the intertropical convergence (ITC) and the south-eastern trade winds to cross over the equator and take south-westerly direction due to Coriolis force. The well-known ‘pulsations’ in the monsoon weather are due to waves of dynamic origin (not frontal cyclones associated with the polar front) which develop in the ITC. These dynamically in­duced waves after coming over India become cy­clone vortices which cause summer monsoon rains in the country. While the development of cyclonic vortices is related to the wet weather their occlusion produces dry weather which continues till new vor­tices are formed.