The Mechanism of the Indian Monsoon
The climate of India is strongly influenced by monsoon winds. This wind system reverses its course seasonally and its pattern is permanent in nature. This helps the traders conventionally sailing over the ocean and Arabs are the first to name this wind system as monsoon.The climate of India is strongly influenced by the monsoon winds. It refers to a season in which the wind system reverses completely. The monsoons are experienced in the tropical area roughly between 20° N and 20° S.Various atmospheric conditions influence the monsoon winds. The first condition is the differential heating and cooling of land and water. This creates low pressure on the landmass, while high pressure is created over the seas around during day time, but is reversed during the night time.The second condition is the shift in the position of Inter-Tropical Convergence Zone (ITCZ). In summer, the equatorial trough normally positioned about 5°N of the equator moves over the Ganga plain creating a monsoon trough during the monsoon season.
The third condition is the presence of the high-pressure area that develops east of Madagascar. It is approximately at 20°S over the Indian Ocean. The intensity and position of this high-pressure area affects the Indian Monsoon.The fourth condition develops during the summer. The Tibetan Plateau gets intensely heated resulting in strong vertical air currents and high pressure over the plateau about 9 km above sea level. The fifth condition develops during the summer due to the movement of the westerly jet streams to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian Peninsula.Changes in pressure over the southern oceans also affect the monsoons. In certain years, there is a reversal in the pressure conditions. This periodic change in pressure conditions is known as the Southern Oscillation, or SO.
The Monsoon is experienced in the tropical area roughly between 20°N and 20°S. The Mechanism of the Indian monsoon can be understood with the help of following important facts. The different heating and cooling of land and water creates low pressure on the landmass of India while the seas around experience comparatively high pressure.
The shift of the position of Inter Tropical Convergence Zone (ITCZ) in summer, over the Ganga plain (this is the equatorial trough normally positioned about 5°N of the equator also known as the monsoon trough during the monsoon season).
The Presence of the high-pressure area, East of Madagascar, approximately at 20°S over the Indian Ocean. The intensity and position of this high pressure are effects the Indian Monsoon. The Tibetan plateau gets intensely heated during summer, which results in strong vertical air currents and the formation of high pressure over the plateau at about 9 km above sea level. The movement of the Westerly jet stream to the North of Himalayas and the presence of the tropical Easterly Jet stream over the Indian Peninsula during summer. Apart from this, it has also been noticed that changes in the pressure conditions over the southern oceans also affects the monsoon.
Te South-West monsoon is the most significant feature of the Indian climate. The season varies from less than 75 days over west Rajasthan, to more than 120 days over the south-western regions of the country contributing to about 75% of the annual rainfall. The monsoon in influenced by global and local phenomenon like El-Nino, Northern hemispheric temperatures, sea surface temperatures , show cover etc.
The monsoon rainfall oscillates between active spells associated with widespread rains over most parts of the country and breaks with little rainfall activity over the plains and heavy rains across the foothills of the Himalayas. Heavy rainfall in the mountainous catchments results flooding over the plains. However, very uncomfortable weather due to high humidity and temperature is the feature associated with the break of monsoon.
Why no precipitation in Kachchh and Western Rajasthan?
There is no mountain barrier to tap the advancing winds. As the Aravallis have an almost north-south axis, they fail to block the passage of these monsoon currents (which rather blow parallel to the Aravallis) and lift them.The monsoon currents heading towards Rajasthan are rather shallow and are superimposed by stable anti-cyclonic air.The hot and dry continental air masses from western Pakistan (Baluchistan) are drawn towards the thermal low developed in this region. These air masses check the ascent of air and absorb its moisture.These conditions are unfavorable for precipitation in Kachchh and western Rajasthan where desert conditions prevail.Some of the currents from the Arabian Sea branch manage to proceed towards Chhotanagpur plateau through the Narmada and Tapti gaps. These currents ultimately unite with the Bay of Bengal branch.Although a few air currents from the main Arabian Sea branch are diverted northward towards Kachchh and the Thar desert, these currents continue upto Kashmir without causing rain anywhere on their way. In fact, an east-to- west line drawn near Karachi in Pakistan practically marks the limit of the monsoon rainfall.
Bay of Bengal Branch:
This branch is active in the region from Sri Lanka to Sumatra Island of the Indonesian archipelago. Like the Western Ghats of India in the case of the Arabian Sea branch, the windward slopes of the West Coast Mountains of Myanmar (Arakan and Tenasserim mountains) get heavy rainfall when the main monsoon currents of this branch strike the Myanmarese coast. Akyab on the west coast records 425 cm during the June-September period. As in case of the leeward sides of the Western Ghats in India, here too, the rain shadow effect is pronounced on the leeward side.A northern current of this branch strikes the Khasi hills in Meghalaya and causes very heavy rains. Mawsynram (near Cherrapunji), situated on the southern slopes of Khasi hills, has the distinction of recording the highest annual average precipitation in the old.This is because of its peculiar geographical location. Mawsynram is flanked on all sides by the Garo, Khasi and Jaintia hills except for a gap through which the rain-bearing winds enter and are forced to rise, thus yielding the heaviest rainfall. Shillong, a mere 40 km away on top of the Khasi hills, receives only about 140 cm of rainfall during June-September.Another current of the Bay of Bengal branch takes a left turn at the eastern end of the low pressure trough (roughly the Bengal delta). From here, it blows in a south-east to north-west direction along the orientation of the Himalayas. This current causes rainfall over the northern plains.The monsoon rainfall over the northern plains is assisted by west-moving monsoon or cyclonic depressions called ‘westerly disturbances’. These are formed in the Bay of Bengal and move along the southern fringe of the northern plains causing copious rains there which are vital for the rice crop.The intensity of rainfall decreases from east to west and from north to south in the northern plains. The decrease westwards is attributed to the increasing distance from the source of the moisture. The decrease in rainfall intensity from north to south, on the other hand, is due to increasing distance from the mountains which are responsible for lifting the moisture-laden winds and causing orogenic rainfall in the plains, especially in the foothills.
The stable, dry anti-cyclonic winds prevailing over the subcontinent after the retreat of the south-west monsoons are not capable of causing precipitation because they are free of moisture. Instead, these winds produce dry and fine weather. However, certain areas in the north get winter precipitation: from sources far away.