Concept of Monsoon and Concepts of Halley
Some of the important concepts about the origin of Monsoon have been given as under
Concept of Halley (1686)
Halley hypothesized that monsoons is the extended land breeze and sea breeze on a large scale, produced by the differential heating of continents and ocean basins. During the summer season in the northern hemisphere, when the sun rays are vertical over the Tropic of Cancer, the huge landmass of Asia heats quickly and develops a strong low pressure center near lake Baikal (Siberia) and Peshawar (Pakistan), but in comparison to this, the pressure over the adjacent water of the Indian and the pacific ocean is relatively high.
Under these conditions, a sea to land pressure gradient develops. Consequently, the surface air flow is from the high pressure over the oceans towards the low pressure areas over the heated landmass. The thermal concept about the origin of monsoon has, however, not been accepted universally as it fails to explain the intricacies of monsoon.
The Dynamic Concept by Flohn
The Dynamic concept about the origin of monsoons was put forward by Flohan in 1951. According to this concept, monsoons is the result of seasonal migration of planetary winds and pressure belts. With in the internal-Tropical convergence, characterized by equatorial Westerlies, covering South and South-East Asia. Thus, equatorial Westerlies are established over these areas.
The equatorial Westerlies become south-west or summer monsoons. On a rotating Earth, the trade winds of the Southern hemisphere after crossing the equator turn towards their right (Carioles effect). The NITC is associated with numerous atmospheric storms (cyclones), which yield heavy rainfall during wet monsoon months (July to September).
Similarly, the North-East or winter monsoon does not originate only due to low pressure in the southern hemisphere during East monsoons are North-East trade winds which are re-established over South-East Asia due to Southward shifting of pressure and winds belts.
It is oblivious that due to Southward movement of the sun at the time of winter solstice, the NITC is withdrawn from over South and South-East Asia, and North-East trade wins occupy their Normal position. These North-East trades, thus, become winter monsoons. The North-East monsoons having their origin on land are generally, dry and devoid of rains.
South-West Monsoons
During the Summer solstice in the Northern hemisphere, the Sun ray’s are vertical over the Tropic of Cancer and the ITCZ, particularly the NITCZ shifts towards the pole-ward and extends upto 30°N latitude, covering the South and South-East Asia. Thus, the equatorial Westerlies become the South-West monsoon winds.
North East Monsoon
On the other hand, during the winter season in the Northern hemisphere, the Sun ray’s are vertical over the Tropic of Capricorn and ITCZ, particularly STCZ shifts towards pole-ward and there is withdrawal of the NTCZ from the South and South-East Asia.
Thus, the North-East trade wind reoccupy their normal position and called a winter monsoon. As this North-East monsoon originate on land and moves towards the sea, therefore, not able to cause rainfall.