Introduction (The Indian Monsoon)
The word ‘monsoon’ has been derived from an Arabic word Mausim, which literally means season. Therefore, the word monsoon denoted a season in which the wind regime is completely reversed. For the people of India, monsoons are indicators of rain. The moist monsoon winds, after crossing the equator in Indian ocean, acquire South-Westerly direction as they are attracted towards the low pressure area in North-Western India and Central Myanmar.
The dry and hot bearing trades are thus, completely replaced by sea bearing trades are thus, completely replaced by sea bearing winds full of moisture. Based on the difference between tropical continental air and equatorial maritime air, the meteorologist say that monsoon is complete replacement of the dry hot air by the equatorial maritime air upto an altitude of 3 to 5 km over the land and water surface.
Monsoon is confined only to the tropical lands lying between 20ºN to 20º S. But in the Indian sub- continent, it is greatly influenced by the Himalayan ranges bringing the whole sub-continent under the sway of these equatorial winds for a season ranging between 2 to 5 months. It accounts for 75 to 90% of the annual rainfall just from June to September.
Theories of the Origin of the Indian Monsoon
Monsoons are the seasonal winds which reverse the direction of wind flow. Over the years many uncertainties of the monsoons have been solved but much remains to be done. There are theories which generally divided into:
Classical Theory
First scientific study was done by Al Masudi, an Arab explorer who gave an account of the reversal of the ocean currents and the monsoon winds over the North Indian Ocean. In the 17th century, sir Edmund Halley explained the monsoon as resulting from thermal contrasts between continents and oceans due to their differential heating. Halley’s ideas basically involved the land and sea between except that in the case of the monsoon, day and night are replaced by summer and winter. He said that the primary cause of the Indian monsoon circulation was the differential heating effects of the land and the sea.
According to him, when the sun rays are vertical over the Tropic of Cancer during summer in the Northern Hemisphere, the huge landmass of Asia heats quickly and develops a strong low pressure center near lake Baikal and Peshawar, but in comparison to this, the pressure over the adjacent water of the Indian and Pacific ocean is relatively high. Under this 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.
Modern Theory
H. Flohn’s dynamic theory was given in 1951. According to this theory, monsoon is the result of seasonal migration of planetary winds and pressure belts. The inter-tropical convergence zone is formed due to the convergence of the North-East and South-East winds near the equator. There is a belt of doldrums within the inter tropical convergence, characterized by equatorial Westerlies. At the time of summer solstice (21st June), when the sun rays are over the Tropic of Cancer, the NITC (North Inter Tropical Zone) is extended upto 30ºN latitude, covering south and south-East Asia.
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. The NITC is associated with numerous cyclones, which yield heavy rainfall during wet monsoon months (July to September). The North-East monsoons are North-East trade winds which are re-established over South-East Asia due to southern shifting of the pressure and wind belts. It is obvious that due to Southward movement of the sea at the time of winter solstice, the NITC is withdrawn from over South and South-East Asia and North-East trades thus become Winter Monsoons.
P. Koteshwaram (1952) tried to establish a relation between the monsoon and the atmospheric conditions prevailing over Tibet plateau. Over India, a sub-tropical Westerly jet develops in the winter season and is replaced by the tropical Easterly jet in the Summer season. The high temperature during the summer over the Tibetan Plateau, is believed to be the critical factor leading to the formation of the tropical Easterly Jet stream over India.
The mechanism affecting the monsoon is that the westerly jet stream causes high pressure over Northern parts of the continent during the winter. This results in the North to South flow of the winds in the form of the North-East monsoon. The high heat of the Tibetan plateau along with high altitude of plateau generates the tropical Easterly jet over central India. It creates a ‘low’ pressure zone over Northern Indian plains and development of South- West monsoon takes place.
Phenomenon of Indian Monsoon
Systematic studies of the causes of rainfall in the South Asian region help to understand the monsoon phenomenon, particularly some of the important aspects, such as:
- The onset and landward advance of the monsoon
- Rain bearing systems and the relationship between their frequency and distribution of monsoon rainfall
- Break in the monsoon i.e. a spell of dry weather
- Retreat of the South-West monsoon and the onset of North-East monsoon.
Characteristics of Indian Monsoon
The rainy season in India begins with the arrival of South-West monsoon season in June and continues till the middle of September. Three fourth of India’s annual rainfall is received during this season. This is the season which is crucial for the economy of India as it controls the yields of main Kharif crops like pulses, rice etc. So, it becomes essential to study the characteristics of Indian monsoon. These are the chief characteristics of the Indian Monsoon
- Rainfall in South-West monsoon is seasonal in nature as it is received between June and September
- Rainfall is largely orographic as the Western Ghats and Himalayas are the main relief features controlling the rainfall. The windward side of Western Ghats receive more than 250 cm of rainfall, which includes the East coast of Indus-Ganga-Brahmaputra plains and North-East India.
- The amount of rainfall declines as we move from the coasts to the interiors.
- The rainfall occurs in wet spells of a few days, dispersed with intervals known as breaks
- There is large spatial variation in distribution of rainfall all over the country
- There is uncertainty in the occurrence of monsoon rains as well as their withdrawal.
Distribution of Rainfall
The regional variation in the distribution of annual rainfall in India is about 125 cm. The highest rainfall occurs along the west coast, on the Western Ghats, as well as in the sub-Himalayan areas in the North-East and the hills of Meghalaya. Mawsynram, located on the crest of the southern range of Khasi hills, receives the highest average annual rainfall in the world also famous for its stalagmite and stalactite caves. Cherapunji, located 16 km East of Mawsynram is another region of very high rainfall.
The rainfall exceeds 1200 cm here. In certain parts of the Khasi and Jaintia hills, the rainfall exceeds about 1100 cm. However, it drops to 110 cm in the Brahmaputra valley and the adjoining hills. The 100 cm isohyets run Southwards from the Gujarat coast, roughly parallel to the crest of the Western Ghats upto Kanyakumari.
The rainfall over the peninsular regions lying to the East of this line, drops abruptly to below 60 cm. Rainfall between 100-200 cm is received in the Southern parts of Gujarat, East Tamil Nadu, North-East peninsula covering Odisha, Jharkhand, Bihar, Eastern Madhya Pradesh, Northern Ganga plain along the sub- Himalayas and the Cachar valley and Manipur. To the North, the 100 cm isohytes trends Eastwards passing over the Southern parts of Jammu and Kashmir, Himachal Pradesh and Northern Uttar Pradesh.
The driest parts which are prone to drought in Rajasthan, adjoining Haryana nd Gujarat. The drought is less intensive in the South-East of a line joining Ahmedabad and Kanpur. Another area liable to frequent drought lies on the leeward side of the Western Ghats. It is estimated that an area of about 1 milion sq. km receives inadequate rainfall and drought prone.