Special Supplement June 2017 Indian Monsoon Indian Monsoon is the most prominent monsoon systems, which affects India, its farming community, economy and water bodies. The monsoon has two phases. The first one carries abundance moisture with it and moves over the country through Bay of Bengal and Arabian Sea. It begins in June and covers most of India with rain until September. In late September these winds reach Himalayas and make a U-turn. From late October to December, the monsoon glides back down the sub continent on its way to Indian Ocean. Indian Monsoon ( 1 )
Indian Monsoon is the most prominent monsoon systems, which affects India, its farming community, economy and water bodies. The monsoon has two phases. The first one carries abundance moisture with it and moves over the country through Bay of Bengal and Arabian Sea. It begins in June and covers most of India with rain until September. In late September these winds reach Himalayas and make a U-turn. From late October to December, the monsoon glides back down the sub continent on its way to Indian Ocean. The later one blows from northeast during cooler months and reverses direction to blow from the southwest during the warmest months of the year. The Southwest monsoon brings more than 75% of the rainfall. It carries a lot of fresh water recharging ground water, filling of ponds and reservoirs which act as buffer and utilised during the lean season. Indian agriculture revolves around monsoon for its prosperity. With 70% of the Indian population depending on farming either directly or indirectly, more than 60% of the area under rain fed and agriculture contributing 18% of the GDP. Any delay or failure of monsoon adversely effects agriculture, Indian economy and importantly livelihoods of two third of the country s population. At times also effect social and political relationships. When it s strong it can even trigger floods. Indian Monsoon: The differential heating of land and sea during the summer months is the mechanism which sets the stage for the monsoon winds to drift towards the sub continent. During April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian Ocean gets immensely heated. This causes an intense low pressure in the north western part of the sub continent. Since the pressure in the Indian Ocean in the south of landmass is high as water gets heated slowly, the low pressure cell attracts the southeast trades across the equator. The Inter Tropical Convergence Zone (ITCZ) is a low pressure zone located at the equator where trade winds converge, and so, it is a zone where air tends to ascend. In July, the ITCZ is located around 20 N-25 N latitudes (over the Gangetic plain), sometimes called the monsoon trough. This monsoon trough encourages the development of thermal low over north and northwest India. Due to the shift of ITCZ, the trade winds of the southern hemisphere cross the equator between 40 and 60 E longitudes and start blowing from southwest to northeast due to the Coriolis force, it becomes southwest monsoon. In winter the ITCZ moves southward, and so the reversal of winds from northeast to south and south west, takes place. They are called northeast monsoons. Inter Tropical Convergence Zone (ITCZ) These conditions help in the northward shift in the position of the ITCZ. The southwest monsoon may thus, be seen as a continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equator. These winds cross the Equator between 40 E and 60 E longitudes. The shift in the position of the ITCZ is also related to the phenomenon of the withdrawal of the westerly jet stream from its position over the north Indian plain, south of the Himalayas. The easterly jet stream sets in along 15 N latitude only after the western jet stream has withdrawn itself from the region. This easterly jet stream is held responsible for the burst of the monsoon in India. Southwest Monsoon: As stated earlier due to rapid increase of temperature in May over the north western plains, the low pressure Indian Monsoon ( 2 )
conditions over there get further intensified. By early June, they are powerful enough to attract the trade winds of Southern Hemisphere coming from the Indian Ocean. These southeast trade winds cross the equator and enter the Bay of Bengal and the Arabian Sea, only to be caught up in the air circulation over India. Passing over the equatorial warm currents, they bring with them moisture in abundance. After crossing the equator, they follow a south westerly direction. That is why they are known as southwest monsoons. The rain in the south west monsoon season begins rather abruptly. One of the result of this is it brings down the temperature substantially. The south west monsoon arrives at Kerala coast by 1st June, by 15th June it covers up to Central India and by mid July it covers entire sub-continent (detailed map). The monsoon approaches land in two branches: 1. The Arabian Sea Branch 2. The Bay of Bengal Branch. 1. The Arabian Sea Branch: The winds over the Arabian Sea further split in to three branches: Its one branch is obstructed by the Western Ghats. These winds climb the slopes of the Western Ghats and soon they become cool, and as a result, the windward side of the Sahyadris and Western Coastal Plain receive very heavy rainfall ranging between 250 cm and 400 cm. After crossing the Western Ghats, these winds descend and get heated up. This reduces humidity in the winds. As a result, these winds cause little rainfall east of the Western Ghats. This region of low rainfall is known as the rainshadow area. Another branch of the Arabian Sea monsoon strikes the coast north of Mumbai. Moving along the Narmada and Tapti River valleys, these winds cause rainfall in extensive areas of central India. The Chotanagpur plateau gets 15 cm rainfall from this part of the branch. Thereafter, they enter the Ganga plains and mingle with the Bay of Bengal branch. A third branch of this monsoon wind strikes the Saurashtra Peninsula and the Kachchh. It then passes over west Rajasthan and along the Aravalis, causing only a scanty rainfall. In Punjab and Haryana it too joins the Bay of Bengal branch. These two branches, reinforced by each other, cause rains in the western Himalayas. 2. The Bay of Bengal Branch: The Bay of Bengal branch strikes the coast of Myanmar and part of southeast Bangladesh. But the Arakan Hills along the coast of Myanmar deflect a big portion of this branch towards the Indian subcontinent. The monsoon, therefore, enters West Bengal and Bangladesh from south and southeast instead of from the south-westerly direction. From here, this branch splits into two under the influence of the Himalayas and the thermal low is northwest India. Its one branch moves westward along the Ganga plains reaching as far as the Punjab plains. The other branch moves up the Brahmaputra valley in the north and the northeast, causing widespread rains. Its sub-branch strikes the Garo and Khasi hills of Meghalaya. Mawsynram, located on the crest of Indian Monsoon ( 3 )
Khasi hills, receives the highest average annual rainfall in the world. Northeast Monsoon: By the end of September, the southwest monsoon becomes weak as the low pressure trough of the Ganga plain. It starts moving southward in response to the southward march of the sun and this commences from extreme west Rajasthan. As Southwest Monsoon approaches the withdrawal phase from a particular region, certain conditions become prevalent. These include change of wind pattern, reduction in cloud cover, cessation of rain or sporadic rain, decrease in humidity and temporary rising tendency of temperatures. This southward movement causes cold winds to sweep down from the Himalayas and Indo-Gangetic plain towards Indian Ocean. This is known as retreating monsoon or north east monsoon. By the beginning of October, the low pressure covers northern parts of the Bay of Bengal and by early November, it moves over Karnataka and Tamil Nadu. By the middle of December, the centre of low pressure is completely removed from the Peninsula. This is the time the state of Tamil Nadu receives more than 50% of its rainfall. The widespread rain in this season is associated with the passage of cyclonic depressions which originate over the Andaman Sea and manage to cross the eastern coast of the southern Peninsula. These tropical cyclones are very destructive. The thickly populated deltas of the Godavari, Krishna and Kaveri are their preferred targets. Every year cyclones bring disaster here and such cyclonic storms are less frequent in the Arabian Sea. Distribution of rainfall in India: The rainfall across India is uneven. On one hand Cherrapunji in the Northeast India receives an annual average rainfall of 1175 cm which is highest in the world. On the other hand Jaisalmer in western India receives an annual average rainfall of 9cm. That is clearly visible that the amount of rainfall received from place to place differs and at times drastically. Based on the amount of rainfall received India is divided in to four zones: High rainfall zone: rainfall above 200 cm. There are two regions of the country which receive more than 200 cm. One along the west coast on the Western Ghats and two along the Sub-Himalayan areas in the north east. Areas of Medium Rainfall : Rainfall between 100-200 cm is received in the southern parts of Gujarat, east Tamil Nadu, north eastern Peninsula covering Odisha, Jharkhand, Bihar, eastern Madhya Pradesh, northern Ganga plain along the sub-himalayas and the Cachar Valley and Manipur. Areas of Low Rainfall: Western Uttar Pradesh, Delhi, Haryana, Punjab, Jammu and Kashmir, eastern Rajasthan, Gujarat and Deccan Plateau receive rainfall between 50-100 cm. Areas of Inadequate Rainfall: Parts of the Peninsula, especially in Andhra Pradesh, Karnataka and Maharashtra, Ladakh and most of western Rajasthan receive rainfall below 50 cm. Indian Monsoon ( 4 )
The average rainfall across the country is shown in the map right below: El-Nino and Its effects on Monsoon: El-Nino means little boy in Spanish. El-Nino is an Oceanic and Atmospheric phenomenon that leads to unusual warming of water in the Peru Coast. This leads to warm water and low pressure condition develops in the Eastern Pacific (Peru) and Cold condition and high pressure in Western Pacific. Since Pressure is inversely related to the amount of rainfall, El-Nino leads to drought situation in Australia and South East Asia. This condition leads to reversal of pressure difference between Indian and Pacific Ocean known as Southern Oscillation. It is a weather system which re- emerges after a gap of about two to five years in the Pacific Ocean and its effects lasts about 12 months on an average. This phenomenon affects rainfall in India during the monsoon months. Trade winds normally blow westward from South America towards Asia during Indian monsoon months. Warming of the Pacific results in weakening of these winds. Moisture and the heat content thereby, gets limited and results in reduction and uneven distribution of rainfall across the Indian sub-continent. The most prominent droughts in India since 1871 have been El Nino triggered droughts, including the recent ones in 2002 and 2009.But during 1997-98 was a strong El Nino year but that did not cause drought in India, in fact, rainfall was in excess. On the other hand, a moderate El Nino in 2002 resulted in one of the worst droughts. From the data it is clear that El Nino adversely effects rainfall in India but it is also important to note that El Nino always doesn t lead to droughts. When poor monsoon or drought like situation happens it leads to decline in agriculture productivity which in turn leads increase in food inflation and eventually increase in commodity prices. During this situation the Government should ensure availability of sugar, pulses, edible oil. Put restrictions on exports and preventing them from hoarding. Indian Monsoon ( 5 )