Rajya Sabha TV In Depth – The Science of Monsoon
Monsoon is the lifeline of Indian economy as 2/3rd of it depends on farm income and rain is the only source of irrigation for over 40% of the country’s cropped area. Over 70% of India’s annual rainfall occurs in July-September monsoon season. A good monsoon increases crop productivity, raises farm income and drives the economy while, a weak monsoon inflates food prices and harms the economy.
- The word “monsoon” is derived from Arabic word mawsim and/or Hindi mausam.
- Monsoon refers to the seasonal reversing wind accompanied by precipitation. It occurs due to a difference in temperature between the landmass and the ocean. Major monsoon of the world are West African and Asian-Australian monsoons.
Map of India showing monsoon advancement (source: NCERT)
In Indian context:
- Southwest monsoon/ Indian monsoon: Indian subcontinent has a large heated landmass during the summer months (April to September), and a cooler water mass in the form of Indian Ocean. This causes a temperature difference, which creates a pressure gradient and drives moisture laden winds over Indian subcontinent. Impact of this monsoon is felt in India, Sri Lanka, Bangladesh and Myanmar and as far north as in China’s Xinjiang. It has two branches:
- The Arabian Sea branch: It first hits the Western Ghats and Kerala gets the first rain in India. It causes rain in the coastal areas of Konkan and Goa
- as it moves northwards, but eastern part of the Western Ghats do not receive much precipitation.
- The Bay of Bengal Branch: It picks up moisture-laden winds from Bay of Bengal and moves towards North-East India and Bengal. The Himalayas act as a barrier for them and thus, rain occurs in Indo Gangetic plains, Meghalaya, etc.
- Northeast monsoon/ Retreating monsoon: The cycle is reversed during colder months (October to April). Wind blows from cooler landmass (Himalayas and Indo-Gangetic plain) towards Indian ocean (south of Deccan). This causes precipitation over the oceans and in regions like Tamil Nadu.
Theory of Monsoon:
- During summer months in the Northern Hemisphere, the ITCZ (Intertropical Convergence Zone) shifts North, pulling southwest monsoon winds onto the land from the sea. Huge landmass of the Himalayas restricts the low-pressure zone onto the Himalayas themselves. When Tibetan plateau heats up more than the Himalayas, the ITCS abruptly and swiftly shifts North. This leads to bursting of monsoon rains over the Indian subcontinent.
- A reverse shift takes place for the Northeast monsoon winds causing minor rainfall over the eastern Indian Peninsula during the Northern Hemisphere winter months.
- The rainfall caused due to Southwest monsoon is a type of orographic rainfall. It occurs when masses of air are pushed by wind upwards along the side of elevated landforms. This results in adiabatic cooling and ultimately condensation and precipitation. Along the leeward side rain shadow is observed.
Diagram of orographic rainfall
Other types of Monsoons:
- Asian-Australian Monsoon: It stretches from North Australia to Russian Pacific coast and stretches into Indian ocean.
- North American Monsoon: Warm, moist airs from Gulf of California blowing northeast and from Gulf of Mexico blowing northwest meet over Sierra Madre Occidental mountains in central Mexico. This brings moisture to the mountain ecosystem and continues North to Arizona, New Mexico and Texas.
- Southwest monsoon accounts for over 80% of rainfall in India. Monsoon impacts Indian economy- agriculture, industry and society as a whole. It brings respite from heat and transforms large part of semi-desert areas into green land. Crops specially like cotton, rice, oilseeds, and coarse grains depends heavily on rains.
- Monsoons are often associated with conditions like ‘El Nino’ (Spanish for ‘Little Boy’) that occurs every 2 to 7 years and La Nina. It is caused by unusual warming in Eastern tropical Pacific Ocean and reversal of prevailing winds in the region. El Nino can trigger above average rains in northern Peru & draught in Southeast Asia, Australia and India.
Thus, monsoon which causes rainfall every year in India due to changing wind directions drives its economy and its timely and adequate occurrence is very important. But, reports of floods and draughts in different regions of the countries due to their erratic patterns in recent past is a cause of concern and, effects of climate change on its changing patterns needs to be studied.