WHY IN NEWS ?
- The India Meteorological Department (IMD) on Thursday declared the onset of monsoon over Kerala.
HOW ONSET OF MONSOON IS DECLARED ?
- Onset is declared when out of 14 selected meteorological stations on the Kerala coast, at least 60 per cent, that is at least nine of them, get at least 2.5 mm rainfall over two consecutive days.
- The onset marks the formal start of the four-month monsoon season that brings about 75 per cent of India’s annual rainfall.
- There are a few other accompanying criteria, relating to wind speed and pressure, which also need to be satisfied simultaneously.
MORE ABOUT 2023 ONSET OF MONSOON:
- This onset is likely to be similar to a ‘soft landing’.
- Monsoon is expected to remain weak for at least one more week, thanks to the development of cyclone Biparjoy in the Arabian Sea.
- The cyclone is also most likely to prevent the strengthening of the monsoon system, and further delaying its penetration over the Indian mainland.
- The northwards progression of the monsoon is unlikely to happen in the normal fashion till the cyclone subsides next week.
- In normal course, monsoon spreads through most of southern India, and at least half of Maharashtra by June 10.
- As of now, however, the northern limit of monsoon continues to remain as far in the south as central Sri Lanka.
ABOUT INDIAN MONSOON :
- A monsoon is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation.
- It is also used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ) between its limits to the north and south of the equator.
- Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern, although technically there is also a dry phase.
- The term is also sometimes used to describe locally heavy but short-term rains.
MECHANISM OF INDIAN MONSOON:
- As the high-sun season moves northward during April, India becomes particularly prone to rapid heating because the highlands to the north protect it from any incursions of cold air.
- There are three distinct areas of relative upper tropospheric warmth—namely, (1) above the southern Bay of Bengal, (2) above the Plateau of Tibet, and (3) across the trunks of the various peninsulas that are relatively dry during this time.
- These three areas combine to form a vast heat-source region.
- The relatively warm area above the southern Bay of Bengal occurs mostly at the 500–100-millibar level.
- The dry surface of Tibet (above 4,000 metres [13,100 feet]) absorbs and radiates heat that is readily transmitted to the air immediately above.
- At about 6,000 metres (19,700 feet) an anticyclonic cell arises, causing a strong easterly flow in the upper troposphere above northern India.
- The subtropical jet stream suddenly changes its course to the north of the anticyclonic ridge and the highlands, though it may occasionally reappear southward of them for very brief periods.
- This change of the upper tropospheric circulation above northern India from westerly jet to easterly flow coincides with a reversal of the vertical temperature and pressure gradients.
- India’s inverted triangular shape, the land is heated progressively as the sun moves northward.
- This accelerated spread of heating, combined with the general direction of heat being transported by winds, results in a greater initial monsoonal activity over the Arabian Sea.
ITCZ AND INDIAN MONSOON:
- The Inter-Tropical Convergence Zone is where these winds collide (ITCZ) and it is the area of ascending air, the greatest clouds, and heavy rainfall.
- During the changing of the seasons, the location of the ITCZ varies north and south of the equator.
- The Monsoon Front is the boundary between the south-west monsoons and the north-east trade winds (ITCZ). Along this front, rain falls.
- The ITCZ shifts to 20°- 25° N latitude in July, where it is positioned in the Indo-Gangetic Plain, and the south-west monsoons blow from the Arabian Sea and the Bay of Bengal.
BURST OF INDIAN MONSOON:
- The term “monsoon break” or “burst” refers to the abrupt onset of moisture-laden winds accompanied by strong thunder and lightning.
- The monsoon may come in the first week of June in coastal parts of Kerala, Karnataka, Goa, and Maharashtra, and in the first week of July in the interior.
- The daily temperature drops by 5°C to 8°C between mid-June and mid-July.
- As these winds approach the land, the relief and thermal low pressure over northwest India change their southwesterly course.
IMPACT OF CLIMATE CHANGE ON INDIAN MONSOON:
- Climate models indicate that global warming is expected to increase monsoon rainfall by 14% by the end of the century if greenhouse gas (GHG) emissions remain high.
- The global ocean has absorbed 90% of the excess heat generated by man in the past 50 years due to which extreme rainfall events of the sort seen in Maharashtra would become very common.
- Impact of Climate Change Delayed Monsoon Withdrawals: Intense warming of Indian peninsula and oceans resulting in the formation of local-low pressure systems are attributed to delayed monsoon withdrawals.
IMPORTANCE OF INDIAN MONSOON :
- The monsoon is responsible for over 70% of India’s annual rainfall.
- It influences the yield of a wide range of cereals and pulses, including rice, wheat, and sugarcane.
- A bad monsoon in India results in decreased income and affects the livelihood of many people as a major amount of them work in the agriculture field.
- A good monsoon in India can result in increased amounts of hydroelectric power generation which accounts for at least 25% of India’s electricity generation.
- Agriculture employs more than half of India’s people, and monsoon rains have a direct impact on their wages and well-being.
- Agriculture contributes to more than 15% of India’s GDP, hence crop failure due to a lack of rain has a negative impact on the economy.
- Monsoonal rainfall helps with water storage for irrigation, electricity generation, and drinking.
SYLLABUS: MAINS, GS-1, GEOGRAPHY
SOURCE: BRITANNICA, IMD, THE HINDU