The Indian Monsoon

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  • Monsoon - 20° N and 20°S.
  • Differential heating and cooling of land and water. Landmass – Low pressure, seas – High pressure.
  • Shift of the position of Inter Tropical Convergence Zone (ITCZ) – Ganga Plane.
  • High-pressure area, east of Madagascar. 20°S over the Indian Ocean, high-pressure area affects the Indian Monsoon.
  • Tibetan plateau gets intensely heated during summer.
  • Movement of the westerly jet stream to the north of the Himalayas and the presence of the tropical easterly jet stream over the Indian peninsula during summer.
  • Changes in the pressure conditions over the southern oceans also affect the monsoons. tropical eastern south Pacific Ocean experiences high pressure. Tropical eastern Indian Ocean experiences low pressure. certain years, there is a reversal in the pressure conditions and the eastern Pacific has lower pressure in comparison to the eastern Indian Ocean.
  • Southern Oscillation or SO - periodic change in pressure conditions.
  • A feature connected with the SO is the El Nino phenomenon in which a warm ocean current that flows past the Peruvian Coast, in place of the cold Peruvian current, every 2 to 5 years. Changes in pressure conditions are connected to the El Nino.
  • phenomenon is referred to as ENSO (El Nino Southern Oscillations).
Nature of Indian Monsoon
The Onset of the Monsoon and Withdrawal
  • 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 subcontinent.
  • April and May when the sun shines vertically over the Tropic of Cancer, the large landmass in the north of Indian ocean gets intensely heated.
  • Causes the formation of an intense low pressure in the northwestern. pressure in the Indian Ocean in the south of the landmass is high as water gets heated. slowly, the low pressure cell attracts the southeast trades across the Equator.
  • Northward shift in the position of the ITCZ. cross the Equator between 40°E and 60°E longitudes.
  • Not steady winds but are pulsating in nature. duration of the monsoon is between 100-120 days from early June to mid-September.
  • Rainfall increases suddenly and continues constantly for several days – ‘burst’ of the monsoon. distinguished from the pre-monsoon showers.
  • Monsoon arrives at the southern tip of the Indian peninsula. first week of June. Arabian Sea branch reaches Mumbai about ten days later on approximately the 10th of June & Bay of Bengal branch also advances rapidly and arrives in Assam in the first week of June.
  • Lofty mountains, monsoon winds to deflect towards the west over the Ganga plains.
  • Mid-June the Arabian Sea branch of the monsoon arrives over Saurashtra-Kuchchh and the central part of the country.
  • Arabian Sea and the Bay of Bengal branches of the monsoon merge over the northwestern part of the Ganga plains.
  • Delhi generally receives the monsoon showers from the Bay of Bengal branch by the end of June (tentative date is 29th of June)
  • First week of July, western Uttar Pradesh, Punjab, Haryana and eastern Rajasthan experience the monsoon. mid-July, the monsoon reaches Himachal Pradesh and the rest of the country.
Withdrawal or the retreat of the monsoon is a more gradual process.
  • Easterly jet stream sets in along 15°N latitude only after the western jet stream has withdrawn.
  • Withdrawal of the monsoon begins in northwestern states of India by early September. Mid-October, withdraws completely from the northern half of the peninsula.
  • Southern half of the peninsula is fairly rapid. Early December, the monsoon has withdrawn from the rest of the country.
  • Islands receive the very first monsoon showers from south to north last week of April to the first week of May.
  • Withdrawal, north to south from the first week of December to the first week of January.
Rain-bearing Systems and Rainfall Distribution.
  • First originate in the Bay of Bengal causing rainfall over the plains of north India.
  • Second is the Arabian Sea current of the south west monsoon which brings rain to the west coast of India.
  • Western Ghats is or ographic as the moist air. Two factors
    • Offshore meteorological conditions.
    • position of the equatorial jet stream along the eastern coast of Africa
Break in the Monsoon
Rain fails to occur for one or more weeks, it is known as break in the monsoon.
  • If the rain-bearing storms are not very frequent along the monsoon trough or the ITCZ.
  • West coast the dry spells are associated with days when winds blow parallel to the coast

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