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Understanding Indian Monsoon

Context

The Indian Meteorological Department has announced normal monsoon this year in 2017 after two drought years in 2014 and 2015, with respite in 2016.

The fact that Indian agriculture is largely dependent on the monsoon reveals the good prospects for Indian economy ahead in the year given agriculture being the base.

Simultaneously, it will help us deal with the water scarcity problems and regional disparities (disputes between states on river waters) and find the solutions thereof when the water is relatively abundant.

The effects on ecology and hydropower potential are also obvious.

Such is the aura of the monsoon and is therefore rightly called the unofficial “Finance Minister” of India.

Geographical factors involved

  • The monsoon which was bewildered by the El-Nino in the drought years of late is insignificant actor this year.

Let us examine the inter-relationship between the mentioned geographical processes.

  • As the summersets in and the sun shifts northwards, the windcirculation over the subcontinent undergoesa complete reversal at both, the lower as wellas the upper levels.
  • By the middle of July, thelow pressure belt nearer the surface [termedas Inter Tropical Convergence Zone (ITCZ)]shifts northwards, roughly parallel to theHimalayas between 20° N and 25° N.
  • By thistime, the westerly jet stream withdraws fromthe Indian region.
  • In fact, meteorologists havefound an interrelationship between thenorthward shift of the equatorial trough (ITCZ)and the withdrawal of the westerly jet streamfrom over the North Indian Plain. It is generallybelieved that there is a cause and effectrelationship between the two.
  • The ITCZ beinga zone of low pressure, attracts inflow of windsfrom different directions. The maritime tropicalairmass (mT) from the southern hemisphere,after crossing the equator, rushes to the lowpressure area in the general southwesterly
  • It is this moist air current which ispopularly known as the southwest monsoon.
  • An easterly jet stream, formed only at the level of troposphere, flows over the southern part of the Peninsula in June, andhas a maximum speed of 90 km per hour. In August, it is confined to 15° Nlatitude, and in September up to 22° N latitudes.The easterlies normally do not extend to the northof 30° N latitude in the upper atmosphere.
  • Theeasterly jet stream steers the tropicaldepressions into India. These depressions playa significant role in the distribution of monsoonrainfall over the Indian subcontinent. Thetracks of these depressions are the areas ofhighest rainfall in India. The frequency at whichthese depressions visit India, their directionand intensity, all go a long way in determiningthe rainfall pattern during the southwestmonsoon period.

Mechanism of monsoon

  • Towards the end of the nineteenth century, itwas believed that the differential heating ofland and sea during the summer months isthe mechanism which sets the stage for themonsoon winds to drift towards the
  • During April and May when thesun shines vertically over the Tropic of Cancer,the large landmass in the north of Indian oceangets intensely heated. This causes theformation of an intense low pressure in thenorthwestern part of the subcontinent.
  • Sincethe pressure in the Indian Ocean in the southof the landmass is high as water gets heated slowly, the low pressure cell attracts thesoutheast trades across the Equator. Theseconditions help in the northward shift in theposition of the ITCZ.
  • The southwest monsoonmay thus, be seen as a continuation of thesoutheast trades deflected towards the Indiansubcontinent after crossing the Equator.
  • Thesewinds cross the Equator between 40°E and60°E longitudes. The shift in the position of the ITCZ is alsorelated to the phenomenon of the withdrawalof the westerly jet stream from its position overthe north Indian plain, south of the Himalayas.
  • The easterly jet stream sets in along 15°Nlatitude only after the western jet stream haswithdrawn itself from the region. This easterlyjet stream is held responsible for the burst ofthe monsoon in India.
  • The southwestmonsoon sets in over the Kerala coast by 1stJune and moves swiftly to reach Mumbai andKolkata between 10th and 13th June. By mid July, southwest monsoon engulfs the entire
  • There seem to be two rain-bearing systems in First originate in the Bay of Bengal
    causing rainfall over the plains of north India. Second is the Arabian Sea current of the southwest monsoon which brings rain to the west coast of India.
  • Much of the rainfall along theWestern Ghats is orographic as the moist air isobstructed and forced to rise along the Ghats.
  • The intensity of rainfall over the west coast ofIndia is, however, related to two factors:
    (i) The offshore meteorological conditions.
    (ii) The position of the equatorial jet stream along the eastern coast of Africa.
  • The frequency of the tropical depressionsoriginating from the Bay of Bengal varies fromyear to year. Their paths over India are mainlydetermined by the position of ITCZ which isgenerally termed as the monsoon trough.
  • Asthe axis of the monsoon trough oscillates, thereare fluctuations in the track and direction ofthese depressions, and the intensity and theamount of rainfall vary from year to year.
  • Therain which comes in spells, displays a decliningtrend from west to east over the west coast, andfrom the southeast towards the northwest overthe North Indian Plain and the northern partof the Peninsula

What is El- Nino?

  • EI-Nino is a complex weather system that appears once every three to seven years, bringing
    drought, floods and other weather extremes to different parts of the world.
  • The system involves oceanic and atmospheric phenomena with the appearance of warm currents off the coast of Peru in the Eastern Pacific and affects weather in many places including India.
  • EI-Nino is merely an extension of the warm equatorial current which gets replaced temporarily by cold Peruvian current or Humbolt current.
  • This current increases the temperature of water on the Peruvian coast by 10°C. This results in:
    (i) the distortion of equatorial atmospheric circulation;
    (ii) irregularities in the evaporation of sea water;
    (iii) reduction in the amount of planktons which further reduces the number of fish in the sea.
  • The word EI-Nino means ‘Child Christ’ because this current appears around Christmas in December. December is a summer month in Peru (Southern Hemisphere).
  • EI-Nino is used in India for forecasting long range monsoon rainfall. In 1990-91, there was a wild EI-Nino event and the onset of southwest monsoon was delayed over most parts of the country ranging from five to twelve days.

Issues associated and solutions

  • Agricultural sector is the highest user of the water resources in India and this therefore warrants serious attention provided its productivity is still very meagre.
  • The new irrigation technologies like drip irrigation etc against the running pump water are required. The water harvesting infrastructure also needs revamping through scientific planning.
  • The concept of “inter-linking of rivers”, notwithstanding its ecological impacts, has to be considered in the light of regional variations in the vast physiographic landmass of India which evokes political conflicts.
  • Lastly, the behavioural changes at the level of individuals can be ingrained through the Swatch Bharat programme to contain the waste of fresh-water.

Practice Questions

  1. Describe the mechanism of the monsoon in Indian subcontinent. Explain.
  2. Why is the Indian agriculture still dependent on the vagaries of the monsoon? Enumerate the factors which perpetuate such dependence. Also briefly point out what can be done.
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