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DAILY NEWS ANALYSIS

  • 21 September, 2021

  • 30 Min Read

Monsoon in India - UPSC

South West Monsoon in India (Jun, July, Aug, Sep):

  • Definition: It is the seasonal reversal in the wind direction. It is experienced in tropical areas (20°N to 20°S). In India, it is experienced in June, July, August and September.

5 Key players in Monsoon:

1) Differential heating and cooling of land (LP) and Sea (HP) in summer.

2) Northward shifting of ITCZ in July over Ganga plains

  • ITCZ is a low pressure zone located at the equator where trade winds converge.
  • This leads to the formation of thermal low over North and Northwest India.
  • Southeast Trade winds from S hemisphere cross the Equator (40°E to 60°E of longitude) and turn right due to Coriolis force in the SW direction.
  • It is also called as Monsoon trough during monsoon year.
  • The location and shift of ITCZ governs the breaks and pulsating nature of Monsoons.

3) The presence of High Pressure (HP) area over Madagascar (20°S of Indian Ocean). The intensity and position of this HP area affects Indian monsoon.

4) Tibetan plateau gets intensely heated

  • This results in strong vertical air currents and the formation of High Pressure over the plateau @ about 9 km above sea surface.
  • This is Tropical Easterly Jetstream which is associated closely with the burst of monsoon.
  • Tropical Easterly Jetstream is also a major reason why there are no cyclones during Monsoon because the presence of an Easterly jet over the Indian landmass in the upper troposphere prevents vertical circulation of air...which is a pre-condition for formation of cyclones.

5) Northward shifting of Sub Tropical Westerly Jet Stream (STWJ) over North of Himalayas:

Jet Stream Theory: Origin of Monsoon

(Reason for the sudden burst of Monsoon)

  • STWJ is a band of fast moving air from West to East usually found in mid-latitudes in the upper troposphere @ the height of 8-15km
  • During Winter, STWJ shifts Southwards. It gets bifurcated coz of Himalayas and 1 branch blows South of Himalayas. There is a presence of HP belt over the Northwest India and Low Pressure over Indian Ocean.
  • Presence of STWJ south of Himalayas also induces HP belt in the upper troposphere. Consequently wind direction is from SE to NW.
  • During summer, sun is in Northern Hemisphere, Northwest India is intensely heated. Therefore, LP belt is created over the surface of Northwest India.
  • Indian Ocean is comparatively cooler and has a HP belt. Ideally, wind should blow from sea to land but there is still presence of STWJ to the South of Himalayas.
  • HP belt over the upper troposphere prevents the upward movement of air from the surface LP and thus, the monsoon is not yet activated.
  • By the start of June STWJ is moved to N of Himalayas and thus upper tropospheric HP is removed. This induces a rapid movement of wind from sea towards land, giving the sudden "burst" or "break" of monsoon.

6) The presence of Tropical Easterly Jetstream over peninsula in summer. It is associated closely with the burst of Monsoon.

  • Role of Tibetan Plateau: It is the highest & largest plateau of the world w avg height of 4000 m and is surrounded by even higher mountain ranges.
  • In summer, Tibetan Plateau acts as a heat source. Air above it is heated & is warmer compared to the surrounding air of mountain ranges. Warm air rises above and creates HP belt in the upper air above Tibetan plateau. Air spreads from HP belt and sinks over Indian Ocean around 30°S and 70°E.
  • This movement from Tibet to IO is known as Tropical Easterly Jet (TEJ). This jet drives monsoonal winds towards Indian subcontinent and intensifies SW monsoon.
  • The area over IO where TEJ sinks is known as Mascarene High.
  • It is a major reason why there are no cyclones over the Indian landmass. It is present in the Upper troposphere & prevents d vertical circulation of air which is a pre-condition for formation of cyclone.

7) . S-E trade winds from Southern hemisphere cross the equator and turn right due to coriolis force.

El Nino/La Nina and the Indian Monsoon

  • EI-Nino is a complex weather system that appears once every 3-7 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 (locate these currents in your atlas).
  • This current increases the temperature of water on the Peruvian coast by 10°C. This results in:
  1. the distortion of equatorial atmospheric circulation;
  2. irregularities in the evaporation of sea water;
  3. 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.

Retreating Southwest (SW) Monsoon Season (Oct, Nov): Retreat of Monsoon and onset of the NE Monsoon:

  • Unlike sudden burst of SW monsoon, retreat is gradual.
  • It starts retreating from North India by the 2nd week of Sep. By the end of Sep, SW monsoon becomes weak as the LP trough (ITCZ) of Ganga plain starts moving southwards in response to southwards in response march of the Sun.
  • The monsoon retreats from the Western Rajasthan by the 1st week of Sep. It withdraws from Rajasthan, Gujarat, Western Ganga plains and Central Highlands by the end of month.
  • By beginning of Oct, LP covers Northern Bay of Bengal and by early Nov, it moves over KN and TN. By mid Dec the ITCZ is completely removed from the Peninsula.
  • It is marked by clear skies and rise in temp. The land is still moist. Owing to the conditions of HT and humidity, the weather becomes rather oppressive. This is commonly known as ‘October Heat’. In the 2nd ½ f Oct, temp ↓ esp in NI.

Tropical depressions or Bay Cyclones in late monsoons:

Why is the retreating monsoon season dry in North India and rain in the Coromandal Coast in Oct & Nov?

  • It is associated w the passage of cyclonic depressions which originate over Andaman Sea and manage to cross the Eastern coast of S peninsula. These tropical cyclones are very destructive.
  • The thickly populated deltas of Godavari, Kaveri and Krishna are their preferred targets. Every year cyclone brings disaster here.
  • A few cyclonic storms also strike the coast of WB, Bangladesh and Myanmar. Such cyclones are less frequent in Arabian Sea. Why?

Why do the cyclones in Arabian Sea not affect India?

  • Tropical cyclones move from east to West.....they are secondary circulations and maintain the larger direction of the planetary winds (i.e Trade winds which blow from East to West).
  • So, any cyclones to form in Arabian Sea are less likely to affect India.
  • Also, another reason that can be given is that the findlater jet of the coast of Somalia causes coastal upwelling and hence cooling of waters in the western Indian ocean and cooler temperatures would not support the formation of tropical cyclones.
  • However, please note that it would happen only if the Findlater Jet is very strong.

What is the difference between Retreating Southwest Monsoon and Northeast Monsoon?

Retreating South West Monsoon

Northeast Monsoon

  • Blow in Oct, Nov
  • Season of transition between hot, rain season and cold, dry season.
  • Characterised by oppressive heat and humidity called ‘October Heat’.
  • They blow from SW direction but are not strong enough to blow right into Northern plain.
  • They withdraw in stages which result in decreasing rain.

  • Dec, Jan, Feb
  • Cold Weather Season.
  • This is very pleasant season with low temperature, low humidity and clear skies.
  • These winds blow from NE direction from land to sea.
  • They do not give rain to any part of India except TN coast.

Mechanism of Monsoon

It can be understood in 4 different points:

1) Onset and Landward advance of Monsoon from Kerala:

  • Differential heating of land (LP) and sea (HP) in summer.
  • Northward shift of ITCZ.
  • Northward shifting of STWJ.
  • Heating of Tibetan plateau and inducement of TEJ.

2) Rain bearing systems:

  • Monsoon rainfall enters in India thr’ 2 branches, Arabian Sea branch of SW Monsoon brings rain to W coast and further interiors of Peninsula (It is related to offshore meteorological conditions along eastern coast of Africa) and BoB Bengal branch gives rainfall over Eastern, NE and NI. Tropical depressions originating in BoB cause rainfall in Northern plains.
  • Arabian Sea Branch of SW Monsoon: The total volume is 3 times more than BoB branch. It approaches Southwest coast of India by the 1st week of June. They further split into 3 branches:
  1. Western Ghats (WG):

  • The high WG leads the moist air to rise quite high. Heavy rain (200-250 cms) falls on the windward slopes of WG.
  • The leeward side receives only 60 cm rainfall & rain shadow areas receive much less.
  • The winds progressively deposit less rainfall from W to E Ex. Mumbai-200 cm & Chennai-40 cm and from S to N Ex. Thiruvan-puram-325 cm, GA-300 cm & Mumbai-200 cm.
  • The intensity of rainfall over the west coast of India is related to 2 factors: The offshore meteorological conditions and The position of equatorial jet stream along the eastern coast of Africa.
  1. North of Mumbai: After crossing the Ghats the monsoon winds cross the plateau & reach MP, JH and OD giving rainfall. Part of this wind enters through Narmada valley & causes good rainfall in the Chota Nagpur plateau. Further North, a part of these winds passes over West and Southeast Rajasthan without depositing much rainfall and goes straight to the sub-Himalayan region giving rainfall to the foothills of the W Himalayas, eastern PN, HR and NE RJ where it meets the BoB branch.

  2. Saurashtra Peninsula and Kachch: Aravallis runs almost parallel to the direction of this branch and do not form a barrier to intercept the winds, and hence do not receive much rainfall from this branch.

  • Bay of Bengal branch of SW Monsoon:

  1. These winds pass over Ganga and Brahmaputra delta strike against Shiwalik ranges, Himalayas, Chittagong and Assam hills, then tise and cause heavy rainfall in WB, SK, AR and southern slopes of Khasi-Jaintia hills. Cherrapunji (1250 cm) receives the highest rainfall in the World.
  2. 1 branch of BoB branch is deflected in Western direction, blowing from SE, between Himalayas and Peninsula. The winds move up in the Indo gangetic plain parallel to Himalayas.
  3. The rainfall decreases up to Kashmir from Ganga plains (250 cm). A small amount of ppt is received on the E Aravalli slopes as they form a barrier, but Thar on the West side lies in the rain shadow for the SW monsoon BoB branch and receives little or no rainfall.
  4. During monsoon, strong winds and wet spells last for a few days followed by a period of weak winds or monsoon lull. This leads to dry spells between two wet spells.
  5. The normal duration of monsoon in most parts of India is 100 days to 120 days but in RJ it is 45-50 days, while it is max of 6 months in Kerala. Temp are < in d wet season due to cloudiness & rainfall.

Why Tamil Nadu dry during this season?

  • TN coast is parallel to BoB branch of SW monsoon.
  • It lies in the rain shadow region of Arabian Sea branch of SW Monsoon.

3) Break in the Monsoon:

  • During SW monsoon, if rain fails to occur for 1 or more weeks, it is known as break in the monsoon. These breaks are quite common during rainy season. The location and shift of ITCZ governs the breaks and pulsating nature of Monsoons. These breaks in diff regions are due to different reasons:
  1. In NI, rains are likely to fail if the rain-bearing storms are not very frequent along monsoon trough or the ITCZ over this region.
  2. Over the W coast, the dry spells are associated with the days when winds blow parallel to the coast.
  3. Dry spells occur over Western RJ due to thermal conditions in the lower atmosphere, such as Inversion of temp.
  4. Cyclonic originating head of BoB and their crossing into the mainland may result in BoB branch of SW monsoon winds getting drawn into cyclonic depression resulting in dry spells in SE parts of Ganga plains.
  • Position of ITCZ over Northern plain, called as monsoon trough determines tracks of tropical depressions. As the axis of monsoon trough oscillates, the tracks of these depressions also vary. This cause wide fluctuation in the direction and path these depressions take intensity of rainfall a.w.a variations in the amount of rainfall from year to year. If the trough lies close to Himalayas. It causes heavy rainfall in the foothills which results in widespread floods and prolonged dry spells in the plains. If the LP trough lies over Ganga plain, it results in heavy rains in the plains and dry spells in the foothills

4) Retreat of Monsoon and onset of the NE Monsoon.

Completed above.

What are the characteristics of Monsoonal Rainfall?

  • Rainfalls received from southwest monsoon is seasonal in character. June - Sep.
  • Governed by Relief or Topography.
  • Monsoon rainfall decreases from the increasing distance from the sea.
  • Monsoon rains occur in wet spells of few days. Then Breaks related to cyclonic disturbances over BoB and their crossing into mainland.
  • Heavy downpour leads to runoff and soil erosion.
  • 75% of the total rain received during SW monsoon. Agri dependent on it.
  • Uneven spatial distribution.
  • Rain sometimes end considerably earlier than usual causing great damage to standing crops and making the sowing of winter crops difficult.

Source: NCERT and Aspire IAS Notes


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26 Oct,2021

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