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Shifting Monsoon Patterns

11 Nov 2022 Shifting Monsoon Patterns

Posted at 11:18h in Daily current-affairs by
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Significance for Prelims: Onset and Withdrawal of Monsoon; ‘triple dip’ La Nina

Significance for Mains: Monsoon and its Characteristics; Shifting monsoon patterns and its impact. 

News: India has witnessed a 7% monsoon rainfall surplus this year. Rainfall increased substantially in central and southern India. Rainfall in southern and central India was 25% and 20% over average, respectively.

About Monsoon: 

  • The word “monsoon” originated from the Arabic word “mausim,”  meaning “season,”.
  • The seasonal reversal in the wind direction during a year is known as a monsoon.

Causes of Monsoon:

  • Difference between annual temperature trends over land and sea.
  • Shift in Intertropical Convergence Zone or ITCZ towards East Asia, sub-Saharan Africa, Australia and the southern parts of North America. ITCZ are low-pressure zone where the northeast and southeast trade winds converge.

Causes of the Indian monsoon: 

  • Summer in the Indian subcontinent led to the heating of the Thar desert and adjoining areas of the northern and central parts of the Indian subcontinent and this rapid solar heating between April and  May develops a low-pressure zone over the subcontinent. 
  • Moisture-laden winds from India rush in to fill up the void developed due to the creation of a low-pressure zone.  
  • Further,  ITCZ  shifts northwards towards the subcontinent causing monsoon rains in the mainland during the last week of May or the first week of June.

Characteristics of Monsoonal Rainfall

  • Seasonal Nature: Southwest monsoon rainfall mostly occurs between June and September.
  • Relief or topography plays an important role in monsoon rainfall: Western Ghat’s windward side receives rainfall over 250 cm and due to hill ranges and the Eastern Himalayas, North-Eastern states receive heavy rainfall. 
  • Monsoon rainfall depends on the distance from the sea: With the increase in distance from the seas monsoon rainfall declines. During the southwest monsoon period, Kolkata receives 119 cm Patna 105 cm, Allahabad 76 cm and Delhi 56 cm.
  • One of the unique characteristics of monsoon is “breaks”:. The wet spells of monsoon rains having a duration of a few days are interspersed with rainless intervals known as ‘breaks’. These breaks in rainfall mainly formed due to the cyclonic depressions at the head of the Bay of Bengal. 
  • Heavy runoff and soil erosion during the summer rainfall period of monsoon.
  • Agrarian economy of India depends on monsoons as three-fourths of the total rain in the country is received during the southwest monsoon season.
  • Uneven spatial distribution: Spatial distribution of monsoon ranges from 12 cm to more than 250 cm.
  • Sometimes there are considerable delays with respect to the onset of the monsoon and sometimes the rains end considerably earlier than usual. This flip-flop led to great damage to standing crops and difficulty in sowing winter crops. 
  • During the June-September period cyclonic systems from the Arabian Sea and the Bay of Bengal fuel the monsoon.

Onset of the Monsoon: 

  • Monsoon winds drift towards the subcontinent during differential heating of land and sea during the summer months.
  • During April and May,  large north Indian landmass gets intensely heated due to the vertical shining of the sun over the Tropic of Cancer.
  • Outcome of intense heating over Tropic of Cancer: Formation of intense low pressure in the northwestern part of the subcontinent. 
  • Due to the slow heating of water pressure in the Indian Ocean in the south of the landmass is high. Hence, the low-pressure cell attracts the southeast trade winds across the Equator.
  • The above conditions led to a northward shift in the position of the Intertropical Convergence Zone ( ITCZ). Hence, the southwest monsoon is the continuation of the southeast trades deflected towards the Indian subcontinent after crossing the Equator.

Monsoon withdrawal: Cessation of rainfall activity over northwest India for five straight days

  • Monsoon withdrawal is the establishment of anti-cyclone in the lower troposphere and reduction in moisture content. 
  • June 1 is the official IMD monsoon onset date in Kerala. But, mostly Monsoon enters mainland India between the last week of May and the first week of June. IMD counts the rainfall between June 1 and September 30 as monsoon rainfall.
  • But, the Monsoon system continues to pour rain over India even after September 30. Monsoon-related rain continues till the first fortnight of October and it only retreats from India by late October. 
  • In November, this southwest monsoon is replaced by the retreating, or northeast monsoon.
  • Northeast monsoon or retreating monsoon is a key source of rainfall in several parts of Tamil Nadu, Andhra Pradesh and north interior Karnataka.
  • Timeline of monsoon withdrawal:  Rajasthan is the last state where the monsoon reaches, from there it starts its withdrawal. Around September 15, cyclonic systems are replaced by an ‘anti-cyclone’ circulation. Due to ‘anti-cyclone’ circulation dry, windless conditions start to prevail over western and northern India. 
Monsoon this year: 

  • By around surplus of 7%  Monsoon rainfall in India with extreme inequity.
  • Sharp surge in rainfall in Central and southern India: Rains were surplus in Central India and southern India by 20% and 25%  respectively. 
  • Several instances of flooding in Kerala, Karnataka and Madhya Pradesh were seen this year On the other hand, large rainfall deficits were seen in parts of U. P., Bihar, and Odisha. 
  • Shortfall in the east and northeast India is 17%  while northwest India reported a shortfall of 2%.

Change in monsoon patterns: 

  • Barring a slight dip in a monsoon last year monsoon in India has returned to surpluses after 2019. In 2021, rainfall over the country as a whole, was 1% less than normal even though September rainfall was  35% above usual. India posts another year of surplus rain mostly due to vigorous September. 
  • Data suggest that three years of above-normal rain in a block of four years is unprecedented in IMD’s record-keeping history. 

Reasons for excessive rains in southern and central India: ‘triple dip’ La Nina

  • La Nina is the opposite of  El Nino. Surplus rainfall in India is associated with La Ninas while reduced rains over India are linked with El Ninos. 
  • One of the characteristics of La Nina is cooler-than-normal sea surface temperatures in the central Pacific.
  • ‘triple dip’ La Nina is an extended spell of the La Nina. The phenomenon of  ‘triple dip’ La Nina lasts across three winter seasons in the northern hemisphere.
  • Due to the ‘triple dip’ La Nina India is seeing surplus rain in September this year instead of the retreat of the monsoon.  

Impact:

  • Negative impact on sowing of the kharif, or summer crop: Paddy planting has sown area 5.51% lower than last year. 
  • Elevated inflation: The centre is expecting a minimum of a six-million-tonne shortfall in rice production. 

Further readings: 

  1. El Nino and La Nina
  2. Inter-Tropical Convergence Zone (ITCZ)
  3. Jet Streams
  4. EI-Nino and the Indian Monsoon

2011 Prelims Question: 

Q. La Nina is suspected to have caused recent floods in Australia. How is La Nina different from El Nino? 

  1. La Nina is characterized by unusually cold ocean temperature in equatorial Indian Ocean whereas El Nino is characterized by unusually warm ocean temperatures in the equatorial Pacific Ocean.
  2. El Nino has adverse effect on south-west monsoon of India, but La Nina has no effect on monsoon climate.

Which of the statements given above is/are correct?

(a ) 1 only

(b) 2 only

(c) Both 1 and 2

(d) Neither 1 nor 2

UPSC Mains Questions.

Q. What characteristics can be assigned to monsoon climate that succeeds in feeding more than 50 percent of the won population residing in Monsoon Asia? (2017)

Source: The Hindu; The Times Of India; INDIA : PHYSICAL ENVIRONMENT

Article: Explained | Shifting monsoon patterns; Learning with the Times: What causes the monsoon?

Article Link:

1.https://www.thehindu.com/sci-tech/energy-and-environment/shifting-monsoon-patterns/article65935590.ece 

2.https://timesofindia.indiatimes.com/india/learning-with-the-times-what-causes-the-monsoon/articleshow/6156778.cms 

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