All eyes on monsoon as cooler summer delays crucial pattern

Context: The large parts of country have logged temperatures that are up to 10°C lower than normal due to rains. An uncharacteristically cooler start to the summer, which is likely to persist for a few more weeks in several parts of the country, may hurt the arrival of the crucial monsoon season, weather scientists have said, at a time when the rainy season is expected to anyway be sapped by the Pacific warming phenomenon El Nino.

How lower temperatures are going to affect monsoon?

  • Relative thermal differences create pressure differences which play very significant role in the onset of monsoon. 
  • More heating over northwest India helps with the onset of monsoon. When the ground heats up, especially in heatwave prone parts of northwest India (which includes Rajasthan, Haryana and Delhi), it creates an area of low pressure. There is something called the heat low which forms over northwest India.
  • Hence low temperatures over NW part will lower the advance of monsoonal winds. 

What is heat low? What are its impacts on monsoon rainfall? 

  • During the northward march of sun in northern hemisphere, the continent surrounding the Arabian Sea begin to receive large amounts of heat; not only in the form of radiation from sun, but also flux of heat from the earth’s surface into atmosphere. 
  • As a result of this large input of power, trough of low pressure forms over this region. 
  • Intense heat low acts as suction devise for moist air along the monsoon trough and to some extend related to good monsoon over India. 
  • During weak heat low monsoon rainfall over India is greatly affected and results in deficient or scanty rainfall over vast area of country.
image 1

What is the role of monsoon trough? 

  • Monsoon Trough is an elongated low-pressure area which extends from heat low over Pakistan to Head Bay of Bengal. 
  • This is one of semi-permanent feature of monsoon circulation. 
  • Monsoon trough may be a characteristic of east west orientation of Himalayan ranges and north south orientation of Khasi-Jaintia Hills. 
  • Generally eastern side of monsoon trough oscillates, sometimes southwards and sometimes northwards.
Shifting positions of monsoon troughImpacts on distribution of rainfall across India
image3• When we say that the trough is in normal position, that means it passes through Ganganagar, Allahabad, Calcutta and Head Bay.

• This gives good rain to Central India.

• As a effect of low formation and it’s passage along the trough, the strong wind currents form over the west coast.

• These currents are blocked by Sahyadri mountains resulting in formation of offshore trough.

• Hence due to this offshore trough west of coast of India receives good rain.

image2• When the trough shifts to the south of it’s normal position, than the monsoon conditions are active giving excessive rainfall in central India and over the west coast of India due to offshore trough.

• This position is best for monsoon condition giving rain to all over Peninsular India.
image4• When the trough shifts to north of normal position it means that it shift to foothills of Himalayas giving good rain to Uttarakhand, Himachal Pradesh, Jammu and to all Northeast states of India.

• This condition is known as “Break Monsoon” where rainfall activity over rest of India ceases.

Practice question

What do you understand by ‘Monsoon Trough’ and how does it affects the rainfall distribution in India? (10 marks, 150 words)

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