Why late onset is not a worry

Context: While the IMD has predicted that the monsoon will be delayed this year, that’s not a cause for concern. Regional variations in rainfall, extreme rainfall events, and the developing El Nino are bigger worries.

What is onset of monsoon?

The monsoon season in India typically lasts from June to September, although its timing and intensity can vary across different regions.

The monsoon in India is primarily influenced by the seasonal reversal of winds, known as the Indian Ocean Dipole (IOD), and the movement of the Inter-Tropical Convergence Zone (ITCZ). 

  • Pre-monsoon period (March to May): During this time, temperatures rise across the country due to the increasing solar radiation. As summer approaches, the landmass of the Indian subcontinent heats up faster than the surrounding oceans, causing a low-pressure area to develop over the region.
  • Arrival of the southwest monsoon (end of May to mid-June): The southwest monsoon is responsible for the majority of the rainfall in India. It begins with the onset of the monsoon over the Andaman Sea and the Bay of Bengal. Moisture-laden winds from the Indian Ocean are drawn towards the low-pressure area over the Indian subcontinent, creating a monsoon trough.
    • Factors affecting onset of monsoon:
      • Intense heating of the Indian landmass and formation of intense low pressure.
      • Shifting of ITCZ over Gangetic plain.
      • Withdrawal of sub-tropical westerly Jetstream.
      • Deflection of SE trades after crossing the equator towards Indian west coast.
  • Advancement of the monsoon (June to July): The monsoon winds gradually advance across the country, starting from the southernmost state of Kerala and progressing northwards. This northward progression is known as the “monsoon onset line” and is closely monitored by meteorological departments.
  • Onset over different regions: The onset of the monsoon occurs at different times across various regions of India. The western coast and northeastern states receive the monsoon rains first, followed by the central and northern parts of the country. The Himalayan region experiences the monsoon last.
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  • Monsoon progression and rainfall: Once the monsoon sets in, it brings heavy rainfall to different parts of India. The amount and distribution of rainfall vary from region to region. The western coast and north eastern states generally receive more rainfall compared to the arid regions in the northwest.

What is Indian ocean dipole?

IOD measures differences in sea surface temperatures between the western and eastern parts of the Indian Ocean. It is basically like the El Nino weather system that develops in the Pacific Ocean. It is characterized by an irregular oscillation of sea-surface temperatures in the eastern and western Indian Ocean

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Impact on weather patterns: IOD alters the wind, temperature, and rainfall patterns in the Indian Ocean region. 

  • Positive IOD event is known to bring floods to eastern Africa and droughts and bushfires to eastern Asia and Australia. Ex. 2020 Australian Bushfires. 
  • Positive IOD is known to increase the intensity of Monsoon in the Subcontinent and leads to above normal rainfall. A simultaneous occurrence of Positive IOD and El Nino balances the negative impact of El Nino on the Indian Monsoon rainfall. Ex. above normal rainfall in India in 2019. 
  • In contrast, Negative IOD coupled with El – Nino leads to poor Monsoon rainfall. Ex. Deficient rainfall in 1992.

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