Context

• The impact of global warming on the monsoons are visible in the onset, withdrawal, its seasonal total rainfall, and its extremes.
• Global warming also affects the cyclones over the Indian Ocean and the cyclone’s position affect monsoon’s onset.

What do you mean by Monsoon?

• A monsoon is a seasonal reversing wind accompanied by corresponding changes in precipitation and associated with annual latitudinal oscillation of the Intertropical Convergence Zone (ITCZ).
  • ITCZ is the region that circles the Earth and where the trade winds of the Northern and Southern Hemispheres converge – creating a broad trough of low pressure.
  • Seasonal shifts (between equator and tropic of cancer) in the location of the ITCZ with the apparent movement of the sun results in the wet and dry seasons of the tropics.
• Usually, the term monsoon is used to refer to the rainy phase of a seasonally changing pattern.
• On the Indian subcontinent, as the rain bearing winds come from the southwest direction, it is known as the southwest (SW) monsoon.

What is Meant by the “Onset of the Monsoon”?

• The onset of the monsoon over Kerala signals the beginning of the four-month (June-September) southwest monsoon season, during which India gets more than 70% of its annual rainfall.
• Contrary to what is sometimes assumed, the onset does not mean the first rain of the season.
• The IMD does not officially declare onset until certain prescribed conditions are met, like,
  • Rainfall: The onset is declared if at least 60% of 14 designated meteorological stations in Kerala and Lakshadweep record at least 2.5 mm of rain for two consecutive days at any time after May 10.
  • Wind field: The depth of westerlies in the area that is bound by the equator to 10ºN latitude (passes through Kochi), and from longitude 55ºE to 80ºE.
  • Heat: The measure of the energy emitted to space by the Earth’s surface, oceans, and atmosphere in the area between the 5ºN and 10ºN latitudes, and 70ºE and 75ºE longitudes.
• Factors influencing the onset of SW monsoons
  • Intense low-pressure formation over the Tibetan Plateau
  • The permanent high-pressure cell in the South of the Indian Ocean
  • Subtropical jet stream
  • African Easterly jet (Tropical easterly jet)
  • ITCZ

The Impact of Global Warming on Indian Monsoon

• Alarming increase in floods and droughts provides direct evidence of how global warming has been impacting the Indian monsoon.
• While summer monsoon rainfall each year is unique, there was a large regional and temporal variability in rainfall last year.
• There is evidence that global warming increases the fluctuations in the monsoon, resulting in both long dry periods and short spells of heavy rains.
• The monsoon is also affected by the three tropical oceans— Indian, Atlantic, and Pacific; the ‘atmospheric bridge’ from the Arctic; and the oceanic tunnel as well as the atmospheric bridge from the Southern Ocean (a.k.a. the Antarctic Ocean).
  • A ‘bridge’refers to two faraway regions interacting in the atmosphere while a ‘tunnel’ refers to two remote oceanic regions connecting within the ocean.

Cyclone and Impact of Global Warming on Cyclones

• Cyclones are rapid inward air circulation around a low-pressure area.
• The air circulates in an anticlockwise direction in the Northern hemisphere and clockwise in the Southern hemisphere.
• Cyclone are forming in the pre-monsoon season, closer to the monsoon onset, arguably due to the influence of a warmer Arctic Oceanon the winds over the Arabian Sea.

How does a Cyclone’s position affect Monsoon’s onset?

• Some cyclones in the North Indian Ocean have had both positive and negative impacts on the onset of the monsoon.
• Since the circulation of winds around the cyclones is in the anticlockwise direction, the location of the cyclone is critical as far as the cyclone’s impact on the transition of the monsoon trough(a low-pressure region) is concerned.
• If a cyclone lies further north in the Bay of Bengal, the back-winds blowing from the southwest to the northeast can pull the monsoon trough forward, and assist in the monsoon’s onset.
• For example, Cyclone Mochawhich developed in the first half of May and intensified briefly into ‘a super cyclonic storm.’
  • Mocha’s north-west to east trajectory was due to unusual anticyclones which rotate clockwise.
  • Mocha dissipated on May 15 and the back-winds helped the monsoon set in on time over the Andaman and Nicobar Islands.
• One severe consequence of the anticyclones since March is that both the Arabian Sea and the Bay of Bengal have warmed by more than 1-degree Celsius in the pre-monsoon season.

Recent Cyclones/Typhoons and their Impact

• Cyclone Biparjoy
  • Cyclone Biparjoy is not interacting much with the monsoon trough currently.
  • However, its late birth as well as the late onset of the monsoon are both closely related to typhoons in the northwestern Pacific Ocean.
• Guchol
  • Tropical storm Guchol is now active just to the east of the Philippines and is likely to continue northwest before turning to the northeast.
  • These powerful typhoons are extremely dry and demand moisture from far and wide.
• Typhoon Mawar
  • On May 19, Typhoon Mawar was born and dissipated by June 3. Mawar qualified as a ‘super typhoon’ and is thus far the strongest typhoon to have taken shape in May.
  • It is also the strongest cyclone of 2023 so far.Mawar pulled winds across the equator into the North Indian Ocean and set up Southwesterly winds (blowing from the southwest) over Arabian sea and Bay of Bengal.

Impact of Southwesterly Winds on Indian Monsoon

• Southwesterly winds over the Arabian Sea are welcome news because they bring large quantities of moisture onto the Indian subcontinent.
• On the other hand, southwesterly winds over the Bay of Bengal are bad news for the monsoon.
  • The monsoon winds over the southern Bay of Bengal sweep in from the southwest and west, but they turn around and head northwest towards India from the southeast.
• The strong southwesterly winds over the Bay of Bengal can be imagined to be a very large highway with heavy traffic heading –
  • From the southwest, over southern peninsular India and Sri Lanka,
  • Towards the South China Sea and the northwestern Pacific Ocean, feeding the monstrous typhoons there.
• The monsoon trough is like a little car trying to cross this busy and wide highway from the Andaman Nicobar Islands to India across the Bay of Bengal.

Conclusion

• This already complicated phenomenon of global warming affecting cyclogenesis adds another complication in the dynamics of Monsoon.
• Fortunately, a late monsoon onset doesn’t necessarily mean a monsoon deficit. Then again with the looming crisis of EL Nino the nation awaits the best and is preparing for the worst.