[Explainer] How does the Indian monsoon develop?

  • The southwest monsoon which begins around the first week of June, making the first landfall in Kerala, is one of the most anticipated events of the year as India receives 70-90% of its annual rainfall during this monsoon.
  • Various factors such as the availability of energy in the atmosphere, the intertropical convergence zone, the Coriolis effect and jet streams, play a role in facilitating the southwest monsoons.
  • Many efforts have been made to understand the southwest monsoon, with two major modulators, the Indian Ocean Dipole and the Equatorial Indian Ocean Oscillation, discovered in the late 1990s and early 1990s. 2000. However, the Indian monsoon develops through a complex set of events, of which much remains to be discovered.

Every year around the first week of June, a vast expanse of swirling gray clouds jut out from the Arabian Sea and make landfall in Kerala to the rhythm of rumbling thunder. The pouring rain soon encompasses the entire state as the southwest monsoon sweeps through it.

From June to September, the southwest or summer monsoon moves across India, bathing the country in rain – during this period India receives 70-90% of its annual rainfall. During the cooler months, from October to November, the retreating monsoon or northeast monsoon sets in and brings rain to the east coast of India, especially Tamil Nadu.

What causes the southwest or summer monsoon?

In the “classical” theory, Sir Edmund Halley at 17e century estimated that the differential heating of land and water caused the Indian summer monsoon. According to him, in the summer, the Asian landmass warmed up to form a low pressure system, which drew winds from the Arabian Sea and the Bay of Bengal, which were at lower temperatures and therefore systems. at high pressure.

“But the classical theory doesn’t explain how or why monsoons are unique to certain places on Earth like India. It also doesn’t explain how the monsoon suddenly kicks in, says Arindam Chakraborty, a professor at the Center for Atmospheric and Oceanic Sciences (CAOS), Indian Institute of Science, who works on the Indian monsoon.

Read more: [Explainer] What factors affect the Indian summer monsoon?

What is the “energetic” theory of the monsoon?

“The more modern ‘energetic’ theory supersedes the classical theory by considering the availability of energy in the atmosphere in the development of the monsoon,” says Chakraborty.

The physics of the Indian summer monsoon is not only affected by the amount of energy available from the sun, but also by the amount of water vapor available in the air and the ability to lift water vapor to form clouds.

The tilt of the Earth’s axis causes different parts of the Earth to receive direct sunlight at different times of the year. During summer in the northern hemisphere, the Tropic of Cancer receives direct sunlight, and the continental landmasses of this hemisphere warm up considerably more than the oceans, creating an area of ​​low pressure over India and India. Central Asia. This causes the Intertropical Convergence Zone (or ITCZ) – a low-pressure zone that forms a band enclosing the Earth – to shift north of the equator toward the Tropic of Cancer. This zone forms at the meeting of the southeast and northeast trade winds, which are winds near the Earth’s surface that blow east to west just north and south of the equator, due to of the Earth’s rotation from west to east.

The Intertropical Convergence Zone is an area of ​​low pressure that forms a band enclosing the Earth. This zone is formed at the meeting of the southeast and northeast trade winds. Photo by Kaidor/Wikimedia Commons.

When this shift occurs, the ITCZ ​​moves north from below India to directly cross the Indian subcontinent and strengthens the depression forming over this area. At the same time, southeast trade winds, which cross the equator due to this movement, are deflected eastward due to the Coriolis effect (a force that causes fluids like air and water to bend). water as they pass through the Earth’s surface). These deflected trade winds are now blowing towards India from the southwest, picking up large amounts of moisture from the Arabian Sea. When they hit the Indian peninsula, they bring about the southwest or Indian summer monsoon.

The summer monsoon winds split into two arms, one moving over the Arabian Sea, while the other moves over the Bay of Bengal. The Arabian Inlet causes rainfall all along the west coast of India. The Bay of Bengal arm runs along the eastern coast and moves over the Bay of Bengal to strike the coast of Bengal and bring rain to the southern slopes of the Shillong Plateau. The Himalayas, which act as a barrier to the advance of this arm towards the interior of the lands, lead it towards the north of India. The two arms converge on Punjab and Himachal Pradesh in mid-July.

The onset of the southwest monsoon in 2022. Photo by Indian Meteorological Department.
The onset of the southwest monsoon in 2022. Photo by Indian Meteorological Department.

The validity of the “air mass” theory for explaining how the Indian summer monsoon forms was proven in a seminal 1980 study by D. R. Sikka and Sulochana Gadgil. They analyzed daily satellite images of clouds and concluded that the intense cloud formations during the Indian summer monsoon and even the variations in rainfall over different years were associated with the movement of the ITCZ ​​in time and direction. space.

However, that’s not the whole story. The seasonal migration of the ITCZ ​​not only affects the surface winds (the trade winds), but also triggers numerous events in the upper layers of the atmosphere.

These events involve jet streams, which are bands of narrow, meandering, fast-moving winds (usually 100-200 km/h but up to 400 km/h) in the upper levels of the atmosphere (between 9 km and 16 km above sea level). Three jet streams are believed to affect the Indian summer monsoon – the subtropical westerly current, the tropical easterly current and the Somali or transequatorial jet stream.

What are the subtropical, eastern tropical, and Somali jet streams? How do they affect the southwest monsoon?

The subtropical jet stream forms when warm air from the equator meets cool air from the polar regions and flows west to east. During the Northern Hemisphere summer, as the Tropic of Cancer begins to receive direct sunlight, two things happen. First, in response to a northward shift in heating patterns during the Indian summer, the subtropical jet stream moves northward just over the Tibetan Plateau from its position over central India . Because of this, the second event occurs – a seasonal jet stream, the tropical east, is set up. As the Tibetan plateau begins to warm, the air rises to meet the subtropical jet stream from the west; the intermingling of these two currents is affected by the Coriolis force, which deflects the newly formed tropical jet stream westward. The tropical jet stream flows east to west (10 to 12 km above the plains of the Ganges) across India and dips over the Indian Ocean, where it then gives l extra energy and ‘pushes’ the southwest monsoon towards India.

The Somali jet stream is created due to the intense heating of the air north of the Bay of Bengal by moist convection, which draws winds from the equatorial Indian Ocean towards the Indian subcontinent forming the westerly winds at low altitude (prevailing winds from west to east in mid-latitudes) over the Arabian Sea. These westerly winds bring moisture to Indian lands, thereby enhancing convection.

“Therefore, the monsoon itself is thought to intensify the movement of southwesterly winds from the lower atmosphere. The buildup of water vapor in the atmosphere is held responsible for the ‘bursting’ or the sudden onset of the Indian summer monsoon in early June, and the rapid movement of the summer monsoon across India,” adds Chakraborty.

What is the retreating monsoon?

In late summer in the northern hemisphere, the ITCZ ​​begins to drift south of the equator causing the trade winds to reverse. Now the Asian landmass, including India, is rapidly cooling and forming a large area of ​​high pressure, while the oceans, which are cooling at a slower rate, are forming areas of low pressure. This causes drier, colder air from the mainland to blow seaward, causing the monsoon to retreat or the northeast monsoon.

The retreating monsoon brings rainfall to Chennai.  Photo by McKay Savage/Wikimedia Commons.
The retreating monsoon or northeast monsoon brings rainfall to Chennai. Photo by McKay Savage/Wikimedia Commons.

In northwestern India, the monsoon withdraws quickly and completely in September. But in southeast India, this retreat is more gradual, as the retreating monsoon absorbs moisture from the Bay of Bengal. This brings December rains to the Tamil Nadu coast, which remained dry during the southwest monsoon.

What other factors affect Indian monsoons?

“The Indian monsoon is an extremely complex climate pattern which is affected by many factors, the best known of which are El Nino and La Nina, the Indian Ocean Dipole (IOD) and the EQUINOO (Indian Ocean Oscillation equatorial),” says Chakraborty.

El Nino and La Nina are large-scale sea surface warming or cooling events along the central and east-central Pacific Ocean around the equator, the effects of which have been widely held to responsible for several droughts in India. The IOD is an alternation of warming and cooling of the equatorial region of the Indian Ocean to the west and east, much like the El Nino and La Nina events, and the EQUINOO refers to an alternation of enhanced and depressed cloud formation between the western equatorial Indian Ocean and the eastern equatorial Indian Ocean.

Over the past 30 years, great strides have been made in understanding the Indian summer monsoon, with two major modulators – IOD and EQUINOO – discovered in the late 1990s and early 2000s .

“But we are still a long way from fully understanding the system; as is the case with such complex systems, there is much to study and explore about the Indian monsoon,” adds Chakraborty.

Banner image: The vast expanse of swirling gray clouds advances from the Arabian Sea and makes landfall in Kerala, around the first week of June, triggering the southwest monsoon season. Photo by Anoop Joy/Flickr.

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