MONSOONS: HISTORY AND WHY AND HOW THEY OCCUR

MONSOONS

Monsoons are strong winds that change direction with the seasons due to differences in land and ocean temperature and can trigger dramatic changes in weather. They have a huge impact on South Asia, India and Southeast Asia. The monsoons in India blow from the sea toward land in a southwest direction from June to September. From October through December winds blow over land toward the sea from a northeast direction, sweeping from the Himalayas to the Indian Ocean. The rains during the summer or "wet" monsoon come down when air that has absorbed heat from the Indian landmass rises and is replaced by cooler air from over the Indian Ocean.

The monsoons provide India and its South Asian neighbors with about 90 centimeters (35 inches or rain), about 80 percent of the annual rainfall. Monsoon rain is an import source of drinking water, agricultural water and irrigation water. It is vital for filing up wells and aquifers. Many dams have in South Asia have been built so its water can be stored and used the entire year and also harnessed for hydroelectric power.

Since temperatures in South Asia and can be very hot preceding the monsoon with temperature often exceeding 38°C (100°F) the "wet" monsoon brings much-needed relief. The region’s agricultural industry and economy rely on the summer monsoon, which provides up to 80 per cent of annual rainfall. Heavy monsoon seasons can bring a lot of rain in a relatively short period of time, producing floods and landslides, and destroying crops and villages. At the same time a failed monsoon can be just as destructive, resulting in severe, long-lasting droughts.

Book: “Chasing the Monsoon” by Alexander Frater (Pan Macmillan, 1990)

Monsoon Seasons and Air Movement

Monsoon describes climatic changes brought about by a change in wind direction that affects half the world's population. If there was no monsoon India and Southeast Asia would resemble the Sahara. The monsoon rains are generated by a massive circulation of air caused by the searing summer heat. The process begins when hot air over the land rises and moisture-laden air over the ocean moves in to takes its place. As this air rises it sheds its moisture on the land. The risen air moves back over the sea on a wind going the opposite direction of the one bringing air from the ocean. After the moisture laden-air has moved off it is displaced by a third wave of air that arrives from the sea and the whole cycle begins again.

There are two distinct monsoon seasons. The first occurs from roughly June to September when a southwest wind brings moisture-laden air from the Indian Ocean, Arabian Sea and Bay of Bengal to the Indian Subcontinent and Southeast Asia. The worst time of the year in these countries are the scorching months preceding the monsoon's arrival. The same wind also brings in moisture from the Atlantic to Central Africa. If some reason the monsoon doesn't come it can mean drought and famine that can affect millions. The second monsoon occurs from December to February, when a northeast wind brings cold dry air from Tibet and Siberia to India and moist rainy air from the Pacific to the Philippines, parts of Indonesia and Australia.

How the Summer Monsoon Progresses Across South Asia

The South Asian monsoon typically reaches Kerala in the southern coast of India in late May or early June and gradually advances across the region over the following six weeks. As it moves inland, the winds divide into two main branches. The Arabian Sea branch moves northward along India's west coast and is forced upward by the Western Ghats, producing heavy rainfall through orographic uplift. The Bay of Bengal branch travels northeastward, bringing intense rainfall to Bangladesh, northeastern India, and parts of Myanmar.

As the monsoon advances farther inland during June and early July, the Himalayan mountain barrier prevents the moist air from moving farther north. The Bay of Bengal branch is deflected westward along the foothills of the Himalayas, where it merges with the Arabian Sea branch. Together, these systems spread rainfall across the Indo-Gangetic Plain, central India, and much of the rest of the subcontinent. By mid-July, the southwest monsoon typically covers nearly all of South Asia, including most of India, Bangladesh, Nepal, Bhutan, and parts of Pakistan.

A visualisation of the South Asian Monsoon
based on the Climate Hazards Group InfraRed Precipitation
with Station data (CHIRPS) 30+ year quasi-global rainfall dataset,
analysed and visualised using Google Earth Engine When the monsoon finally arrive they can come with vengeance. Some area receive 20 to 23 centimeters (eight to nine inches) of rain in a single day. In some places, waist-high water fills the streets and mixes with sewage, cattle dung, drowned dogs, rotting grain and swollen carcasses. During heavy monsoons cars are submerged or stranded and only pedicabs can negotiate the roads. The monsoons can be very erratic. Sometimes they start normally, peter out after a month and then produce deluges, causing floods and crop damage.

The monsoons rains can be so strong they sometimes pit houses like hail. Flash floods wash buildings off of hillsides and trains off of tracks. Muddy water flows from the taps and beds are placed on the tops of tables inside houses to keep them dry. Dams overflow; 110 kilometers per hour (70 mph) winds tear off the roofs of houses; military helicopters have to be dispatched to bring supplies; and hundreds of people die.┾

Monsoons at Their Full Fury in August

Monsoons generally reach their peak in August. In late August 2020; AccuWeather reported: Monsoon rains to impact India from coast to coast into next week Heavy rain is forecast for both the east and west coasts of India into early next week, threatening more flooding for communities. As showers and thunderstorms spread across much of India and southeastern Asia this weekend into early next week, two separate lows will target the coasts of India with widespread heavy rainfall. [Source: Courtney Spamer, AccuWeather, August 22, 2020]

“One such low will swirl in the northern part of the Bay of Bengal this weekend through early next week. “This monsoon low is forecast to stay rather stationary in the northern part of the bay, bringing round after round of heavy, tropical rainfall to parts of eastern India, Bangladesh and western Myanmar," said AccuWeather Meteorologist Tony Zartman.

rainfall amounts and dates of the arrival of the South Asia monsoon Dierke

“Despite the presences of tropical moisture within this low, it is not expected to strengthen into a more organized tropical system at this time. “Rainfall amounts into Tuesday are likely to reach 50-100 mm (2-4 inches), with isolated rainfall amounts as high as 200 mm (8 inches). The highest rainfall totals will be in areas that get hit with heavy downpours repetitively into next week.

“Without much movement from the low, the same areas could be hit with downpours several days in a row, making flash flooding a large concern into early next week, including in Odisha, Jharhand and West Bengal through Bangladesh and into Assam and Meghalaya. Assam and parts of Bangladesh have already had widespread flooding issues so far this monsoon season, including one wave of moisture that forced 200,000 people to evacuate.

Southeast Asian Monsoons

The monsoon season in India more less coincides with the rainy seasons in Vietnam, Cambodia, Thailand, China and Japan but is different from the rainy seasons on the west coast of Malaysia (from September to November) and rainy season in Singapore, Borneo, Indonesia and the east coast of Malaysia (November to January). The cyclone season is from June to October. Most cyclones hit the Bay of Bengal between Bangladesh and the east coast of India.

The climate of Southeast Asia in strongly influenced by the southwest monsoon (mid-May to September), which brings rainy weather, and northeast monsoon (November to mid-March), which and dry and cool weather.

The summer monsoon in Southeast Asia, generated by winds blowing from the southwest, brings heavy rainfall to much of mainland and maritime Southeast Asia, including Thailand, Vietnam, Cambodia, Laos, Myanmar, the Philippines, Malaysia, and Indonesia. It generally begins in May and lasts for about 120 to 160 days, extending into September or October. Moisture-laden winds from the South China Sea, western Pacific Ocean, and surrounding tropical waters produce widespread precipitation that supports agriculture, river systems, and water supplies throughout the region. [Source: Woods Hole Institute, Science Direct]

The monsoon plays a crucial role in sustaining major rivers such as the Mekong, Chao Phraya, Irrawaddy, and Red rivers. Rainfall during the wet season replenishes reservoirs, supports rice cultivation, and maintains freshwater ecosystems. Annual precipitation in many parts of Southeast Asia exceeds 1,500 millimeters (59 inches), while mountainous and coastal areas may receive more than 3,000 millimeters (118 inches).

Linkages Between the Southeast Asian and South Asia Monsoons

rainfall amounts during the South Asia monsoon Dierke

The Southeast Asian and South Asian monsoons are closely connected components of a broader Asian monsoon system. The Southeast Asian monsoon generally develops first, with warm, moist air accumulating over the South China Sea and western Pacific. As atmospheric circulation strengthens, moisture is transported westward into the Bay of Bengal, contributing to the development of the Indian summer monsoon, which typically reaches the Indian subcontinent in early June.

Both monsoon systems are influenced by large-scale pressure patterns and upper-atmosphere circulation. An important feature is the South Asian High, a high-pressure system in the upper troposphere that helps regulate convection and rainfall across both South and Southeast Asia. Changes in its position and strength can affect the timing and intensity of monsoon rainfall over a vast region.

Because the two monsoon systems are linked, they are often affected by the same climate influences. Events such as El Niño can weaken monsoon circulation, delay the onset of seasonal rains, and increase the risk of drought across both South and Southeast Asia. Variations in monsoon rainfall have significant impacts on agriculture, water resources, energy production, and food security for hundreds of millions of people throughout the region.

Monsoons and History

The word monsoon is derived from the Arabic word for season — mausim. Around 150 B.C. the Greek sailor Hippalus recognized and identified monsoon winds In ancient times and even today dhows steered by Arab, Persian and Indian mariners utilized the monsoon winds to travel across the Arabian Sea between the Middle East, the Persian Gulf and India. Sinbad the sailor is said to have reached China from Arabia by riding the monsoons and Islam reached India as well as Malaysia and Indian on the same winds.

In the 17th century, Sir Edmund Halley, the man after which Halley’s comet is named, reasoned that the differential heating of land and water caused the Indian summer monsoon. He postulated that the the Asian landmass heated up to form a low-pressure system, which attracted winds from the Arabian Sea and Bay of Bengal, which were at lower temperatures and thus high-pressure systems. The problem with this “classical theory” is that it doesn’t explain how or why monsoons are unique to certain places on Earth like India. Nor does it explain how the monsoon sets as a sudden burst,” says Arindam Chakraborty, a professor at the Centre for Atmospheric and Oceanic Sciences (CAOS) in the Indian Institute of Science, who works on the Indian monsoon. [Source: Anusha Krishnan, Quartz, May 31, 2022]

Did a Failure of the Monsoons Cause the End of Indus Valley Civilization

Scientists theorize that a weakened monsoon may have contributed to the collapse of the Indus Valley civilisation around 2000 B.C. The Indus Valley civilization is the oldest known large civilization in Asia. Stretching from the Arabian Sea to the Himalayas and from the deserts of India to what is now Iran, it embraced 1,500 or so settlements and covered 280,000 square miles, an area roughly the size of Texas, or twice the size of ancient Egypt and Mesopotamia. The Indus Valley civilization was founded roughly around 3000 B.C. and flourished from 2600 to 1900 B.C.

According to PBS: “Climate change is emerging as a primary reason for its gradual demise. Geological evidence shows that the region's climate grew colder and drier, in part perhaps because of a weakened monsoon. By 1800 B.C., the Ghaggar-Hakra River, a river in the region that paralleled the Indus system and that some scholars suggest is the Saraswati, the lost sacred river of Rig Veda, was severely diminished. As a result, cities were abandoned and though some of the population remained, many migrated to more fertile lands in the east around the Ganges and Jumna River. [Source: PBS, The Story of India, pbs.org/thestoryofindia]

The team of researchers from the Archaeological Survey of India (ASI), Institute of Archaeology, Deccan College Pune, and IIT Kharagpur wrote in Nature’s Scientific Reports journal: “‘Our study suggests that the climate was probably not the cause of Harappan decline.” While the ancient people relied upon heavy and regular monsoons between 9,000 and 7,000 years ago to water their crops, after this period, evidence at Bhirrana shows people continued to survive despite changing weather patterns. [Source: Sarah Griffiths for MailOnline, June 2, 2016]

Monsoons and the Maritime Silk Road

Everyone knows the Silk Road but it is widely believed that more silk and Silk Road goods reached the West via sea routes — often collectively referred to as the Maritime Silk Road — than by overland routes. Much of the trade on the Maritime Silk Road was carried out by Arab, Persian and Indian ships not Chinese ones and monsoons were key to this trade.

Small boats hugged the shore. Large boats sailed the seasonal monsoon winds, which carried boats eastward to India in July, August and September and westward from India to the Middle East in December, January and February. The largest teak-hull ships that plied these routes may have been 180 feet long and capable of carrying cargos of 1,000 tons.

Monsoon winds have been utilized for trade since ancient times. One of the greatest ancient Middle East ports was Bernelike on the Red Sea. In the 1990s archaeologists discovered this ancient city under the sands about 600 miles south of modern-day Suez, near the border of Egypt and Sudan. They found evidence of trade with Thailand and Java, and inscriptions in 11 languages including Greek, Hebrew Coptic and Sanskrit. It was surmised that the ships and crews mostly came from India based on the presence of lots teak, a wood native to India and Southeast Asia.

Monsoons and the Arrival of Islam in Southeast Asia

Monsoons also played a role in bringing Islam to Southeast Asia. The Indian Ocean served as both a commercial and a cultural link between Southeast Asia and the countries to the west. Thus Islam, which was established on the Arabian Peninsula by the Prophet Muhammad in the seventh century A.D.. Like the introduction of Indian civilization, the process of Islamization is obscure because of the lack of adequate historical records and archeological evidence. Southeast Asia was not invaded by outsiders and forcibly converted. [Source: Library of Congress *]

Over the centuries, merchants from Arabian Sea and Indian Ocean ports and mystics and literary figures propagated the faith. Because commerce was more prevalent along the coasts than in inland areas it is not surprising that Islamization proceeded more rapidly in the former than the latter. According to historian M.C. Ricklefs, legends describe the conversion of rulers to Islam in coastal Malay regions as a "great turning point" marked by miracles (including the magical circumcision of converts), the confession of faith, and adoption of Arabic names. Javanese chroniclers tended to view it as a much less central event in the history of dynasties and states. But the Javanese chronicles mention the role of nine (or ten) saints (wali in Arabic), who converted rulers through the use of supernatural powers. *

Doubtless small numbers of Muslims traveled through and resided in the archipelago at a very early date. Historical records of the Chinese Tang Dynasty (A.D. 618-907) tell of Arab traders who must have stopped at Indonesian ports along the way to Guangzhou and other southern Chinese ports. Yet the conversion of rulers and significant numbers of indigenous peoples to Islam apparently did not begin until around the late thirteenth century. Many areas of the archipelago resisted the religion's spread. Some, such as Ambon, were converted to Christianity by Europeans. Others preserved their distinctiveness despite powerful Islamic neighbors. These included small enclaves on Java and the adjacent island of Bali, where animist and Hindu beliefs created a distinct, inward-looking culture. *

Why the Monsoons Occur

Theories about why monsoons occur vary. Conventionally, scientists have attributed monsoons to thermal changes in the Asian landmass. Contemporary theory cites other factors — the barrier of the Himalayas and the sun's northward tilt (which shifts the jet stream north). The hot air that rises over South Asia during April and May creates low-pressure areas into which the cooler, moisture-bearing winds from the Indian Ocean flow. These circumstances set off a rush of moisture-rich air from the southern seas over South Asia. [Source: Library of Congress *]

The southwest monsoon occurs in two branches. After breaking on the southern part of the Peninsula in early June, the branch known as the Arabian Sea monsoon reaches Bombay around June 10, and it has settled over most of South Asia by late June, bringing cooler but more humid weather. The other branch, known as the Bay of Bengal monsoon, moves northward in the Bay of Bengal and spreads over most of Assam by the first week of June. On encountering the barrier of the Great Himalayan Range, it is deflected westward along the Indo-Gangetic Plain toward New Delhi. Thereafter the two branches merge as a single current bringing rains to the remaining parts of North India in July. *

The withdrawal of the monsoon is a far more gradual process than its onset. It usually withdraws from northwest India by the beginning of October and from the remaining parts of the country by the end of November. During this period, the northeast winds contribute to the formation of the northeast monsoon over the southern half of the Peninsula in October. It is also known as the retreating monsoon because it follows in the wake of the southwest monsoon. The states of Tamil Nadu, Karnataka, and Kerala receive most of their rainfall from the northeast monsoon during November and December. However, 80 percent of the country receives most of its rainfall from the southwest monsoon from June to September. *

Anusha Krishnan wrote: The monsoon is an extremely complex climate pattern that is affected by many factors, of which the most well-known are the El Nino and La Nina, the Indian ocean dipole (IOD), and the EQUINOO (Equatorial Indian ocean oscillation)”, says Chakraborty. The El Nino and La Nina are large scale warming or cooling events of the sea surface, along the central and east-central Pacific Ocean around the Equator, the effects of which have largely been held responsible for several droughts in India. The IOD is an alternate warming and cooling of the equatorial region of the Indian Ocean in the west and east, much like the El Nino and La Nina events, and the EQUINOO refers to alternating enhanced and depressed cloud formation between the western equatorial Indian Ocean and the eastern equatorial Indian Ocean.

Between the 1990s and 2020s, great strides were been made in understanding the Indian summer monsoon, with two major modulators—the IOD and the EQUINOO—discovered in the late 1990s and early 2000s. “But we’re still a long way off from fully understanding the system; as is the case in such complex systems, there is much that needs to be investigated and explored about the Indian monsoon,” adds Chakraborty. [Source: Anusha Krishnan, Mongabay India, Quartz, May 31, 2022]

Energetics Theory on How Monsoon Develop

Modern ‘energetics’ theory attributes the development of monsoons to the availability of energy in the atmosphere. Anusha Krishnan wrote: The physics of the South Asian summer monsoon is not only affected by the amount of energy available from the sun but also how much water vapour is available in the air and how well the water vapour can be lifted upward to form clouds. [Source: Anusha Krishnan, Mongabay India, Quartz, May 31, 2022]

The tilt in the Earth’s axis causes different parts of the Earth to receive direct rays from the sun during different times of the year. During summer in the northern hemisphere, the Tropic of Cancer receives direct rays from the sun, and the continental landmasses in this hemisphere heat up considerably more than the oceans, creating a low-pressure zone over India and Central Asia. This causes the intertropical convergence zone (or ITCZ)—an area of low pressure that forms a band girdling the Earth—to shift northwards from the Equator towards the Tropic of Cancer. This zone is formed at the meeting of the southeast and northeast trade winds, which are winds close to the Earth’s surface that blow from east to west just north and south of the Equator, due to the Earth’s rotation from west to east.

The Inter-Tropical Convergence Zone is an area of low pressure that forms a band girdling the Earth. This zone is formed at the meeting of the southeast and northeast trade winds. When this shift occurs, the ITCZ shifts northwards from below India to run directly through the Indian subcontinent and strengthens the low pressure forming over this area. At the same time, the southeast trade winds, which cross the Equator due to this movement, become deflected towards the east due to the Coriolis effect (a force that causes fluids like air and water to curve as they travel across the Earth’s surface). These deflected trade winds now blow towards India from the southwest, picking up large amounts of moisture from the Arabian sea. As they hit the Indian peninsula, they cause the southwest or Indian summer monsoon.

The summer monsoon winds split into two arms with one travelling over the Arabian sea, while the other moves over the Bay of Bengal. The Arabian sea arm causes rainfall all along India’s western coast. The Bay of Bengal arm skirts the eastern coast and moves over the Bay of Bengal to strike against the Bengal coast and brings rain to the southern slopes of the Shillong plateau. The Himalayas, which act as a barrier towards the further inland movement of this arm, herd it towards northern India. The two arms converge over Punjab and Himachal Pradesh by mid-July.

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 analysed daily satellite images of clouds and concluded that intense cloud formations during the Indian summer monsoon and even variations in rainfall over different years were associated with the movement of the ITCZ in time and space. However, this is not the entire story. The seasonal migration of the ITCZ not only affects surface winds (the trade winds), but also sets in motion many events in the upper levels of the atmosphere.

These events involve jet streams, which are bands of narrow, meandering, and fast-moving winds (usually 100-200 kilometers/h but can go up to 400 kilometers/h) in the upper levels of the atmosphere (between 9 kilometers and 16 kilometers above sea-level). There are three jet streams that are thought to affect the Indian summer monsoon — the subtropical westerly, the tropical easterly, and the Somali or cross-equatorial jet stream.

Contributions of Subtropical, Tropical Easterlies and Somali Jet Streams to Monsoons

Anusha Krishnan wrote: The subtropical jet stream is formed when warm air from the equator meets the cool air from the polar regions and flows from west to east. During summer in the northern hemisphere, as the Tropic of Cancer begins to receive the sun’s direct rays, two things happen. One, in response to a northward shift in heating patterns during the Indian summer, the subtropical jet stream moves northwards, right over the Tibetan plateau from its position over central India. Due to this, the second event occurs — a seasonal jet stream, the tropical easterly, is set up. As the Tibetan plateau begins to heat up, the air rises to meet the subtropical westerly jet stream; the intermingling of these two currents is affected by the Coriolis force, which deflects the newly formed tropical jet stream towards the west. The tropical jet stream flows from east-to-west (10-12 kilometers above the Gangetic plains) across India and subsides above the Indian Ocean, where it then lends extra energy to and ‘pushes’ the southwest monsoon towards India. [Source: Anusha Krishnan, Mongabay India, Quartz, May 31, 2022]

The Somali jet stream is set up due to the intense heating of the air over the northern Bay of Bengal from moist convection, which attracts winds from the equatorial Indian Ocean toward the Indian subcontinent forming the low-level westerlies (prevailing winds from the west toward the east in the middle latitudes) over the Arabian Sea. These westerly winds bring moisture over Indian land, thus further enhancing the convection.

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

What is the retreating monsoon? As summer wanes in the northern hemisphere, the ITCZ begins to drift down towards the south of the Equator, which causes a reversal in the movements of the trade winds. Now, the Asian landmass, including India, cools rapidly, and forms a large area of high pressure, while the oceans, which cool at a slower rate, form low pressure zones. This causes drier and colder air from the continent to blow offshore causing the retreating monsoon or the northeast monsoon.

In northwest India, the monsoon withdraws rapidly and completely by September. But in Southeast India, this withdrawal is more gradual, as the retreating monsoon picks up moisture form the Bay of Bengal. This brings December rains to the Tamil Nadu coast, which remained dry during the southwest monsoon.

Image Sources: Wikimedia Commons

Text Sources: “Encyclopedia of World Cultures Volume 3: South Asia,” edited by Paul Hockings, 1992, National Geographic, Ministry of Tourism, Government of India,, CIA World Factbook, New York Times, Washington Post, Los Angeles Times, Smithsonian magazine, Encyclopedia.com, Library of Congress, Live Science, The Conversation, The New Yorker, Time, BBC, CNN, Reuters, Associated Press, AFP, Lonely Planet Guides, Google AI, Wikipedia, The Guardian and various websites, books and other publications.

Last updated June 2026