The Indian Monsoons: Understanding South-West and North-East Monsoons

India’s climate revolves around one of the most powerful weather systems on Earth – the monsoon. For billions of people across South Asia, monsoons are not just a season; they determine food security, water availability, and economic stability. India experiences two distinct monsoon systems: the South-West Monsoon, which dominates from June to September, and the North-East Monsoon, which takes over from October to December. Together, these two systems shape the rhythm of life across the subcontinent.

Mechanism of the South-West Monsoon 🔗

The South-West Monsoon is driven by a fundamental principle: land heats up faster than water. During the summer months of April and May, the sun shines almost directly over the Tropic of Cancer, causing the Indian landmass to heat intensely. This creates a large low-pressure zone over the northwestern parts of the subcontinent. Meanwhile, the Indian Ocean, Arabian Sea, and Bay of Bengal remain relatively cooler and maintain higher pressure. According to Britannica, this differential heating between the continental landmass and the surrounding ocean is the primary driver that pulls moisture-laden winds toward the Indian subcontinent.

As these winds travel northward from the ocean, they cross the equator and get deflected to the right due to the Coriolis effect, transforming into south-westerly winds – which is precisely why this monsoon is called the South-West Monsoon. The onset typically occurs over Kerala around June 1st, and the monsoon then progresses across the country through July. The arrival is often sudden and dramatic – a phenomenon known as the “burst” of the monsoon, bringing a sharp drop in temperature of 5°C to 8°C within just a few weeks.

Once the monsoon winds reach the tip of peninsular India, the country’s geography divides them into two major branches: the Arabian Sea Branch and the Bay of Bengal Branch.

The Arabian Sea Branch 🔗

The Arabian Sea Branch is the more powerful of the two. It strikes the Western Ghats almost perpendicularly, forced to rise rapidly along the windward slopes. This orographic lifting cools the air, causing intense condensation and extremely heavy rainfall – between 400 to 500 cm annually on the windward side of the Western Ghats. The leeward side, by contrast, receives significantly less rain, creating a pronounced rain-shadow zone. This branch then splits into further streams – one moving through the Narmada and Tapi river valleys to bring rainfall to central India, and another reaching Gujarat’s Saurashtra Peninsula before converging further north.

The Bay of Bengal Branch 🔗

The Bay of Bengal Branch moves northeastward, picking up additional moisture as it travels over the Bay. It brings heavy rainfall to West Bengal, Bangladesh, and Assam. The Arakan Hills along Myanmar’s coast deflect a large portion of this branch back toward eastern India. Upon reaching the Himalayas, the branch is deflected westward and advances up the Gangetic Plains. A sub-branch of this system strikes the Khasi and Garo Hills of Meghalaya – making Mawsynram one of the wettest locations on Earth. The two branches – Arabian Sea and Bay of Bengal – eventually converge around Delhi by mid-July to form a unified current that extends rainfall to Punjab, Haryana, Rajasthan, and finally Kashmir.

Together, the South-West Monsoon delivers approximately 75% of India’s total annual rainfall, making it the most critical climatic event of the year for the entire subcontinent.

Retreating monsoon and North-East Monsoon 🔗

As September arrives, the sun begins shifting southward. The landmass of northern India cools rapidly while the Indian Ocean retains its warmth. This reverses the pressure gradient: high pressure develops over the Eurasian continent and low pressure persists over the ocean. The monsoon starts withdrawing from northwest India first, retreating progressively southward through October. This phase is known as the retreating monsoon.

By October, the wind direction over the subcontinent has completely reversed. Dry, cool winds now blow from the northeast – from the land toward the sea. These winds are called the North-East Monsoon (NEM), and they operate from October to December. Unlike the South-West Monsoon, which brings moisture from the open ocean, north-easterly winds begin as dry continental air. However, as they pass over the Bay of Bengal, they pick up significant moisture and release it over southeastern India.

Impact of the North-East Monsoon on Tamil Nadu 🔗

The coastal areas most affected by the NEM are Tamil Nadu, parts of Andhra Pradesh (particularly Rayalaseema), Puducherry, and northern Sri Lanka. Tamil Nadu is especially dependent on this system: the North-East Monsoon provides around 50% of the state’s annual rainfall, as the region lies largely in the rain-shadow zone of the South-West Monsoon. While the rest of India winds down from rain by October, Tamil Nadu enters its primary rainfall season. The average rainfall received during the NEM in Tamil Nadu stands at about 443 mm. The NEM period is also when cyclonic weather systems forming over the southern Bay of Bengal intensify, sometimes bringing exceptionally heavy rain – and, at times, destructive flooding – to coastal districts.

Impact of monsoons on the Indian subcontinent 🔗

The monsoons are not just a weather event – they are the backbone of India’s agricultural and economic cycle. Around 60% of Indian agriculture is rain-fed, and the produce from these farms supports roughly 40% of the country’s population. A well-distributed monsoon season directly translates to good harvests, replenished groundwater, full reservoirs, and rural prosperity. Conversely, a poor monsoon sets off a cascade of consequences – from crop failures to rural distress and macroeconomic slowdown.

Agricultural dependence and water security 🔗

The South-West Monsoon fills the country’s rivers, lakes, and groundwater systems after long dry summer months. Major irrigation reservoirs like Bhakra, Hirakud, and Nagarjuna Sagar depend almost entirely on monsoon runoff. Crops like rice, sugarcane, cotton, and pulses are sown at the beginning of the monsoon and harvested in its aftermath. The North-East Monsoon, meanwhile, sustains the second (Rabi) cropping cycle across southeastern India. In Tamil Nadu, the entire cropping system is oriented around the NEM period, with paddy cultivation heavily reliant on October-December rainfall for both irrigation and drinking water supply.

Variability: droughts and floods 🔗

The monsoon’s reliability has always been a challenge. Rainfall across India is highly variable – both from year to year and from region to region. In the same season, one state may face severe flooding while another is in drought. Between 1981 and 2011, dry spells during the monsoon season increased by 27% compared to the period from 1951 to 1980, pointing to a troubling trend of growing climatic unpredictability.

When the South-West Monsoon is weak – often linked to El Niño conditions in the Pacific Ocean – large parts of the country face drought. Agricultural losses spike, groundwater levels fall, and power generation from hydroelectric dams drops. During excessive monsoon years, the reverse holds: floods inundate fields, damage infrastructure, and displace millions. The 2005 Mumbai floods and the 2015 Chennai floods, for instance, caused billions of dollars in economic damage and directly highlighted how inadequate urban infrastructure compounds monsoon-driven disasters.

For Tamil Nadu specifically, studies show that both the South-West and North-East monsoons have become increasingly erratic since 2015, alternating between severe drought years and extreme flood events – a pattern many researchers associate with climate change. Research published via Springer Nature confirms that interannual variability in the NEM has increased significantly between 1959 and 2016, with more frequent high-intensity rainfall events replacing steady, moderate rains.

The role of El Niño, La Niña, and climate change 🔗

The Indian monsoon does not operate in isolation. Global ocean-atmosphere patterns strongly modulate its strength. El Niño – the periodic warming of the central Pacific Ocean – typically weakens the South-West Monsoon and increases drought risk across India. La Niña, on the other hand, tends to strengthen the monsoon and bring above-normal rainfall, though it can also lead to flooding in vulnerable areas. Beyond these natural cycles, climate change is increasingly altering rainfall patterns, with a shift toward fewer but more intense rainfall events – meaning half of a season’s total rainfall may now fall within just 20 to 30 hours, making both flood management and water conservation significantly harder.

The Indian monsoon system is a dynamic, interconnected mechanism that sustains one of the world’s most densely populated regions. Understanding both its South-West and North-East dimensions is essential not just for meteorology, but for agriculture, water resource management, disaster preparedness, and long-term climate adaptation planning. As climate change continues to reshape these patterns, the stakes for getting that understanding right keep growing.

What do you think? Given that India’s agriculture depends so heavily on an increasingly unpredictable monsoon, what kinds of policy or technological solutions should be prioritized to build resilience? And should regions like Tamil Nadu – which depends on the NEM rather than the SWM – receive a different approach to water management and climate planning than the rest of the country?

References

1. https://www.britannica.com/science/Indian-monsoon
2. https://www.pmfias.com/south-west-monsoon-season-south-west-monsoons-arabian-sea-branch-bay-of-bengal-branch/
3. https://india.mongabay.com/2022/05/explainer-how-does-the-indian-monsoon-develop/
4. https://www.pib.gov.in/PressNoteDetails.aspx?id=154892&NoteId=154892&ModuleId=3
5. https://www.orfonline.org/research/monsoon-variability-and-agricultural-drought-management-in-india
6. https://india.mongabay.com/2024/01/why-south-tamil-nadu-received-record-breaking-rain-during-the-northeast-monsoon/
7. https://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1057&context=droughtnetnews
8. https://link.springer.com/chapter/10.1007/978-981-15-4327-2_3