The retreating monsoon and the northeast monsoon

Detail

1. What the retreating monsoon is

• Retreating monsoon means the slow withdrawal of the southwest monsoon winds from India. It happens in October–November, which is also called the post-monsoon season.
• Why it happens:
• The sun's overhead position moves south after the September equinox (about 23 September).
• The monsoon trough (the low-pressure belt over the Ganga plain that pulls in the moist southwest winds) gets weaker and moves south with the sun.

• Weaker low pressure means weaker winds. The southwest monsoon slowly withdraws.

• The monsoon weakens by the end of September. Northern India no longer gets moist winds from the sea.

2. How IMD declares withdrawal

• IMD (India Meteorological Department) has used these rules since 2006. They are checked only after 1 September [2]:
• no rain over an area for 5 days in a row [2];
• an anticyclone (a high-pressure system in which winds blow outward) forms in the lower atmosphere, at 850 hPa (about 1.5 km up) and below [2];

• much less moisture, as seen in satellite water-vapour images and tephigrams (charts of temperature and humidity at different heights) [2].

• After this, withdrawal is declared region by region. IMD checks that the dry area is continuous, that moisture has dropped and that the weather has stayed dry for 5 days [2].

• The monsoon leaves the southern Peninsula, and so the whole country, around 15 October. This is when the southwesterly winds change to another wind pattern [2].

3. The order of retreat (north-west → south-east)

Stage	Time	Area the monsoon leaves / where the low lies
Withdrawal starts	1st week of September (NCERT); about 17 September (IMD revised normal, 2020)	Western Rajasthan (north-west India)
End of September	~30 September	Rajasthan, Gujarat, western Ganga plain, Central Highlands
Early October	1st week of October	The low moves over the northern Bay of Bengal
Mid-October	~15 October (IMD)	Southwest monsoon leaves the whole country [2]
Early November	1st week of November	The low moves over Karnataka and Tamil Nadu
Mid-December	~15 December	The low leaves the Peninsula (NCERT)

• Why the retreat is slower than the onset: the monsoon arrives in sudden pushes ("bursts") in June. It leaves slowly from the north-west to the south-east, over about 3 months.
• Class 9 NCERT (Fig. 3.11) shows the normal withdrawal dates as isochrones (lines joining places where an event happens on the same date).
• Western Rajasthan gets the monsoon last and loses it first. This is why it has the shortest rainy season in India.

4. October heat

• October heat: the hot, sticky weather of October, after the rains stop and before winter begins.
• How it happens:
• The monsoon leaves → skies become clear → strong sunlight → day temperatures rise again.
• The land is still wet from the monsoon → water evaporates → the air stays humid.

• High heat + high humidity → the weather feels oppressive (very uncomfortable).

• When it ends: temperatures fall quickly in the second half of October, mostly in north India. Long nights and clear skies let the land lose heat quickly at night.

• Who is dry and who is wet:
• North India becomes dry.

• The eastern Peninsula gets its rainiest months in October–November.

• 2026 example: IMD expects above-normal maximum temperatures over most of India in October 2026. It also expects below-normal rain for the country as a whole (<85% of the October LPA of 75.4 mm, base period 1971–2020) [5]. LPA (Long Period Average): the average rainfall of a place over a long base period, used as "normal".

5. The northeast (winter) monsoon

• Northeast monsoon: the dry northeasterly winds that blow from land to sea (offshore winds) over India in winter. They blow when the ITCZ (Inter Tropical Convergence Zone: the low-pressure belt near the equator where the trade winds of the two hemispheres meet) moves south with the sun.
• Why it is mostly dry: these winds start over the cold, dry land of Central Asia and north India. So they carry little moisture and give little rain over most of India.
• The exception: the south-east coast:
• The winds cross the Bay of Bengal → they pick up moisture over the warm sea.
• They hit the Coromandel coast (the east coast of Tamil Nadu, about 8°N–13.5°N) → the moist air rises and cools → rain in October–December.

• The areas that get rain: the Tamil Nadu coast, southern Andhra Pradesh, south-east Karnataka and south-east Kerala (NCERT).

• Why Tamil Nadu is unusual: in June–September, Tamil Nadu lies in the rain-shadow of the Western Ghats (the dry side of a mountain, away from the wind). So it gets more rain in October–December than in the southwest monsoon [3].

6. Northeast monsoon: IMD data

Parameter	Value
Easterly winds set in over south-east Peninsula (normal)	14 October [3]
Normal onset over coastal Tamil Nadu and south coastal Andhra Pradesh	20 October [3][4]
Actual onset	15 October 2024; 16 October 2025 [3][4]
Climatological withdrawal	4 January, ± about 11 days [3]
Average length of the season	~75 days [3]
Tamil Nadu normal October–December rain	~443 mm (442.8 mm) [3]
Share of annual rain, Tamil Nadu, Puducherry and Karaikal	~48% [5]
Share of annual rain, south Peninsular India (5 subdivisions*)	~30% [5]
October–December LPA, south Peninsular India (1971–2020)	334.1 mm [5]

*South Peninsular India in IMD terms = Tamil Nadu, Puducherry & Karaikal; Coastal Andhra Pradesh & Yanam; Rayalaseema; Kerala & Mahe; South Interior Karnataka [5].

• 2026 outlook (issued 30 September 2026): October–December rain over south Peninsular India is likely to be normal (88–112% of LPA) [5].
• Strong El Niño conditions now exist over the equatorial Pacific and are expected to grow stronger until the end of 2026 [5]. El Niño: unusual warming of the eastern Pacific Ocean, part of ENSO (El Niño–Southern Oscillation).

• Neutral IOD conditions are expected through the season [5]. IOD (Indian Ocean Dipole): a difference in sea-surface temperature between the western and eastern Indian Ocean.

• IMD has issued seasonal forecasts for the northeast monsoon since 1998 using statistical models. Since 2021 it has added a Multi-Model Ensemble (MME) system, which combines many coupled climate models, including IMD's own Monsoon Mission Climate Forecast System [5].

7. Cyclones of the retreating monsoon season

• Where they form: cyclonic depressions (areas of low pressure with winds spiralling inward) form over the Andaman Sea (east of the Andaman and Nicobar Islands, south of Myanmar). They also form over the south-east Bay of Bengal.
• Track: they move west / north-west and cross the east coast of India.
• Main targets: the densely populated deltas of the Godavari, Krishna (Andhra Pradesh) and Kaveri (Tamil Nadu).
• A few turn north to hit West Bengal, Bangladesh and Myanmar.
• Why they matter: these cyclones and depressions give the Coromandel coast most of its rain. Chennai's climograph (Class 9 NCERT) shows October–November as its wettest months.

• On average there is about 1 day of very heavy rain per northeast monsoon season over this region. These heavy spells come with the landfall of cyclones, depressions and lows on the east coast [5].

• Arabian Sea: cyclones are rarer here than in the Bay of Bengal (NCERT). This is now changing:

• Extremely severe cyclonic storms (lifetime maximum winds >46 m/s) were first seen in the Arabian Sea in the post-monsoon season in 2014 and 2015 [6].
• Model studies link this rise to human-caused warming, not to natural variation [6].
• The cyclone Genesis Potential Index (a measure of how favourable conditions are for cyclones to form) over the Arabian Sea increased from 1998 to 2021 in the post-monsoon season [7].

• Warming of the Arabian Sea has made it an emerging cyclone hotspot [8].

• Cyclone formation and naming: see air-masses-fronts-cyclones and natural-hazards-disasters.

8. The two monsoons compared

Feature	Southwest monsoon	Northeast (retreating/winter) monsoon
Months	June–September	October–December
Wind direction	South-west to north-east (from sea to land)	North-east to south-west (from land to sea)
Moisture source	Arabian Sea and Bay of Bengal branches	Picked up only while crossing the Bay of Bengal
ITCZ / trough position	North, over the Ganga plain	Moves south, towards the equator
Main rain areas	Western Ghats, north-east India, Ganga plain	Coromandel coast, south Andhra Pradesh, south-east Karnataka and Kerala
Tamil Nadu	Rain-shadow, little rain	Main rainy season (~48% of annual rain) [5]
Cyclones	Fewer; monsoon depressions over the Bay	Andaman Sea depressions hit the east-coast deltas

Prelims Hooks

• The southwest monsoon starts to withdraw from western Rajasthan. Normal date: ~17 September (IMD 2020 revised normal); NCERT says the first week of September.
• Order of withdrawal: western Rajasthan → Gujarat / western Ganga plain / Central Highlands → northern Bay of Bengal → Karnataka and Tamil Nadu → leaves the Peninsula.
• IMD's withdrawal rules (since 2006): 5 dry days, an anticyclone at 850 hPa, and less moisture. They apply only after 1 September [2].
• The southwest monsoon leaves the whole country around 15 October [2].
• October heat = clear skies + high temperature + high humidity over wet land. It is not caused by moist winds from the sea.
• The northeast monsoon gets its rain from the Bay of Bengal, not the Arabian Sea. Trap: the "dry" northeast winds still bring rain to the Coromandel coast.
• Tamil Nadu, Puducherry and Karaikal get ~48% of their annual rain in October–December. South Peninsular India gets ~30% [5].
• Normal onset of the northeast monsoon over coastal Tamil Nadu: 20 October [3].
• Post-monsoon cyclones from the Andaman Sea hit the Godavari, Krishna and Kaveri deltas. Chennai's wettest months are October–November.
• The Arabian Sea had its first post-monsoon extremely severe cyclonic storms in 2014 and 2015 [6].

Mains Points

• Why the south-east coast gets winter rain (GS-I):
• The ITCZ moves south → dry offshore northeast winds blow.
• They gain moisture over the Bay of Bengal.
• Moisture-laden winds plus Andaman Sea depressions bring rain to the Coromandel coast, which is in rain-shadow in summer.

• Result: Tamil Nadu's farming, tanks and reservoirs depend on October–December rain (~48% of its annual rain) [5].

• Disaster risk on the east-coast deltas (GS-III):

• Post-monsoon cyclones hit the crowded, low-lying Godavari, Krishna and Kaveri deltas.
• They bring storm surge, floods and crop loss.

• Responses: impact-based forecasts and early warnings from IMD, and preparation in advance [5].

• Climate change and cyclone patterns (GS-I/III):

• The Arabian Sea is warming. Its post-monsoon cyclone potential rose from 1998 to 2021 [7], and very severe storms now form there [6].
• This exposes the west coast (Gujarat, Maharashtra, Kerala) to cyclones that used to be rare.

• Coastal planning, shelters and warning systems must now cover both seas.

• Rabi season and water planning:

• Weak post-monsoon rain (for example, below-normal October 2026 rain over most of India [5]) dries the soil, lowers reservoir and groundwater recharge, and raises irrigation demand for rabi crops.
• Responses: contingency crop plans and water conservation [5].