The onset of the Southwest Monsoon over the Indian subcontinent is not a gradual climatic transition but a sudden, highly synchronized atmospheric event known as the “Monsoon Burst.” This phenomenon marks the structural replacement of the hot, dry subtropical air mass over the Indian landmass with a deep, moisture-laden equatorial maritime air mass. The India Meteorological Department (IMD) relies on specific criteria spanning precipitation, wind velocity, and longwave radiation to formally declare the onset.
Principal Meteorological Triggers of Onset
Shift of the Subtropical Westerly Jet Stream
During the winter and pre-monsoon months, the Subtropical Westerly Jet Stream flows south of the Himalayas at an altitude of 9 to 12 kilometers, maintaining high pressure over the northern plains. The monsoon cannot advance until this upper-air jet stream shifts entirely northward across the Tibetan Plateau. This physical displacement removes the anti-cyclonic subsidence over northern India, creating a low-pressure vacuum that allows lower-tropospheric moist winds to surge forward.
Thermal Anti-cyclonogenesis over the Tibetan Plateau
The Tibetan Plateau acts as an elevated heat source nearly 4 to 5 kilometers above sea level. Intense solar heating during summer causes the plateau to radiate heat into the upper troposphere, establishing a powerful, warm thermal anti-cyclone. The rising air from this system flows southward and descends over the western Indian Ocean near Madagascar, intensifying the Mascarene High and pushing cross-equatorial winds toward India.
Establishment of the Tropical Easterly Jet Stream
The southward outflow from the Tibetan upper-air anti-cyclone forms the Tropical Easterly Jet Stream (TEJ). This jet stream flows from east to west in the upper troposphere across Peninsular India, centered around 14° N latitude. A robust TEJ creates lower-level cyclonic vorticity, pulling moist air masses inland.
The Somali Jet (Findlater Jet)
This low-level, high-velocity cross-equatorial wind current flows off the coast of East Africa. It channels moisture-laden air directly toward India’s western coast. An intensification of the Somali Jet leads to heavy, widespread rainfall along the Konkan and Malabar coasts.
Shifting of the Inter-Tropical Convergence Zone (ITCZ)
Continuous thermal heating over South Asia causes the ITCZ to shift northward from the equator to a position parallel to the Indo-Gangetic plains (20° N to 25° N). In this position, it is designated as the Monsoon Trough. This intense low-pressure belt acts as the ultimate destination for global trade winds.
IMD Criteria for Declaring Monsoon Onset over Kerala
The IMD officially declares the onset of the Southwest Monsoon over Kerala only when specific criteria are simultaneously met after May 10:
- Rainfall: At least 60% of the 14 designated weather stations in Kerala, Lakshadweep, and coastal Karnataka (including Thiruvananthapuram, Punalur, Kollam, Alappuzha, Kottayam, Kochi, Vellanikkara, Amini Divi, Kannur, Kudulu, Kozhikode, Thalassery, Mangaluru, and Minicoy) must record a minimum of 2.5 mm of rainfall for two consecutive days.
- Wind Field: The depth of the westerly winds must extend up to 600 hectopascals (hPa) in the equator to 10° N latitude box, and 55° E to 80° E longitude box. The zonal wind speed at 925 hPa must exceed 15 knots.
- Outgoing Longwave Radiation (OLR): The OLR value, measured via satellite, must drop below 200 W/m2 in the area between 5° N to 10° N latitude and 70° E to 75° E longitude, indicating dense, deep convective cloud cover.
Chronological Advancement of the Monsoon
The Southwest Monsoon follows a highly structured timeline as it advances across the country, split into the Arabian Sea and Bay of Bengal branches.
May 15 to May 20
The monsoon makes its first entry into Indian territory over the Andaman and Nicobar Islands in the southeast Bay of Bengal.
June 1
The official normal date for the monsoon to strike the mainland of India along the southern tip of Kerala (Malabar Coast).
June 5 to June 10
The Arabian Sea branch advances along the Western Ghats to reach Mumbai, while the Bay of Bengal branch sweeps into Northeast India and gets deflected by the Himalayas toward West Bengal and Bihar.
June 15
The monsoon covers central India, including Madhya Pradesh, Chhattisgarh, and parts of Gujarat.
July 1 to July 5
The winds penetrate into Rajasthan, Haryana, and Punjab.
July 8
The normal date by which the southwest monsoon covers the entire country, including the extreme northwestern parts of Rajasthan.
Core Dynamics of Monsoon Withdrawal
Monsoon withdrawal, or the retreat phase, marks the systematic replacement of moist equatorial monsoonal air with dry, cold continental air. This process begins in northwestern India during September and is driven by the apparent southward migration of the Sun.
Principal Meteorological Triggers of Withdrawal
Weakening of the Monsoon Trough
As the Sun migrates south toward the Equator, solar insulation over the northern hemisphere drops sharply. The thermal low-pressure Monsoon Trough over the Indo-Gangetic plains loses its intensity and begins to collapse.
Establishment of Anti-cyclonic Circulation
Cooling temperatures over northwestern India lead to the subsidence of air, establishing a lower-tropospheric anti-cyclonic circulation centered over Rajasthan. This anti-cyclone drives out moisture-laden winds and initiates dry, continental air flow.
Southward Migration of the ITCZ
The ITCZ begins its steady retreat toward equatorial latitudes. As the axis of this low-pressure belt shifts south, the rain-bearing southwest monsoon winds are pulled out of the subcontinent.
IMD Criteria for Declaring Monsoon Withdrawal
The IMD utilizes separate criteria for declaring withdrawal from northwestern India and the rest of the country.
Criteria for Withdrawal from Northwest India (after September 1)
- Synoptic features must show the establishment of an anti-cyclonic circulation in the lower troposphere (at 850 hPa level).
- There must be a continuous reduction in moisture content over the region for at least five consecutive days, as indicated by precipitable water vapor data.
- The region must experience a complete cessation of rainfall for five consecutive days.
Criteria for Withdrawal from the Rest of India (South of 15° N)
- The withdrawal is declared based on the systematic cessation of southwest monsoon rainfall and the structural reversal of surface winds from southwesterly to northeasterly.
Chronological Timeline of Withdrawal
September 17
The normal date for the commencement of monsoon withdrawal from extreme northwestern Rajasthan.
October 1
The monsoon completely withdraws from Jammu & Kashmir, Punjab, Haryana, Uttarakhand, Uttar Pradesh, and Rajasthan.
October 15
The southwest monsoon withdraws from central India, Gujarat, Bihar, and Jharkhand, clearing the entire northern half of the country. This coincides with the complete cessation of the Southwest Monsoon over the Indian mainland.
October 15 to October 25
The simultaneous withdrawal of the Southwest Monsoon from the southern peninsula and the formal onset of the Northeast Monsoon over Tamil Nadu, coastal Andhra Pradesh, and adjoining parts of Karnataka and Kerala.
Comparative Summary: Onset vs. Withdrawal Mechanics
| Feature | Monsoon Onset Phase | Monsoon Withdrawal Phase |
| Primary Period | June to July | September to October |
| Initial Impact Zone | Andaman Islands (May 15), Kerala (June 1) | Northwest Rajasthan (September 17) |
| Upper Air Circulation | Subtropical Westerly Jet retreats north of Tibet; TEJ establishes at 14° N. | Subtropical Westerly Jet returns south of the Himalayas; TEJ collapses. |
| Lower Air Circulation | Cyclonic vortex over northern plains; cross-equatorial Somali Jet active. | Anti-cyclonic vortex over Rajasthan; continental winds establish. |
| ITCZ Position | Positioned over Indo-Gangetic Plains (20° N – 25° N). | Migrating south toward the Equator and Southern Hemisphere. |
| Moisture / Humidity | Abrupt rise in relative humidity; drop in Outgoing Longwave Radiation. | Sharp drop in relative humidity; clear, cloudless skies over land. |
| Associated Hazard | Torrential rainfall, lightning strikes, and flash floods along the coast. | “October Heat” over plains; severe tropical cyclones in the Bay of Bengal. |
High-Yield Facts and Trivia for UPSC Prelims
The Mascarene High Connection
The Mascarene High is a semi-permanent high-pressure cell located over the southern Indian Ocean near Madagascar (30° S). A stronger, intensely developed Mascarene High creates a sharper pressure gradient across the equator, driving a more forceful and timely monsoon onset over Kerala.
Monsoon Breaks vs. Early Withdrawal
A temporary cessation of rainfall over the plains during July or August is a “Monsoon Break,” caused by the monsoon trough moving up to the Himalayan foothills. This is distinct from “Monsoon Withdrawal,” which only occurs after September when lower-level anti-cyclonic circulation permanently establishes over northwest India.
The Madden-Julian Oscillation (MJO) Effect
The MJO is an eastward-moving band of rain-bearing clouds that travels along the equator every 30 to 60 days. If the active convective phase of the MJO aligns with the Indian Ocean during late May or early June, it accelerates cloud formation, leading to an early or intense monsoon onset. Conversely, its presence in the Pacific Ocean can delay the onset.
The Tropospheric Reversal
The onset of the Indian monsoon is unique because it involves a complete vertical reversal of winds. In the lower troposphere (up to 6 km), winds blow from the southwest (Southwest Monsoon), while in the upper troposphere (above 9 km), winds blow from the east (Tropical Easterly Jet).
Last Modified: June 5, 2026