Monsoon Mechanism Explained: SW vs NE Monsoon for UPSC Geography

Quick Answer: The Indian monsoon mechanism UPSC aspirants must master rests on four drivers: differential heating of land and sea, the seasonal northward shift of the ITCZ to about 25°N, the behaviour of jet streams (subtropical westerly jet, Somali jet, Tropical Easterly Jet), and the Madden-Julian Oscillation governing active-break cycles. The southwest monsoon sets in over Kerala around 1 June and withdraws from northwest India from about 17 September, giving way to the northeast monsoon that drenches Tamil Nadu.

Monsoon Mechanism in One Paragraph: The Direct Answer

The monsoon is a seasonal reversal of winds caused by the differential heating of the Asian landmass and the Indian Ocean. In summer, intense heating builds a low-pressure cell over northwestern India; the Inter-Tropical Convergence Zone (ITCZ) migrates north to roughly 25°N over the Ganga plain. Moist southwesterlies, strengthened by the cross-equatorial Somali Jet, surge in — a surge sustained aloft by the Tropical Easterly Jet. In winter the pattern reverses: pressure gradients flip, cool dry northeasterlies flow out of the land, and the retreating phase brings rain to Tamil Nadu. The MJO and phenomena such as ENSO and the IOD modulate the strength of both regimes.

Why India Has a Monsoon: The Basic Mechanism

The word monsoon derives from the Arabic mausim (season). India’s monsoon exists because of a giant pressure seesaw:

• Summer (June–September): rapid heating of the Tibetan Plateau and northwest India creates a thermal low; air flows from the high-pressure zone over the southern Indian Ocean toward land as moisture-laden southwest winds.
• Winter (October–December): the land cools faster than the sea; a high-pressure cell forms over Central Asia, and dry northeasterlies blow out toward the Bay of Bengal.

This is why the monsoon is best understood not as rain alone but as a wind regime that reverses direction twice a year.

ITCZ Shift and the Tropical Easterly Jet

The ITCZ — the equatorial trough where trade winds converge — normally sits near the equator, but during the Indian summer it migrates dramatically northward to about 25°N, riding over the Ganga plain. This unusually poleward shift (unique to South Asia, thanks to the Himalaya blocking cold polar air) is the single most important trigger of the SW monsoon. The Tibetan Plateau’s heating at 4–5 km altitude warms the upper air, and the pressure gradient at upper levels drives the Tropical Easterly Jet (TEJ) — an easterly current at ~14–16 km that streams from the South China Sea toward Africa, with its core over peninsular India near 10–15°N. The TEJ’s divergence aloft pumps up convection and sustains monsoon rainfall; a weak TEJ usually means a weak monsoon.

Role of Jet Streams: Subtropical Westerly Jet, Somali Jet and TEJ

Three jet streams shape the monsoon calendar:

1. Subtropical Westerly Jet (STWJ): in winter it flows south of the Himalaya, keeping the ITCZ suppressed and causing western disturbances over northwest India. Its withdrawal north of the Himalaya (typically by early June) is the classic herald of monsoon onset.
2. Somali (Findlater) Jet: a low-level (~1.5 km), high-speed cross-equatorial current that races along the East African coast, gathering moisture from the southwest Indian Ocean and delivering it to the Arabian Sea branch of the monsoon.
3. Tropical Easterly Jet (TEJ): forms because of the upper-tropospheric warm high over Tibet; maintains organized convection over India throughout the season.

Somali Jet and Indian Ocean Dipole (IOD)

The Somali Jet and the IOD are two sides of Indian Ocean behaviour. The IOD is measured by the difference in sea-surface temperature between the western Indian Ocean (Arabian Sea side) and the eastern Indian Ocean (near Sumatra):

• Positive IOD: warmer west, cooler east — strengthens moisture convergence over India, generally enhancing monsoon rainfall (e.g., 2019, 2020).
• Negative IOD: warmer east — suppresses convection over India, often dampening the monsoon.

Madden-Julian Oscillation (MJO) and Active-Break Cycles

The MJO is a 30–60 day pulse of enhanced and suppressed cloud and rainfall that travels eastward along the tropics, circling the globe from the Indian Ocean to the Pacific. When the MJO’s convectively active phase lies over the Indian Ocean (phases 2–4), the monsoon is in an active spell — heavy rain over central India. When the suppressed phase sits over us (phases 6–8), rainfall collapses over the core monsoon zone and a break sets in. For official monitoring of such oscillations, see the India Meteorological Department.

Onset of the Southwest Monsoon: Step-by-Step Sequence

1. STWJ retreats north of the Himalaya; heating over Tibet peaks.
2. ITCZ shifts north to ~25°N over the Ganga plain.
3. The Somali Jet strengthens, driving cross-equatorial moisture flow.
4. TEJ establishes over peninsular India.
5. Onset over Kerala around 1 June (with an IMD-declared normal window of about a week either side), marked by a sustained rainfall “burst.”
6. The current splits into the Arabian Sea branch (striking the Western Ghats by ~10 June) and the Bay of Bengal branch (entering the northeast and curving west over the Ganga plain by mid-June); the whole country is covered by roughly mid-July.

Break Monsoon: What It Is and Why It Happens

A break monsoon is a dry spell of several days to weeks during July–August when the monsoon trough (the surface manifestation of the ITCZ) shifts north to the foothills of the Himalaya. Rainfall over the Indo-Gangetic plain and central India collapses, while the northeastern states and the Himalayan belt actually receive more rain. The Monsoon Trough’s mean position — the axis from Sri Ganganagar through Allahabad/Prayagraj to the Bay of Bengal — is the reference line UPSC loves to test.

Withdrawal of the SW Monsoon and Onset of the NE Monsoon

Withdrawal begins from extreme northwest India around 17 September, progressing south and east, completing by mid-October as per IMD’s revised normals. As the ITCZ retreats south, the “October heat” — hot, humid, oppressive weather with clouding — grips northern India. By late October the northeasterlies establish, and the cyclonic storm tracks shift to the Bay of Bengal, feeding Tamil Nadu’s rainy season.

Northeast (Retreating) Monsoon Mechanics

Unlike the SW monsoon, the NE monsoon blows from land to sea and is dry by origin. But while crossing the Bay of Bengal, these winds pick up moisture and deposit it on the Tamil Nadu coast after the easterly trade winds are deflected by the topography. Key contrasts for Prelims:

El Niño, La Niña and ENSO Impact on Indian Monsoon

El Niño warms the central-eastern Pacific, weakening the Walker circulation and the trade winds; convection shifts away from the Indian Ocean region, and the SW monsoon over India typically weakens — most Indian drought years (e.g., 2002, 2009, 2015) were El Niño years. La Niña generally favours a stronger monsoon. However, a positive IOD can offset El Niño’s effect, as in 2018–19 era seasons — a nuance UPSC increasingly probes. Track ENSO updates via NOAA Climate.gov and PIB releases.

Prelims Question Patterns: How UPSC Frames Monsoon Questions

UPSC’s favourite monsoon angles over the past decade:

• Statement-based MCQs on ITCZ: e.g., the 2019 question asking where the ITCZ shifts during the monsoon (correct: over the Ganga plain during summer).
• Jet streams and Tibetan Plateau heating: roles of STWJ and TEJ.
• Break monsoon: which regions get rain during a break (NE India, Himalayan foothills).
• Retreating monsoon: why Tamil Nadu rains in October–December; “October heat.”
• El Niño/IOD: mechanism-level cause-and-effect rather than definitions.

Elimination strategy: strike out any option claiming the ITCZ moves south in July, that the TEJ is a westerly current, or that break monsoon rains increase over central India. Anchor the three numbers: 1 June (Kerala onset), 17 September (withdrawal begins), ~48% (Tamil Nadu’s NE-monsoon share).

Quick Revision Table

Frequently Asked Questions

Q: When does the southwest monsoon normally set in over Kerala and withdraw from India?

The SW monsoon normally sets in over Kerala around 1 June (IMD allows a normal window of roughly a week on either side). Withdrawal begins from northwest India around 17 September and completes by mid-October, per IMD normals.

Q: Why does Tamil Nadu receive rainfall during the retreating monsoon?

NE monsoon winds are dry over land but pick up moisture while crossing the Bay of Bengal, striking the Tamil Nadu coast. Additionally, during the SW monsoon, Tamil Nadu lies in the rain-shadow of the Western Ghats, so its annual rainfall is concentrated in October–December.

Q: What happens during a break monsoon phase?

The monsoon trough/ITCZ shifts to the foothills of the Himalaya. Rainfall over the Indo-Gangetic plain and central India stops, while rainfall increases over northeast India and the Himalayan belt.

Q: What is the MJO in simple terms?

It is a 30–60 day eastward-moving pulse of clouds and rainfall along the tropics. Its active phase over the Indian Ocean brings heavy monsoon rain; its suppressed phase causes breaks.

Q: How does El Niño affect the Indian monsoon?

El Niño weakens the Walker circulation and trade winds, shifting convection to the central Pacific; this generally reduces SW monsoon rainfall over India and is associated with many drought years.

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