Indian Monsoon Mechanism Explained: ITCZ, Jet Streams and MJO

In one line: The Indian monsoon bursts in June when the ITCZ shifts north over the subcontinent, the Somali Jet ferries moisture across the equator, and Tibetan Plateau heating builds an upper-air low that together pull moist southwesterlies onto the Kerala coast.

Read the revision table at the end first, then work through the mechanisms in order, then attempt the PYQ drill. That sequence fixes the Indian monsoon mechanism — ITCZ, jet streams and MJO — permanently in your memory.

Direct Answer: Why Does the Monsoon Burst in June?

By late May, three forces align. The Inter-Tropical Convergence Zone (ITCZ) migrates north with the overhead sun and sits over the Ganga plain by mid-June. The Somali Jet, a cross-equatorial low-level current off the East African coast, accelerates and pumps Arabian Sea moisture toward Kerala. Meanwhile, intense heating of the Tibetan Plateau creates a powerful upper-air low-pressure zone that intensifies the whole circulation. The southwest monsoon “bursts” when all three lock into place — normally around 1 June over Kerala.

The Monsoon as a System: Seasonal Reversal of Winds

The word monsoon comes from the Arabic mausim (season). At its base lies differential heating: land heats faster than sea, so summers build a thermal low over the Asian landmass while oceans stay comparatively cool and high-pressure. Pressure gradients drive winds from sea to land (southwest in summer), and the pattern reverses in winter (northeast winds). Examiners love this framing: differential heating is the engine, but the ITCZ, jet streams, Tibetan heating and ocean couplings are the gears that determine onset timing and strength. Never write “monsoon = differential heating alone” in Mains — that is a half-mark answer.

ITCZ: The Inter-Tropical Convergence Zone and Its Northward Shift

The ITCZ is the belt where northeast and southeast trade winds converge, forcing moist air to rise and generating heavy convection. In June-July, following the apparent movement of the sun, the ITCZ shifts dramatically northward — to around 20–25°N over the Ganga plain (sometimes called the monsoon trough over India). This northward draw pulls in moisture-laden southeast trades from the southern Indian Ocean, which cross the equator, deflect right (Coriolis), and arrive as the southwest monsoon. The position of the monsoon trough day-to-day decides whether the plains get rain or a break spell — a favourite UPSC angle.

Tibetan Heating: The ‘Heat Engine’ of the Monsoon

The Tibetan Plateau, averaging over 4,500 m, sits like a giant radiator under the summer sun. By May-June it heats intensely, forming a strong low-pressure zone in the upper troposphere (around 200 hPa). This upper-air cyclonic circulation does two things: it strengthens the meridional pressure gradient driving the monsoon, and it sets up the Tropical Easterly Jet on its southern flank. Strong Tibetan snow cover in winter = weaker summer heating = weaker monsoon; this is why snow cover over Tibet is tracked as a monsoon predictor. This is why researchers call Tibet the monsoon’s “heat engine.”

Somali Jet (Findlater Jet): The Onset Accelerator

The Somali Jet — named after meteorologist John Findlater — is a low-level, cross-equatorial jet blowing at speeds exceeding 45 knots at roughly 1–1.5 km altitude, hugging the Horn of Africa. The East African highlands act as a channel, accelerating the flow as it crosses the equator and turns northeast over the Arabian Sea. Its role in onset: it transports enormous quantities of moisture toward the Kerala coast in June, converting a sluggish wind reversal into a sudden “burst.” A stronger Somali Jet means earlier, more vigorous onset; its interannual variability is directly linked to onset dates.

Subtropical Westerly Jet Streams and the Onset Timing

During winter, the subtropical westerly jet stream flows south of the Himalayas, roughly along 25–30°N, effectively “capping” northern India with westerly, dry flow. The monsoon cannot establish while this jet sits over the region. As summer progresses, the jet migrates north of the Himalayas (by late May/early June), removing the obstruction and allowing the monsoon circulation and ITCZ to sweep north. If the westerly jet lingers — as happens in disturbed years — onset is delayed or weak over northwest India. This timing relationship is a repeated SSC CGL and UPSC Prelims trap.

Tropical Easterly Jet and Upper-Air Circulation

Tibetan heating produces a strong upper-level temperature gradient, and under thermal wind balance this generates an easterly jet near 14–15°N at about 200 hPa over peninsular India — the Tropical Easterly Jet (TEJ), active June–September. The TEJ’s core (strongest around 15°N) drives upper-level divergence (air spreading out and sinking away) above the monsoon trough, which reinforces rising motion below — sustaining rainfall. Its strength influences rainfall distribution across central and peninsular India; a weak TEJ often accompanies drought years. Remember the pairing: Tibetan heating (cause) → Tropical Easterly Jet (effect) → monsoon burst and rainfall distribution (outcome).

Madden-Julian Oscillation (MJO): The 30-60 Day Pulse

So far we’ve explained onset. But why does the monsoon rain come in two-week bursts followed by dry spells? Enter the MJO — an eastward-moving pulse of clouds, rainfall and circulation that circles the globe along the tropics every 30–60 days. When the MJO’s convectively active phase passes over the Indian Ocean (phases 2–3 of the standard 8-phase tracking), India gets active monsoon spells — heavy rain, strong westerlies. When the suppressed phase arrives, break monsoon conditions set in — the monsoon trough shifts to the Himalayan foothills, rain largely stops over the plains. The MJO explains intra-seasonal variability that neither ENSO nor differential heating can. Keep your explanation this simple in Prelims; elaborate with phase diagrams only in Mains.

Indian Ocean Dipole (IOD) and ENSO: Ocean Coupling

Two ocean-atmosphere couplings modulate seasonal strength:

• IOD: a temperature seesaw between the western and eastern Indian Ocean. A positive IOD (warm west, cool east near Sumatra) shifts convection westward toward India — generally a good monsoon. Negative IOD does the opposite.
• ENSO (El Niño/La Niña): El Niño warms the central-eastern Pacific, shifts Walker circulation east, and typically weakens the Indian monsoon; La Niña generally strengthens it. A strong positive IOD can, however, neutralise an El Niño’s damage — as in 2018-19.

Reference authoritative tracking at the IMD Pune and NOAA Climate.gov ENSO pages.

Onset and Withdrawal: Normal Dates and IMD Definition

Onset over Kerala: 1 June (normal date, with a standard deviation of about 8 days). The IMD declares onset using objective criteria: rainfall of 2.5 mm or more for two consecutive days over a defined Kerala grid, plus wind field (westerly speed ≥ 600 hPa threshold) and outgoing longwave radiation values. The monsoon then advances northwestward, covering the whole country by around 15 July. Withdrawal begins from northwest India around 17 September, completing the retreat from the peninsula by mid-October. Memorise both date pairs — SSC asks them directly.

Previous Year Questions (PYQs): UPSC and SSC Patterns

• UPSC Prelims-style: “The Tropical Easterly Jet at 200 hPa forms due to — (a) ENSO (b) Tibetan Plateau heating (c) Somali Jet (d) IOD.” Answer: Tibetan Plateau heating.
• UPSC Prelims 2012 pattern: Questions on the normal date of monsoon onset over Kerala and the criteria used by IMD.
• UPSC Mains (GS1): “Why is the Tibetan Plateau considered the heat engine of the Indian monsoon? Explain.” and “Discuss the role of the Somali Jet in the onset of the southwest monsoon.”
• SSC CGL pattern: The ITCZ over India in July is also called the — monsoon trough; MJO period — 30–60 days; withdrawal start — Rajasthan/northwest India, mid-September.

Exam-Ready Summary: Mnemonics and Quick Revision Table

Mnemonic — “SIT-JEM“: Somali Jet, ITCZ shift, Tibetan heating, Jet streams (westerly retreat + easterly formation), ENSO/IOD, MJO. Six controls, one monsoon.

Read this table once tonight and once after your mock test. Then answer the FAQs below without looking back.

Frequently Asked Questions

Q: When does the monsoon normally burst over Kerala?

Around 1 June per IMD normal dates. Onset is declared using objective criteria — two consecutive days of rainfall ≥2.5 mm over the Kerala grid, supported by wind field and outgoing longwave radiation thresholds.

Q: Is the monsoon caused only by differential heating?

No. Land-sea differential heating is the base engine, but the ITCZ shift, Somali Jet, Tibetan heating, jet stream migration, MJO and ENSO/IOD all modulate onset timing and rainfall strength.

Q: How does MJO affect monsoon rainfall in India?

The MJO’s eastward-moving pulse enhances convection during its active phase (heavy rain spells over India) and suppresses it during the break phase, cycling every 30–60 days through the monsoon season.

Q: What happens if the subtropical westerly jet stays over the Himalayas?

It blocks the northward shift of monsoon circulation and the ITCZ, delaying onset and weakening rainfall, especially over northwest India.

Q: Which PYQs have asked about jet streams and monsoon?

UPSC Prelims and Mains have repeatedly asked on the Tropical Easterly Jet’s Tibetan origin, ITCZ/monsoon trough position, Somali Jet role and IMD onset criteria; SSC focuses on normal onset/withdrawal dates and the MJO’s 30–60 day period.

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