# Indian Monsoon: Mechanism, El Niño, IOD and the Northeast Monsoon — UPSC Notes

By Shivam Singh, Founder & Mentor, Minimalist IAS · Published 14 Oct 2026
Source: https://www.minimalistias.com/blog/indian-monsoon-el-nino-iod-northeast-monsoon-upsc-notes

> A one-stop GS1 guide to the Indian monsoon: how it works, why it fails, how El Niño and the IOD change it, and why Tamil Nadu gets its rain in October–December. Includes the 2026 season, PYQs, MCQs and Mains questions.

**The Indian monsoon is a seasonal reversal of winds, driven by the heating of the Asian landmass and the northward shift of the ITCZ, and it brings India about three-quarters of its yearly rain.** El Niño and the Indian Ocean Dipole (IOD) decide whether a year is good or bad; the 2026 season ended at 87% of the long period average (LPA).

> **Why in news:** The 2026 southwest monsoon closed about 12.6% below normal (759.4 mm against an LPA of 868.6 mm) in an El Niño year, and attention has now turned to the October–December northeast monsoon, which gives Tamil Nadu about 48% of its annual rain (IMD).

**Updated (October 2026):** merged the three El Niño sections into one, added the northeast monsoon outlook and corrected the Prelims 2017 question wording.

## Syllabus mapping

- **Prelims:** Indian and World Geography — physical geography, climate; Environment (climate change).
- **GS Paper I (Mains):** important geophysical phenomena such as cyclones; Indian climate and monsoon; salient features of world physical geography.
- **Also useful in:** GS Paper III (agriculture, disasters) and Essay.

## What is the Indian monsoon and where does it rain?

Before the mechanism, the facts every question starts from.

- **Meaning:** from Arabic ‘mausim’ (season); winds reverse seasonally, south-westerlies in summer and north-easterlies in winter.
- **Scale:** the southwest monsoon (June–September) gives roughly 70–75% of India's annual rain; LPA 868.6 mm (1971–2020).
- **Uneven in space:** over 200 cm on the Western Ghats' windward side, the west coast and the north-east; 100–200 cm in the Ganga plain and central India; 50–100 cm in the Deccan interior; under 50 cm in west Rajasthan, Kutch and Ladakh.

## How does the southwest monsoon work? Classical and modern views

That uneven pattern has a physical cause: land heating that pulls the whole wind system northward.

*Figure: A six-step flowchart shows how land heating over Asia and the Tibetan plateau shifts the ITCZ north, reorganises the jet streams, draws the southeast trades across the equator as south-westerlies, and then brings onset, burst and withdrawal.*

- **Step 1 — land heats:** summer sun builds a deep low over north-west India and the Tibetan plateau.
- **Step 2 — ITCZ shift:** the zone of rising air moves from the equator to about 25°N over the Ganga plain, forming the monsoon trough.
- **Step 3 — jets:** the subtropical westerly jet leaves the Himalayan front and moves north of the plateau; the tropical easterly jet sets in over the peninsula.
- **Step 4 — trades cross the equator:** southeast trades deflect right (Coriolis) and become south-westerlies; the Somali (Findlater) jet carries moisture.
- **Steps 5–6 — onset, burst, withdrawal:** moist winds meet the Western Ghats and rain bursts over the west coast; as the sun moves south the low weakens.

| View | Idea | What it explains |
| --- | --- | --- |
| Classical (thermal) | Land and sea heat unequally (Halley) | Wind reversal |
| ITCZ shift | Equatorial trough migrates over the Ganga plain | Rain belt position |
| Jet stream and Tibetan plateau | Westerly jet moves north; easterly jet forms; plateau heats the air | Sudden burst in June |
| Walker circulation (ENSO) | East–west Pacific pressure see-saw | Why some years fail |

**Analogy:** think of the monsoon as a vast sea breeze with a calendar: the land heats up, the ocean's moist air rushes in, and the ocean's own moods (El Niño, IOD) decide how much arrives.

## Onset, branches, breaks and withdrawal

Once the system is running, it follows a fairly fixed calendar and two moisture-carrying branches.

*Figure: A timeline shows IMD's normal monsoon dates: onset over Kerala on 1 June, coverage of the whole country by 8 July, withdrawal from west Rajasthan from 17 September, and complete withdrawal by 15 October.*

- **Normal dates (IMD, revised 2020):** Kerala 1 June; whole country by 8 July (earlier 15 July); withdrawal from west Rajasthan about 17 September; complete by 15 October.
- **Arabian Sea branch:** hits the Western Ghats first; heavy windward rain, a rain shadow on the leeward Deccan.
- **Bay of Bengal branch:** turned west by the Arakan Yoma and the north-east hills, it moves up the Ganga plain; the Garo–Khasi funnel makes Meghalaya (Mawsynram, Cherrapunji) among the wettest places on earth.
- **Break:** a rainless spell of a week or more when the trough shifts towards the Himalayan foothills; the Himalayan belt and north-east get rain, the plains stay dry.
- **IMD categories (share of LPA):** deficient below 90%; below normal 90–95%; normal 96–104%; above normal 105–110%; excess over 110%.

## Why do some years fail? El Niño, La Niña, IOD and MJO

The calendar is regular; the rainfall is not, because the oceans change the moisture supply year to year.

- **ENSO:** El Niño–Southern Oscillation is the coupled ocean–atmosphere cycle of the tropical Pacific; El Niño and La Niña are its warm and cold phases.
- **Normal Walker circulation:** easterly trades pile warm water near Indonesia; air rises there and sinks over the eastern Pacific.
- **El Niño:** trades weaken, the warm pool shifts east, air sinks over India and Indonesia, so the southwest monsoon is usually weaker (droughts in 2009 and 2015). **La Niña** is usually the opposite.
- **ONI:** the Oceanic Niño Index is a three-month running average of sea-surface temperature anomaly in the Niño 3.4 region (5°S–5°N, 170°W–120°W); +0.5°C or more for at least five consecutive overlapping seasons marks El Niño, −0.5°C or less La Niña (NOAA).
- **IOD:** an east–west sea-surface temperature difference across the tropical Indian Ocean, measured by the Dipole Mode Index (western minus south-eastern box). A positive IOD often supports the monsoon and can offset El Niño; a negative IOD is the reverse. 1997: strong El Niño, yet a normal monsoon (about 102% of LPA), often credited to a positive IOD (debated).
- **MJO (Madden–Julian Oscillation):** an eastward-moving pulse of cloud and rain around the tropics, repeating in about 30–60 days; its wet phase over the Indian Ocean favours active spells, its dry phase breaks.

| Feature | El Niño | La Niña | Positive IOD | Negative IOD |
| --- | --- | --- | --- | --- |
| Ocean | Central and eastern Pacific warmer | Same region cooler | Western Indian Ocean warmer, east cooler | Western Indian Ocean cooler, east warmer |
| Walker circulation / winds | Weaken | Strengthen | — | — |
| Usual effect on Indian monsoon | Weaker, drought risk | Stronger, flood risk | Favourable; can offset El Niño | Unfavourable; can add to El Niño |
| Index | ONI +0.5°C or more | ONI −0.5°C or less | DMI positive | DMI negative |

**Do not confuse:** Western Disturbances are storms from the Mediterranean region that bring winter rain and snow to north-west India, not the northeast monsoon.

## Retreating and northeast monsoon: why Tamil Nadu is wet in October–December

The same El Niño that weakens the summer monsoon does not act the same way once the winds reverse, which is why the south-east coast gets its rain later.

- **Cause:** from June to September Tamil Nadu lies in the Western Ghats' rain shadow; in winter high pressure builds over land to the north; north-easterlies cross the warm Bay of Bengal, pick up moisture and meet the south-east coast.
- **Share of rain:** Tamil Nadu, Puducherry and Karaikal get about 48% of annual rain in this season (IMD); Bay depressions and cyclones, at their peak in Oct–Dec, are the rain makers.

| Feature | Southwest monsoon | Northeast monsoon |
| --- | --- | --- |
| Season | June–September | October–December |
| Wind | South-westerly, sea to land | North-easterly, land to sea (picks up moisture over the Bay) |
| Main rain regions | Western Ghats, central and north India, north-east | Tamil Nadu, Puducherry, coastal Andhra, south Karnataka, Kerala |
| El Niño effect | Usually weaker | Often normal or more; not guaranteed |

*Figure: A comparison shows El Niño weakening the southwest monsoon, which ended 2026 at 87% of normal, while the northeast monsoon gives Tamil Nadu about 48% of its annual rain and IMD's 2026 outlook for the south peninsula is normal.*

- **One El Niño, two monsoons:** El Niño weakens the Walker circulation over India in summer, but a pressure gap between the Bay and the Arabian Sea and warmer southern Bay water can feed moist air and low-pressure systems into the south-east coast in winter.
- **Precedent:** 2015 had a weak southwest monsoon but devastating Chennai floods in December.
- **IMD outlook 2026:** October–December rain over the south peninsula is most likely normal (88–112% of LPA). Treat ‘extreme rain’ as a risk, not a forecast.

## How is the monsoon forecast, and what is at stake?

Because El Niño and the IOD make the monsoon uncertain, forecasting and impact planning matter as much as the physics.

- **Forecast:** IMD issues a seasonal outlook in April and an update around the end of May; for 2026 it forecast a below-normal season and the actual was 87.4% of LPA. Models: statistical plus, since 2021, a multi-model ensemble (MMCFS).
- **Mission Mausam:** approved by the Union Cabinet in 2024 with an outlay of ₹2,000 crore over two years; led by the Ministry of Earth Sciences.
- **Impacts:** poor rain cuts kharif sowing and reservoirs and moves food prices and rural demand; drought in deficient years, floods and landslides in the Himalaya and north-east, cyclones on the east coast in Oct–Dec.
- **Local vs national:** 2026 was deficient overall, yet [Odisha had excess rain from Bay lows](https://www.minimalistias.com/blog/odisha-wettest-monsoon-bay-of-bengal-deep-depression).

## Using the monsoon in a Mains answer

- **Introduction:** seasonal wind reversal and its drivers.
- **Body:** heating, ITCZ, jets; ENSO, IOD, MJO, breaks; SW vs NE with the 2026 example.
- **Way forward and conclusion:** impact-based forecasting, district advisories, crop insurance (Mission Mausam); a linked ocean–atmosphere system, not a single wind.

## Prelims traps and 60-second revision

> **Prelims traps:**
> **(1) IOD is about the Indian Ocean (west minus east), not the Pacific.**
> **(2) El Niño is the warm phase of ENSO, not a monsoon wind.**
> **(3) Tamil Nadu's winter rain is the north-east monsoon; north-west India's winter rain is from Western Disturbances.**
> **(4) Positive IOD usually helps the monsoon; negative IOD usually does not.**
> **(5) Normal band is 96–104% of LPA.**

- **Q: Which three drivers create the monsoon?** Land heating, ITCZ shift, jet-stream change.
- **Q: What does a weak Walker circulation do to India?** Air sinks, so the southwest monsoon weakens.
- **Q: What did 2026 deliver?** 87.4% of LPA, a 12.6% deficit, deficient category.
- **Q: Share of Tamil Nadu's rain in Oct–Dec?** About 48%.

## Related PYQs

- **UPSC CSE Prelims 2017:**
**“With reference to ‘Indian Ocean Dipole (IOD)’ sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct?**
**1. IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.**
**2. An IOD phenomenon can influence an El Nino’s impact on the monsoon.”**
Select the correct answer using the code given below:
(a) 1 only
(b) 2 only
(c) Both 1 and 2
(d) Neither 1 nor 2
Answer: (b) 2 only.
- **UPSC CSE Mains 2014, GS Paper I (10 marks, 150 words):**
**“Most of the unusual climatic happenings are explained as an outcome of the El-Nino effect. Do you agree?”**
Use: partial agreement — ENSO plus the IOD offset and local systems (Odisha 2026).
- **UPSC CSE Mains 2017, GS Paper I (15 marks, 250 words):**
**“What characteristics can be assigned to monsoon climate that succeeds in feeding more than 50 percent of the world population residing in Monsoon Asia ?”**
Use: seasonal reversal, rain with warmth, river water and the kharif calendar.

**See the detailed explanations:** [Prelims 2017](https://www.minimalistias.com/pyqs/prelims/2017) · [Mains 2017](https://www.minimalistias.com/pyqs/mains/2017) · [Mains papers](https://www.minimalistias.com/pyqs/mains) in the free PYQ vault, with year-wise analysis. Free to browse; free sign-up to download.

## Mentor’s note

- **Don’t write that El Niño always means drought.** Say ‘usually weaker’ and name the modifiers (IOD, local Bay systems); 1997 is the counter-example.
- **Quote IMD’s categories exactly.** The 2026 October–December outlook for the south peninsula is ‘normal’ (88–112% of LPA), not ‘above normal’. The 2017 Prelims IOD option swapped the south-eastern Indian Ocean for the ‘Eastern Pacific’.

## Target Prelims MCQs

**Consider the following statements:**

1. **During an El Niño, the Walker circulation over the equatorial Pacific weakens.**
2. **The Oceanic Niño Index is based on sea-surface temperature anomalies in the Niño 3.4 region.**
3. **La Niña is usually associated with a weaker Indian southwest monsoon.**

How many of the above statements are correct?  
(a) Only one  
(b) Only two  
(c) All three  
(d) None

**Answer: (b) Only two**

**Explanation:**

**Statement 1 is correct:** the trades weaken as warm water shifts east.

**Statement 2 is correct:** it is a three-month running mean of the Niño 3.4 anomaly.

**Statement 3 is incorrect:** La Niña usually favours a stronger monsoon.

**Consider the following statements:**

**Statement-I: Winter rainfall in north-west India is brought by the northeast monsoon.**

**Statement-II: Western Disturbances are extra-tropical storms that originate in the Mediterranean region.**

Which one of the following is correct in respect of the above statements?  
(a) Both Statement-I and Statement-II are correct and Statement-II explains Statement-I  
(b) Both Statement-I and Statement-II are correct, but Statement-II does not explain Statement-I  
(c) Statement-I is correct, but Statement-II is incorrect  
(d) Statement-I is incorrect, but Statement-II is correct

**Answer: (d) Statement-I is incorrect, but Statement-II is correct**

**Explanation:**

**Statement-I is incorrect:** that rain comes from Western Disturbances; the northeast monsoon rains the south-east peninsula.

**Statement-II is correct:** they come from the Mediterranean region.

**Consider the following pairs:**

**Term : Description**

1. **Somali (Findlater) jet : a fast low-level cross-equatorial south-westerly flow that carries moisture to India**
2. **Madden–Julian Oscillation : an eastward-moving pulse of cloud and rain that repeats in about 30–60 days**
3. **Mascarene High : a high-pressure area over the Bay of Bengal**

How many of the pairs given above are correctly matched?  
(a) Only one  
(b) Only two  
(c) All three  
(d) None

**Answer: (b) Only two**

**Explanation:**

**Pair 1 is correctly matched:** it feeds the Arabian Sea branch.

**Pair 2 is correctly matched:** its phases shape active and break spells.

**Pair 3 is incorrectly matched:** the Mascarene High lies over the southern Indian Ocean near the Mascarene Islands.

## Mains practice questions

> **El Niño does not affect every part of India in the same way. Discuss with reference to the southwest and northeast monsoons. (Answer in 150 words; 10 Marks)**

> **Critically examine the role of ocean–atmosphere interactions, such as ENSO and the Indian Ocean Dipole, in shaping the Indian monsoon, and discuss how the forecasting system has responded to this variability. (Answer in 250 words; 15 Marks)**

## Frequently asked questions (FAQs)

### What was the monsoon rainfall in 2026?

The 2026 southwest monsoon gave 759.4 mm against an LPA of 868.6 mm: about 87% of normal, a 12.6% deficit, in an El Niño year (IMD).

### How does El Niño affect the Indian monsoon?

El Niño weakens the Walker circulation, so air sinks over India and the southwest monsoon is usually weaker. A positive IOD can soften this: a tendency, not a rule.

### What is the Indian Ocean Dipole?

The sea-surface temperature difference between the western and south-eastern tropical Indian Ocean. A positive phase usually helps the Indian monsoon.

### Why does Tamil Nadu get rain in October–December?

Tamil Nadu is in the Western Ghats' rain shadow in June–September. In October–December north-easterlies and Bay of Bengal systems bring about 48% of its annual rain (IMD).

## Related reading

- [Why Odisha got excess rain when India was deficient in 2026](https://www.minimalistias.com/blog/odisha-wettest-monsoon-bay-of-bengal-deep-depression)
- [Revise the NCERT basics of the Indian monsoon in NCERT Foundations](https://www.minimalistias.com/resources/ncert)
- [Practise previous-year Geography questions in the free PYQ vault](https://www.minimalistias.com/pyqs)
- [All Daily UPSC Briefs](https://www.minimalistias.com/resources/daily-brief)

## Sources

- [IMD press release: seasonal outlook for the post-monsoon season 2026](https://mausam.imd.gov.in/Forecast/marquee_data/Press_Release%20OND_30-09-2026.pdf)
- [IMD press release: new normal dates of monsoon onset and withdrawal](https://mausam.imd.gov.in/backend/assets/press_release_pdf/IMD_Press_release_new_onset_and_withdrawal_dates%28Final%29.pdf)
- [PIB: Cabinet approves Mission Mausam](https://www.moes.gov.in/sites/default/files/PIB2053898.pdf)
- [NOAA Climate.gov: Oceanic Niño Index](https://www.climate.gov/print/8428)
- [UPSC: previous question papers](https://www.upsc.gov.in/examinations/previous-question-papers)

**Minimalist IAS static notes — by Shivam Singh.**

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