El Niño, La Niña and IOD: Quick Answer
El Niño is the warm phase of the El Niño–Southern Oscillation (ENSO): sea surface temperatures in the central and eastern equatorial Pacific become unusually warm, and the Indian summer monsoon usually weakens. La Niña is the cold phase of the same cycle and usually strengthens the monsoon. The Indian Ocean Dipole (IOD) is a separate see-saw of sea temperatures between the western and eastern Indian Ocean; a positive IOD (warmer west, cooler east) tends to support monsoon rain and can partly offset El Niño.
Key exam takeaway: El Niño → weaker monsoon, drought risk. La Niña → stronger monsoon, flood risk. Positive IOD → helps the monsoon. Negative IOD → hurts it. IMD watches all three before its monsoon forecast.
The 2026 monsoon is a live example. IMD says 2026 June–September rainfall was 87% of the Long Period Average (759.4 mm against an LPA of 868.6 mm), the 4th lowest since 2001, in a year when strong El Niño conditions built up over the Pacific. As of IMD's press release of 5 November 2026, very strong El Niño conditions prevail and the IOD is neutral, expected to stay neutral till December 2026.
This note is part of our geography series. Start with the basics in Indian Climate Notes for SSC (monsoon mechanism and Koppen) and then come back here for ENSO and IOD.
What Is ENSO? (The Three Phases)
The El Niño–Southern Oscillation is a natural climate cycle produced by the ocean and atmosphere over the tropical Pacific working together. IMD lists three phases:
| Phase | Pacific sea surface temperature | Usual effect on Indian summer monsoon |
|---|---|---|
| El Niño (warm phase) | Unusually warm in the central and eastern equatorial Pacific | Weaker monsoon, more dry spells, higher temperatures |
| La Niña (cold phase) | Unusually cool in the same region | Stronger monsoon, more widespread rain, flood risk |
| ENSO-neutral | Close to the long-term average | No strong push either way |
"Southern Oscillation" is the atmospheric half of the cycle: a see-saw in surface air pressure between the western Pacific (around Indonesia and northern Australia) and the eastern Pacific. When the pressure pattern flips, the trade winds weaken and warm water spreads east, which is El Niño.
How often does ENSO happen?
As per IMD's ENSO FAQ, ENSO events usually develop every 2 to 7 years, begin in spring, strengthen through the year and peak between November and February. An El Niño usually lasts 9 to 18 months, while a La Niña can persist for two or even three years.
How El Niño Weakens the Indian Monsoon
In a normal year, strong easterly trade winds push warm surface water towards Indonesia. Air rises over this warm western Pacific and the eastern Indian Ocean, and the large-scale circulation helps carry moisture towards India.
During El Niño:
- The trade winds weaken and warm water shifts towards the central and eastern Pacific.
- The zone of rising air and heavy rain moves east with it, over the open Pacific.
- Over the Indian region, the large-scale circulation that carries moisture becomes weaker, so monsoon rainfall tends to fall short.
IMD's FAQ lists the typical impacts: below-normal monsoon rainfall, more frequent dry spells, higher temperatures and heatwaves, lower reservoir inflows and lower farm output. It also names 1982, 1987, 2002, 2009 and 2015 as major Indian drought years that occurred during El Niño events.
Does every El Niño cause drought?
No, and this is a favourite statement-type question. The Ministry of Earth Sciences told Parliament (PIB, 11 December 2024) that since 1950 there have been 16 El Niño years, and in 7 of them Indian monsoon rainfall was below normal. The link is stronger in the second half of the season, especially September. Other drivers such as the IOD, the Madden–Julian Oscillation (MJO), Eurasian snow cover and the Atlantic can soften or worsen the effect.
How La Niña Affects India
La Niña is the opposite: the eastern Pacific cools, trade winds strengthen and the monsoon circulation is usually stronger. As per IMD and PIB, La Niña years generally bring normal to above-normal monsoon rainfall over most of India, which helps rain-fed farming and groundwater but can also bring floods. PIB adds that extreme north India and parts of Northeast India can still see below-normal rain in La Niña years, and winters tend to be colder.
What Is the Indian Ocean Dipole (IOD)?
The Indian Ocean Dipole is a difference in sea surface temperature between two "poles" of the tropical Indian Ocean: the western pole (Arabian Sea, near the Horn of Africa) and the eastern pole (near Indonesia). IMD notes that it is sometimes called the "Indian Niño" and that it was identified by climate researchers, including an Indian meteorologist, in 1999.
| IOD phase | Sea temperature pattern | Usual effect on Indian monsoon |
|---|---|---|
| Positive IOD | Western Indian Ocean warmer than normal, eastern cooler | Supports monsoon rainfall; can partly offset El Niño |
| Negative IOD | Eastern Indian Ocean warmer, western cooler | Less favourable for monsoon rainfall |
| Neutral IOD | No strong difference | No extra push |
IMD treats an IOD index value of +0.4 °C as the positive threshold, and a value must stay above it for some time before a positive IOD event is declared.
Memory trick: "Positive IOD = Positive for India." Warm water near India's side (west) means more moisture-laden air over the Arabian Sea.
El Niño vs La Niña vs IOD: Difference Table
| Point | El Niño | La Niña | Indian Ocean Dipole |
|---|---|---|---|
| Ocean | Pacific | Pacific | Indian Ocean |
| What changes | Central and east Pacific warms | Central and east Pacific cools | West–east temperature difference in Indian Ocean |
| Trade winds | Weaken | Strengthen | Not the defining feature |
| Effect on monsoon | Usually negative | Usually positive | Positive IOD helps, negative IOD hurts |
| Typical duration | 9–18 months | Up to 2–3 years | Develops in summer, peaks in autumn |
| Index used | ONI / RONI (Niño-3.4 region) | ONI / RONI | Dipole Mode Index |
How Strength Is Measured: ONI and RONI
IMD's FAQ explains that ENSO is tracked in the Niño-3.4 region (5°N–5°S, 170°W–120°W). The classic Oceanic Niño Index (ONI) is the three-month running mean of sea surface temperature anomaly there; ONI of +0.5 °C or more indicates El Niño conditions and −0.5 °C or less indicates La Niña.
| Index value (°C) | Classification |
|---|---|
| 0.5 to below 1.0 | Weak El Niño |
| 1.0 to below 1.5 | Moderate El Niño |
| 1.5 to below 2.0 | Strong El Niño |
| 2.0 and above | Very strong El Niño |
The same bands with a minus sign apply to La Niña. IMD also notes that there is no universally accepted definition of a "super" El Niño. Because the whole tropical ocean is warming, agencies now use the Relative Oceanic Niño Index (RONI), which subtracts the average tropical warming. According to IMD's FAQ, NOAA's Climate Prediction Center adopted RONI as its operational index in February 2026.
ENSO and IOD in 2026 (As of 7 November 2026)
| Item | Official figure | Source |
|---|---|---|
| 2026 SW monsoon rainfall, all India | 759.4 mm, 87% of LPA (deficient) | IMD, Salient Features of 2026 SW Monsoon, 30.09.2026 |
| Long Period Average (1971–2020) | 868.6 mm | IMD |
| Onset over Kerala | 4 June 2026 (normal date 1 June) | IMD |
| Monsoon covered entire country | 9 July 2026 (normal 8 July) | IMD |
| Region-wise | NW India 94%, Central 97%, South Peninsula 76%, East and NE 74% of LPA | IMD |
| ENSO status | Very strong El Niño conditions; likely during November–December 2026 | IMD press release, 05.11.2026 |
| IOD status | Neutral; likely to stay neutral till December 2026 | IMD press release, 05.11.2026 |
These are time-sensitive figures; check IMD's latest weekly press release before quoting them in a current affairs answer.
El Niño, La Niña and IOD: 10 Practice MCQs
Q1. El Niño is associated with unusual warming of sea surface temperatures in the: (a) Western Indian Ocean (b) Central and eastern equatorial Pacific (c) North Atlantic (d) Arabian Sea Answer: (b). El Niño is the warm phase of ENSO over the central and eastern equatorial Pacific.
Q2. Which phase of ENSO generally strengthens the Indian summer monsoon? (a) El Niño (b) La Niña (c) Neutral (d) None Answer: (b). La Niña usually brings normal to above-normal monsoon rain.
Q3. A positive Indian Ocean Dipole means: (a) Eastern Indian Ocean warmer than western (b) Western Indian Ocean warmer than eastern (c) Pacific warmer than Indian Ocean (d) Bay of Bengal cooler than Arabian Sea Answer: (b). Positive IOD = warm west, cool east; it tends to support the monsoon.
Q4. ENSO events typically peak during: (a) March–May (b) June–August (c) November–February (d) September only Answer: (c). IMD says ENSO peaks between November and February.
Q5. The Niño-3.4 region used to monitor ENSO lies in the: (a) Indian Ocean (b) Atlantic Ocean (c) Equatorial Pacific Ocean (d) Southern Ocean Answer: (c). 5°N–5°S, 170°W–120°W.
Q6. An Oceanic Niño Index value of 1.7 °C indicates: (a) Weak El Niño (b) Moderate El Niño (c) Strong El Niño (d) La Niña Answer: (c). 1.5 to below 2.0 is "strong".
Q7. As per the Ministry of Earth Sciences, out of 16 El Niño years since 1950, Indian monsoon rainfall was below normal in: (a) All 16 (b) 12 (c) 7 (d) 2 Answer: (c). This is why El Niño raises the risk of a poor monsoon but does not guarantee drought.
Q8. The Indian Ocean Dipole is also called: (a) Indian Niño (b) Southern Oscillation (c) Walker Cell (d) Mascarene High Answer: (a).
Q9. Which of these was NOT an El Niño drought year in India named by IMD? (a) 1987 (b) 2002 (c) 2009 (d) 2020 Answer: (d). IMD lists 1982, 1987, 2002, 2009 and 2015.
Q10. The 2026 southwest monsoon rainfall for India as a whole was what percentage of its LPA? (a) 106% (b) 98% (c) 87% (d) 75% Answer: (c). 759.4 mm against 868.6 mm (IMD, 30 September 2026).
For more geography practice, attempt the SSC CGL geography mock test 01 and the SSC CHSL geography mock test 01, or try our Indian climate MCQs for SSC.
El Niño, La Niña and IOD FAQs
What is the difference between El Niño and La Niña?
El Niño is the warm phase of ENSO, when the central and eastern Pacific becomes unusually warm and the Indian monsoon tends to weaken. La Niña is the cold phase, when the same region cools and the monsoon tends to strengthen.
How does El Niño affect India?
It usually weakens the southwest monsoon, increasing the chance of below-normal rain, dry spells, heatwaves and lower crop output. The effect is not certain: since 1950, only 7 of 16 El Niño years had below-normal monsoon rain.
What is a positive IOD?
A positive Indian Ocean Dipole is when the western Indian Ocean is warmer than normal and the eastern part (near Indonesia) is cooler. It generally supports monsoon rainfall over India.
Can a positive IOD cancel El Niño?
It can partly offset El Niño's drying effect, as IMD's FAQ notes, but it does not always fully cancel it. In 2026, the IOD stayed neutral during the season.
Which body forecasts El Niño for India?
The India Meteorological Department (IMD) uses its Monsoon Mission Coupled Forecast System (MMCFS) and other global models, and publishes ENSO and IOD status in its weekly press releases and monsoon forecasts.
Practice Next for Indian Geography
- Core notes: Indian Climate Notes for SSC
- Related: Cyclones in India: IMD categories and names, Northeast monsoon in India, Kharif, Rabi and Zaid crops list
- Also read: Types of soil in India, Indian rivers origin and end list
- Mocks: SSC CGL geography mock test 02, SSC GD geography mock test 01
Sources: IMD FAQ on ENSO, IOD and Indian Summer Monsoon; PIB, Ministry of Earth Sciences, 11 December 2024; IMD, Salient Features of the 2026 Southwest Monsoon; IMD extended range press release, 05.11.2026; IMD@150 science innovations (IOD). Last checked 7 November 2026.