El Niño, IOD and Monsoon: Risk, Not a Verdict

El Niño, Indian Ocean Dipole and the monsoon: risk, not verdict - symbolic image

Why in the News

According to the India Meteorological Department's (IMD) October 2026 ENSO-IOD bulletin, the Pacific has a strong to very strong El Niño: by September 2026 the temperature anomaly of the Niño 3.4 region reached about +2.0 degrees Celsius and, according to the Monsoon Mission Coupled Forecast System (MMCFS), there is a high probability of this condition persisting till January-March 2027. According to news reports (2 November 2026), this "fierce" El Niño is estimated to peak in December, and according to the IMD's official season-closing report of 30 September, the 2026 south-west monsoon was 87 per cent of the long-period average (759.4 mm) - the fourth lowest since 2001 and the 13th lowest since 1901; in east and north-east India it was 1017.1 mm (74 per cent of the long-period average) - the lowest since 1901. Meanwhile, on 31 October the IMD forecast above-normal temperatures and a weak north-east monsoon for November. The question is whether a strong El Niño necessarily means a poor monsoon - history says, no.

El Niño, La Niña and the Indian Monsoon's Relationship

The unusual warming of the surface temperature of the central-eastern part of the equatorial Pacific (the Niño 3.4 region) is El Niño, and the unusual cooling is La Niña; this whole cycle is called ENSO. In El Niño, the Walker circulation over the Pacific weakens, which puts pressure on the Indian monsoon's winds and rainfall system - which is why El Niño years have tended to bring less rain in India. According to the IMD bulletin, this time's sequence: La Niña from September 2025 to February 2026, neutral in March 2026, the El Niño threshold crossed in May 2026, and +2.0 degrees Celsius by September.

History: Tendency, Not a Mechanism

According to news analysis, since 1950 the monsoon season has collided with a moderate or strong El Niño seven times; in five of these years (including 2026), rainfall stayed 10 per cent or more below the long-period average - the IMD calls this "deficient". But the exceptions matter: in 1997, despite one of the strongest El Niños of the century, rainfall stayed almost normal (the IMD had forecast an 8 per cent shortfall), and in 2023 rainfall stayed in the "below normal" category. That is, El Niño raises the risk; it does not decide the outcome.

Indian Ocean Dipole (IOD): The Second Wheel of Balance

The IOD is an index of the temperature difference between the western and eastern equatorial parts of the Indian Ocean. A positive IOD (west warmer) is considered favourable for the Indian monsoon and can balance El Niño's ill effect to some extent - this very factor came to the fore in years like 1997. According to the IMD's October 2026 bulletin, the IOD is currently neutral: both parts are warmer than normal, but the western part more so; the MMCFS estimates the neutral state will persist, while some international centres expect a positive phase in the near term. The Arabian Sea's temperature remains consistently high.

The Picture of 2026's Weather

According to the IMD's season-closing report of 30 September, the 2026 south-west monsoon was 87 per cent of the long-period average (759.4 mm); east-northeast India got 1017.1 mm (74 per cent, the lowest since 1901) and the south peninsula 542.0 mm (76 per cent, the fourth lowest since 1901), while the north-west stayed at 94.1 per cent and central India at 97 per cent; 17 of 36 sub-divisions (42 per cent of the area) had deficient rainfall; according to the IMD, the monsoon had withdrawn from the whole country on 15 October. Now the north-east monsoon (October-December) is important for Tamil Nadu and the south peninsula, which El Niño can affect in a different way - the IMD's 31 October review signals above-normal temperatures and a weak north-east monsoon for November. In the coming seasonal forecasts, the watch will stay on El Niño's December peak, the IOD's direction and the north-east monsoon's actual rainfall.

Exam Angle

Mains (GS-I/GS-III): the climate system (ENSO, IOD, Walker circulation), determinants of the Indian monsoon, agriculture and water security in the context of climate change.

Prelims: the Niño 3.4 region, the ENSO cycle (El Niño/La Niña/neutral), IOD phases, MMCFS, the IMD's rainfall classification (deficient = shortfall of more than 10%), the north-east monsoon.

Prelims Practice Questions

Question 1. With reference to the IMD's October 2026 ENSO-IOD bulletin, which statement is correct?

(a) La Niña conditions prevail in the Pacific Ocean

(b) The temperature anomaly of the Niño 3.4 region was about +2.0 degrees Celsius in September 2026, and a strong-to-very-strong El Niño is likely to persist till January-March 2027

(d) The IOD is currently in a firmly positive phase

Answer and Explanation

Answer: (b). According to the IMD bulletin, the Niño 3.4 anomaly was about +2.0 degrees Celsius in September 2026; the MMCFS gives a high probability of the strong-to-very-strong El Niño persisting till January-March 2027. The IOD is neutral, not firmly positive.

Question 2. Consider the following statements about the Indian Ocean Dipole (IOD):

Statement 1: In a positive IOD, the western equatorial Indian Ocean is warmer than the eastern part

Statement 2: A positive IOD can balance El Niño's anti-monsoon effect to some extent

Statement 3: According to the IMD's October 2026 bulletin, the IOD is in a firmly negative phase

Which of the above statements are correct?

(a) Only 1 and 2

(b) Only 2 and 3

(d) 1, 2 and 3

Answer and Explanation

Answer: (a). In a positive IOD the western part is warmer and it is considered favourable for the monsoon (examples like 1997). In the October 2026 bulletin the IOD is neutral, not negative, so Statement 3 is wrong.

Mains Practice Question

"A strong El Niño is a risk for the Indian monsoon, not a verdict." Analyse this statement with reference to the role of other climate factors including the IOD. (Write the answer in 250 words. 15 marks)

Answer Outline

Introduction: in October 2026, between the strong-to-very-strong El Niño (Niño 3.4 +2.0 degrees Celsius) and the 2026 monsoon staying 13 per cent below the long-period average (87 per cent), this question is central.

Main Body: the mechanism: with El Niño the Walker circulation weakens, putting pressure on the monsoon; the tendency: of seven collisions since 1950, five were deficient years. The counter: in 1997, despite the strongest El Niño, rainfall stayed almost normal - the balancing of factors like a positive IOD; in 2023, a limited shortfall. The present: the IOD is neutral, the MMCFS sees neutrality likely to continue; some international centres expect a positive phase. Implications: risk-based, not destiny-based, preparedness for agricultural planning, reservoir management and regional inequalities such as east-northeast India.

Conclusion: without understanding the ENSO-IOD combination and regional inequality, treating El Niño as the monsoon's sole determinant is incomplete from both the scientific and the policy view.

Way Forward

El Niño's peak in December, its persistence in January-March 2027, the IOD's direction and the north-east monsoon's actual rainfall - these four points will have to be watched in the next IMD bulletins; the rabi season and the early signals of the 2027 south-west monsoon will be built from these.

Sources

Note: El Niño's intensity, the Niño 3.4 anomaly, the MMCFS forecast and the IOD's status are from the IMD's official October 2026 bulletin. All rainfall figures of the 2026 monsoon (87 per cent LPA, 759.4 mm, regional percentages, since-1901/2001 ranks, sub-division split) are directly verified from the IMD's official document of 30 September 2026, "Salient Features of the 2026 Southwest Monsoon Season". The post-1950 comparative analysis and the "December peak" estimate are from The Hindu's analysis.

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