Decode Civils CAShorts
Current affairs Open app
ExplainerEnvironmentPrelimsGS3

El Niño, La Niña and the Indian monsoon: how ENSO works

By CAShorts DeskPublished Sources: WMO; IMD; NOAA Climate Prediction Center; Bureau of Meteorology; The Indian Express

Why in news: WMO's Global Seasonal Climate Update of 21 October 2026 forecast a Niño 3.4 anomaly near +3.8°C, peaking around December 2026.

El Niño, La Niña and the Indian monsoon: how ENSO works
Illustration (AI-generated)

The El Niño–Southern Oscillation (ENSO) is a coupled ocean-atmosphere cycle in the equatorial Pacific. It has three phases: warm El Niño, cool La Niña and neutral. El Niño usually weakens India's southwest monsoon by slowing the trade winds. On 21 October 2026 the World Meteorological Organization forecast a Niño 3.4 anomaly near +3.8°C for November–January, with effects into 2027.

Syllabus: Prelims · Geography; GS3 · Agriculture & Irrigation; GS1 · World Physical Geography

Why this matters for UPSC

ENSO is core GS I physical geography and GS III agriculture. Prelims can ask about the Niño 3.4 region, RONI, the Southern Oscillation and the Indian Ocean Dipole; Mains, why El Niño weakens the southwest monsoon and how India prepares.

In short

The El Niño–Southern Oscillation is a natural ocean-atmosphere cycle in the equatorial Pacific whose warm phase usually weakens India's monsoon, while the Indian Ocean Dipole can soften it.

How El Niño dries themonsoonweakereasterlytrade windswarm watershift eastWalkercirculationweakenslessmoisturetowards Indiabelow-normalmonsoonrainfall

ENSO cannot be stopped, but its monsoon risk can be managed: India must read El Niño with the IOD and act on forecasts before sowing.

Why it matters

A farmer in a rain-fed district times kharif sowing by the monsoon, and nearly half of India's farming still depends on rain, IMD notes. When El Niño thins the monsoon, sowing slips, reservoirs fill less, hydropower and drinking water run short and food prices can climb, while heatwaves push up power demand. A forecast months ahead lets States plan contingency crops, water releases and drought relief before the damage shows. The larger issue is climate risk: warmer oceans feed more heat and moisture into extremes, so even a familiar natural cycle can hit harder, and one weak season ripples into rural incomes and inflation.

Prelims pointers

  • Niño 3.4 region: 5°N–5°S and 170°W–120°W in the east-central equatorial Pacific; its three-month temperature anomaly decides the ENSO phase.
  • RONI adoption: NOAA's CPC made it the operational ENSO index in February 2026; Australia's Bureau of Meteorology moved to a relative index in September 2025.
  • ENSO strength (IMD): weak 0.5 to below 1.0°C, moderate 1.0 to below 1.5, strong 1.5 to below 2.0, very strong 2.0°C or more; no agreed definition of 'super'.
  • Southern Oscillation Index: Tahiti minus Darwin pressure anomaly; strongly negative in El Niño, positive in La Niña.
  • Dipole Mode Index: western box 50°E–70°E, 10°S–10°N minus eastern box 90°E–110°E, 10°S–Equator; +0.4°C or more marks a positive IOD.
  • IMD rainfall categories: deficient below 90% of LPA, below normal 90–95, normal 96–104, above normal 105–110, excess above 110; LPA 87 cm (1971–2020).
  • IMD's El Niño drought years: 1982, 1987, 2002, 2009 and 2015; yet 1997, a strong El Niño, brought a normal monsoon.
  • WMO El Niño/La Niña Update: issued quarterly with the International Research Institute for Climate and Society; the Global Seasonal Climate Update is monthly.

Practise a UPSC-style question

With reference to the El Niño–Southern Oscillation, consider the following statements:

1. During El Niño, easterly trade winds across the equatorial Pacific weaken below their usual strength.

2. During El Niño, surface air pressure over Indonesia falls below normal while it rises over the eastern Pacific.

3. La Niña brings unusually cool surface water to the central and eastern equatorial Pacific.

Which of the statements given above is/are correct?

  1. (a) 1 and 2 only
  2. (b) 2 and 3 only
  3. (c) 1 and 3 only
  4. (d) 1, 2 and 3
Show the answer

Answer: (c) 1 and 3 only

Answer: (c) 1 and 3 only. Statement 2 reverses the pressure pattern of El Niño.

Statement 1: correct. NOAA's Climate Prediction Center lists weaker-than-average easterly trade winds as a typical El Niño condition.

Statement 2: incorrect. In El Niño, pressure is above average over Indonesia and the western Pacific and below average over the eastern Pacific.

Statement 3: correct. WMO and IMD define La Niña as unusually cool surface water in the central and eastern equatorial Pacific.

Eliminate: Statement 2 is wrong because El Niño raises pressure over Indonesia. Out: (a), (b), (d).

Trap: Reversed direction. The right places appear, but the pressure change at each is turned around.

Remember: El Niño: high pressure and drought over Indonesia, low pressure and rain in the east-central Pacific.

More Explainers

All Explainers

More Environment current affairs