El Niño, La Niña and the Indian Ocean Dipole

Prerequisites

• Mechanism of the Indian Monsoon — ENSO and IOD act on top of the basic monsoon mechanism.
• Rainfall Distribution — LPA and the IMD categories used to judge a “bad” year.
• Ocean currents: the cold Humboldt (Peru) current off South America and the idea of upwelling.

The Hook: A Warm Ocean 15,000 km Away

Why it matters: ENSO and the IOD are among the most asked climatology topics in Prelims (statement-based questions on warm and cold phases, indices and effects). Mains GS-III asks you to link them to drought, food security and the economy.

The Normal State: Walker Circulation

Under normal (neutral) conditions in the equatorial Pacific:

The easterly trade winds blow along the equator from east to west, pushing warm surface water towards Indonesia. Air rises over the warm west, flows east at height, sinks over the cool east and returns west at the surface. This loop is the Walker circulation (वॉकर परिसंचरण), named after Sir Gilbert Walker.

El Niño and La Niña

El Niño and La Niña recur irregularly, typically every 2 to 7 years, and an event usually lasts around 9–12 months, sometimes longer. Between them are ENSO-neutral years.

Southern Oscillation and ENSO

• In the 1920s Sir Gilbert Walker, then head of the India Meteorological Department, discovered the Southern Oscillation (दक्षिणी दोलन): a see-saw in surface air pressure between the eastern and western tropical Pacific. When pressure is high over the east, it is low over the west, and vice versa. He was looking for ways to forecast the Indian monsoon.
• In 1969 Jacob Bjerknes showed that the ocean warming (El Niño) and the pressure see-saw (Southern Oscillation) are two sides of one coupled ocean–atmosphere system: ENSO (El Niño–Southern Oscillation).

graph TD
    A["Trade winds weaken"] --> B["Warm water spreads east
Upwelling off Peru stops"]
    B --> C["Rising air shifts to
central and east Pacific"]
    C --> D["Walker circulation weakens;
sinking air over Indonesia and India"]
    D --> E["Weaker Indian monsoon,
higher drought risk"]
    B --> A

The loop from B back to A is the Bjerknes feedback: warmer east Pacific water weakens the pressure difference, which weakens the trade winds further.

How ENSO Is Measured

The Indian Ocean Dipole (हिंद महासागर द्विध्रुव, IOD)

The IOD is the Indian Ocean’s own see-saw of sea surface temperature, described by N.H. Saji, Toshio Yamagata and colleagues in 1999.

• The IOD is measured by the Dipole Mode Index (DMI): the western-box SST anomaly minus the eastern-box SST anomaly.
• IOD events usually develop around June–July, peak in September–November and decay by December.
• A positive IOD can offset an El Niño. The classic example is 1997: one of the strongest El Niños of the century, yet India’s monsoon was about normal, helped by a positive IOD. In 2019 a strong positive IOD accompanied an excess monsoon.
• Positive IOD years also tend to bring good north-east monsoon rain to Tamil Nadu and drought and bushfire risk to Australia.

Madden–Julian Oscillation (MJO)

The MJO is an eastward-moving pulse of clouds and rain that travels around the tropics in roughly 30 to 60 days. When its wet phase is over the Indian Ocean, it can trigger monsoon onset, active spells and low-pressure systems; its dry phase can deepen a break. ENSO and IOD act over seasons; the MJO acts over weeks.

ENSO and the Indian Monsoon: The Record

Analysis: Why ENSO Matters for India (Mains material)

1. Agriculture and food security: a weak monsoon hits rain-fed kharif crops (paddy, pulses, oilseeds, coarse cereals), reduces reservoir storage for rabi, and raises food prices. Buffer stocks with the Food Corporation of India and procurement policy act as shock absorbers.
2. Economy: lower rural income reduces demand for tractors, two-wheelers and consumer goods; food inflation influences RBI monetary policy; drought relief strains state budgets.
3. Water and energy: less hydropower and drinking water stress; more electricity demand for pumping and cooling in hotter El Niño years.
4. Heat: El Niño years tend to bring stronger heat waves, and global temperature records often follow strong El Niños.
5. Preparedness: IMD’s long-range forecasts (April and late May/June), district contingency crop plans, drought manuals, crop insurance (PMFBY), rural employment guarantee (VB-G RAM G, formerly MGNREGA) work demand, and seed banks for short-duration varieties.

Memory Tricks

Don’t Confuse

Common Mistakes

Practice

All questions below are original practice questions written in CGPSC style. They are not previous-year questions.

Level 1: Foundation

1. The pressure see-saw between the eastern and western tropical Pacific is called the ______.
2. During a positive Indian Ocean Dipole, the ______ (western/eastern) Indian Ocean is warmer than normal.
3. The Southern Oscillation Index is based on the pressure difference between ______ and Darwin.

Level 2: Prelims MCQs

Q1. Consider the following statements:

1. During El Niño, the easterly trade winds over the equatorial Pacific weaken.
2. During El Niño, upwelling of cold water off the coast of Peru becomes stronger.
3. A strongly negative Southern Oscillation Index is associated with El Niño conditions.

Which of the statements given above is/are correct?

• (a) 1 only
• (b) 2 only
• (c) 1 and 3 only
• (d) 1, 2 and 3

Q2. Match List I (Person) with List II (Contribution):

• (a) A-4, B-1, C-2, D-3
• (b) A-1, B-4, C-2, D-3
• (c) A-4, B-2, C-1, D-3
• (d) A-3, B-1, C-2, D-4

Q3. Which of the following conditions is generally most favourable for a good Indian south-west monsoon?

• (a) El Niño with a negative IOD
• (b) El Niño with a neutral IOD
• (c) La Niña with a positive IOD
• (d) La Niña with a negative IOD

Q4. The Relative Oceanic Niño Index (RONI), adopted by NOAA’s Climate Prediction Center as its official ENSO index in 2026, differs from the older Oceanic Niño Index mainly because it:

• (a) Uses air pressure at Tahiti and Darwin instead of sea temperature
• (b) Measures the Niño 3.4 temperature anomaly relative to the average of the whole tropical oceans
• (c) Measures sea temperature in the Indian Ocean instead of the Pacific
• (d) Uses rainfall over India as the main input

Level 3: Mains Answer Writing

Q1. (150–200 words) What is ENSO? Explain its influence on the Indian monsoon, with suitable examples, and suggest how India can reduce its impact on agriculture.

Q2. (60 words) Distinguish between a positive and a negative Indian Ocean Dipole.

Quick Revision Table

Key Takeaways

What’s Next

• Climate and the Indian Monsoon — Quick Revision — the whole chapter on one page
• Agricultural Economy — drought, food inflation and farm policy
• Environment and Ecology — climate change and extreme events
• Agriculture and Irrigation in Chhattisgarh — rain-fed paddy and drought response
• Mechanism of the Indian Monsoon — revisit the basic mechanism