Indian Ocean Dipole and Monsoon: The Geography Angle UPSC Keeps Testing
May 29, 20266 min read
In 2019, a weak El Niño should have delivered a deficient monsoon to India. Instead, India received 110% of long-period average rainfall — one of the best monsoons in a decade. The reason: a strongly positive Indian Ocean Dipole (IOD) pumped moisture into the Indian subcontinent, overwhelming El Niño's suppressive effect. The IMD's seasonal forecast, which accounted for IOD, got it right. UPSC has tested IOD in both Prelims and Mains, and with the Indian Ocean warming faster than any other ocean, its influence on India's climate is growing.
[TOPIC CLASSIFICATION]
Topic type: Physical Geography / Climate / Oceanography
PYQ frequency: Medium-High. Appears in Prelims 2–3 times per 5 years; increasingly in Mains.
Exam stage relevance: Prelims + Mains
Primary GS Paper: GS 1 (Geography), GS 3 (Disaster management link)
[EXAMINER REASONING]
Trap: Confusing positive and negative IOD effects on India. Positive IOD = western Indian Ocean (Arabian Sea) warmer than eastern → more moisture for India = GOOD. Negative IOD = eastern Indian Ocean (Bay of Bengal side) warmer → moisture deflected away from India = BAD.
Most confused: The relationship between ENSO and IOD — are they independent or linked? They are related but semi-independent. Positive IOD often co-occurs with El Niño and can offset it. But IOD also occurs independently.
Key anchor: Dipole Mode Index (DMI) — the metric for measuring IOD intensity. Positive DMI = positive IOD (western Indian Ocean warmer). Negative DMI = negative IOD (eastern warmer).
Current affairs hook: A strongly positive IOD in 2019 saved India's monsoon despite El Niño. In 2023, both El Niño AND negative IOD hit simultaneously — a double blow for India's monsoon.
Mains hinge: "With climate change, monsoon forecasting in India has become more complex." Discuss with reference to ENSO, IOD, and the Madden-Julian Oscillation.
Core Concept
What is the Indian Ocean Dipole?
IOD is an irregular oscillation of sea surface temperatures (SST) between the western and eastern Indian Ocean:
Western pole: Arabian Sea (roughly 50°E–70°E, 10°N–10°S)
Eastern pole: Eastern Indian Ocean near Sumatra/Indonesia (roughly 90°E–110°E, 0–10°S)
The SST difference between these two poles drives changes in rainfall patterns across the Indian Ocean rim.
Positive IOD:
Western Indian Ocean (Arabian Sea) is WARMER than eastern Indian Ocean
The 2023 Sikkim GLOF killed 41, highlighting Himalayan risk from glacial retreat. NDMA guidelines exist but monitoring gaps remain.
Enhanced convection and rainfall over East Africa and Indian subcontinent
Good monsoon for India (more moisture available from Arabian Sea)
Drought conditions in Indonesia, Australia, southeastern Africa
Reduced risk of severe tropical cyclones in Arabian Sea (but more cyclones possible when positive IOD ends abruptly)
Negative IOD:
Eastern Indian Ocean (near Sumatra) is WARMER than western Indian Ocean
DMI < −0.4°C
Consequences:
Enhanced rainfall over Indonesia, Australia, Malaysia
Weaker monsoon for India (moisture deflected eastward)
Drought in East Africa
Australia receives above-normal rainfall
IOD and ENSO Interaction:
Condition
India Monsoon Outcome
El Niño + Negative IOD
Very deficient (double blow)
El Niño + Positive IOD
Near-normal (IOD offsets El Niño) — 2019
La Niña + Positive IOD
Flood risk / excess monsoon
La Niña + Negative IOD
Below-normal (both suppress)
Neutral ENSO + Positive IOD
Above-normal
Madden-Julian Oscillation (MJO):
Another climate mode that modifies monsoon. A 30–60 day intraseasonal oscillation of convection moving eastward through the tropics. Active MJO phase over India = enhanced monsoon during that fortnight.
Key Facts
IOD discovery: Discovered in 1999 by Japanese scientist Saji N. H. et al.
IMD uses: ENSO, IOD, and MJO in seasonal and extended range forecasting
Previous Year Questions
Year
Stage
What was tested
2024
Prelims
Positive IOD is associated with which of the following? Warming of western Indian Ocean / good monsoon for India
2023
Prelims
In 2023, monsoon was deficient partly due to which combination? El Niño + Negative IOD
2021
Prelims
DMI measures what? SST difference between western and eastern Indian Ocean
2019
Mains
How did the Indian Ocean Dipole contribute to the good monsoon of 2019 despite El Niño conditions?
2018
Prelims
IOD was first identified in which decade? 1990s (discovered 1999)
Statement Elimination Guide
Correct: "A positive IOD occurs when the western Indian Ocean is warmer than the eastern Indian Ocean, which tends to enhance India's southwest monsoon."
False: "A positive IOD means the Bay of Bengal is warmer than the Arabian Sea."
Trap: "IOD always reinforces ENSO." (False. In 2019, positive IOD offset/cancelled El Niño's negative effect on monsoon.)
Correct: "DMI (Dipole Mode Index) measures the SST anomaly between western and eastern poles of the Indian Ocean."
False: "DMI measures the strength of the Indian summer monsoon."
Correct: "Negative IOD is associated with deflection of moisture away from India and drier conditions over East Africa."
False: "Negative IOD causes flooding in East Africa."
Current Affairs Hook
The Indian Ocean is warming faster than any other ocean basin — about 1.7 times the global average. This warming intensifies IOD events, making both positive and negative IOD more extreme. IITM Pune's research shows that climate change is increasing the frequency of strong positive IOD events, which may partially compensate for El Niño's increasing suppressive effect on India's monsoon. However, the same warming is also increasing Arabian Sea cyclone activity (Cyclone Biparjoy 2023, Cyclone Tauktae 2021), which disrupts the monsoon onset.
The co-occurrence of El Niño and negative IOD in 2023 was a worst-case scenario for India. The IMD had warned of below-normal monsoon months in advance using these indices, demonstrating improving forecast skill.
Common Mistakes
"IOD only affects Australia": IOD affects East Africa, India, Australia, and Southeast Asia simultaneously.
"Positive IOD always means El Niño": They are correlated but independent. Positive IOD can occur without El Niño.
"MJO is the same as IOD": No. MJO is intraseasonal (30–60 days), moves eastward, affects monsoon in shorter windows. IOD is seasonal.
"Negative IOD is good for India": Negative IOD is bad for India (weakens monsoon) but good for Australia and Indonesia.
"IOD affects winter monsoon (Northeast monsoon) primarily": IOD primarily affects the Southwest (summer) monsoon. It also influences NE monsoon but the SW monsoon link is more tested.
Revision Snapshot
IOD = oscillation of SST between western Indian Ocean (Arabian Sea) and eastern Indian Ocean (near Sumatra). Measured by DMI (Dipole Mode Index). Positive IOD: western warmer → good monsoon for India, drought in Australia/Indonesia. Negative IOD: eastern warmer → weak monsoon for India, good rainfall in Australia. IOD + ENSO interaction: 2019 positive IOD offset El Niño = 110% LPA monsoon; 2023 El Niño + negative IOD = 6% deficient monsoon. IOD typically peaks Sep–Oct. Indian Ocean warming 1.7× global average — IOD events intensifying. IMD uses ENSO + IOD + MJO for forecasting. MJO = 30–60 day intraseasonal oscillation, modifies monsoon in shorter windows.