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Q66 (IAS/2017) Geography › World Physical Geography › Ocean-atmosphere oscillations Official Key

With reference to 'Indian Ocean Dipole (IOD)' sometimes mentioned in the news while forecasting Indian monsoon, which of the following statements is/are correct ? 1. IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean. 2. An IOD phenomenon can influence an El Nino's impact on the monsoon. Select the correct answer using the code given below :

Result
Your answer: —  ·  Correct: B
Explanation

The correct answer is option B (2 only).

**Statement 1 is incorrect** because the Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas — a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia[1]. The statement wrongly mentions "tropical Eastern Pacific Ocean" instead of the eastern Indian Ocean, confusing IOD with ENSO characteristics.

**Statement 2 is correct** because an IOD phenomenon can influence an El Nino's impact on the monsoon[2]. This is further supported by evidence that a positive IOD index often negated the effect of ENSO, resulting in increased Monsoon rains in several ENSO years like 1983, 1994 and 1997[3]. The IOD can modify or counteract El Niño's typically adverse impact on Indian monsoons.

Therefore, only statement 2 is correct, making option B the right answer.

Sources
  1. [1] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
  2. [2] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Impact of IOD On Cyclogenesis In Northern Indian Ocean > p. 417
  3. [3] Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 416
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Don’t just practise – reverse-engineer the question. This panel shows where this PYQ came from (books / web), how the examiner broke it into hidden statements, and which nearby micro-concepts you were supposed to learn from it. Treat it like an autopsy of the question: what might have triggered it, which exact lines in the book matter, and what linked ideas you should carry forward to future questions.
Q. With reference to 'Indian Ocean Dipole (IOD)' sometimes mentioned in the news while forecasting Indian monsoon, which of the following st…
At a glance
Origin: From standard books Fairness: High fairness Books / CA: 10/10 · 0/10

This is a classic 'Definition Trap' question. UPSC loves to take a standard concept (IOD) and subtly swap a geographic location in the definition (Pacific instead of Indian Ocean). If you visualize the map while studying, this is a sitter; if you rote learn text, you get tricked.

How this question is built

This question can be broken into the following sub-statements. Tap a statement sentence to jump into its detailed analysis.

Statement 1
Which sea surface temperature difference characterizes the Indian Ocean Dipole (IOD) phenomenon?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
Presence: 5/5
“IOD starts to develop in the equatorial region of the Indian Ocean in April and is best devolved in October.• The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) ― a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia.• Similar to ENSO, the atmospheric component of the IOD was later discovered and named as Equatorial Indian Ocean Oscillation (EQUINOO: oscillation of warm water and atmospheric pressure between the Bay of Bengal and the Arabian Sea).”
Why this source?
  • Explicitly defines IOD as the difference in sea surface temperature between a western pole (Arabian Sea/western Indian Ocean) and an eastern pole (eastern Indian Ocean south of Indonesia).
  • Names the two poles (western and eastern Indian Ocean), directly answering which SST difference characterizes the IOD.
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Impact of IOD On Cyclogenesis In Northern Indian Ocean > p. 416
Presence: 4/5
“• Positive IOD (the Arabian Sea is warmer than the Bay of Bengal) results in more cyclones than usual in the Arabian Sea.• Negative IOD results in stronger than usual cyclogenesis in the Bay of Bengal. Cyclogenesis in the Arabian Sea is suppressed. [UPSC Prelims 2017] With reference to 'Indian Ocean Dipole (IOD)' sometimes mentioned in the news while forecasting the Indian monsoon, which of the following statements is/are correct? [2017-I] 1. IOD phenomenon is characterised by a difference in sea surface temperature between tropical Western Indian Ocean and tropical Eastern Pacific Ocean.”
Why this source?
  • Describes the sign of the SST difference (positive IOD: Arabian Sea warmer than Bay of Bengal; negative IOD: opposite), illustrating the west–east SST contrast.
  • Links regional warming patterns to the dipole concept, reinforcing that the phenomenon is an SST difference across the Indian Ocean.
Statement 2
Can the Indian Ocean Dipole (IOD) influence an El Niño event's impact on the Indian monsoon?
Origin: Direct from books Fairness: Straightforward Book-answerable
From standard books
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Impact of IOD On Cyclogenesis In Northern Indian Ocean > p. 417
Presence: 5/5
“2. An IOD phenomenon can influence an El Nino's impact on the monsoon.”
Why this source?
  • Explicit sentence states an IOD phenomenon can influence an El Nino's impact on the monsoon.
  • Directly links IOD as a modifier of ENSO–monsoon interactions (clear, unambiguous claim).
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 416
Presence: 5/5
“• During positive IOD, winds over the Indian Ocean blow from east to west (from the Bay of Bengal towards the Arabian Sea). This results in the Arabian Sea (the western Indian Ocean near African Coast) being much warmer and the eastern Indian Ocean around Indonesia becoming colder and dry. In the negative dipole year (negative IOD), the reverse happens to make Indonesia much warmer and rainier. • It was demonstrated that a positive IOD index often negated the effect of ENSO, resulting in increased Monsoon rains in several ENSO years like 1983, 1994 and 1997.”
Why this source?
  • Describes mechanism of positive vs negative IOD (wind/SST changes) that alter regional rainfall patterns.
  • Gives concrete examples where positive IOD negated ENSO effects, increasing monsoon rains in ENSO years (1983, 1994, 1997).
Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
Presence: 4/5
“• In recent decades, the El Nino/ENSO-Monsoon relationship seemed to have weakened in the Indian subcontinent. For e.g. in 1997, strong ENSO failed to cause drought in India. It was discovered that just like ENSO was an event in the Pacific Ocean, a similar seesaw ocean-atmosphere system in the Indian Ocean was also at play. It was discovered in 1999 and named the Indian Ocean Dipole (IOD).”
Why this source?
  • Explains historical observation that strong ENSO did not always produce expected monsoon drought (1997), leading to discovery of IOD.
  • Provides context that IOD can explain variations in ENSO–monsoon relationship.
Pattern takeaway: The 'Geographic Swap' pattern is rampant in Geography MCQs. Whenever a statement defines a boundary or location (e.g., 'Eastern Pacific'), pause and verify if they swapped it with 'Eastern Indian Ocean' or 'Western Pacific'. Statement 1 is a textbook example of this trap.
How you should have studied
  1. [THE VERDICT]: Sitter. Covered in every standard Geography text (NCERT Class 11, PMF IAS, GC Leong updates).
  2. [THE CONCEPTUAL TRIGGER]: Climatology > Ocean-Atmosphere Interactions > Factors affecting the Indian Monsoon.
  3. [THE HORIZONTAL EXPANSION]: Study the 'Big 4' Monsoon modifiers: 1. ENSO (Pacific), 2. IOD (Indian Ocean), 3. MJO (Global propagation), 4. EQUINOO (Atmospheric component of IOD). Know the specific 'Warm Pool' location for each.
  4. [THE STRATEGIC METACOGNITION]: When reading about a 'Dipole' or 'Oscillation', immediately draw the two poles on a blank map. UPSC tests your spatial memory (Where is the warm water?) more than the theoretical definition.
Concept hooks from this question
📌 Adjacent topic to master
S1
👉 IOD definition: west–east SST contrast in the Indian Ocean
💡 The insight

The IOD is defined as the sea-surface temperature difference between a western pole (Arabian Sea/western Indian Ocean) and an eastern pole (eastern Indian Ocean south of Indonesia).

High-yield for monsoon and climate questions: UPSC often asks definition/essence of climate oscillations. Understanding this distinction helps separate IOD from ENSO and answer MCQs or short-answer questions about regional SST anomalies and monsoon impacts.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
🔗 Anchor: "Which sea surface temperature difference characterizes the Indian Ocean Dipole (..."
📌 Adjacent topic to master
S1
👉 Positive vs Negative IOD (regional warming pattern)
💡 The insight

Positive IOD means the western Indian Ocean (Arabian Sea) is warmer than the eastern side; negative IOD is the reverse — directly tied to the SST difference concept.

Important for questions on monsoon variability and cyclogenesis: knowing the sign of IOD helps predict impacts on Arabian Sea/Bay of Bengal cyclone formation and rainfall anomalies, enabling answers on cause–effect and mitigation.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Impact of IOD On Cyclogenesis In Northern Indian Ocean > p. 416
🔗 Anchor: "Which sea surface temperature difference characterizes the Indian Ocean Dipole (..."
📌 Adjacent topic to master
S1
👉 Seasonality of IOD development (April to October peak)
💡 The insight

IOD development timing (starts in April, best developed in October) is given, linking the SST difference to a seasonal cycle relevant to monsoon timing.

Useful for temporal questions in prelims/mains about when climate phenomena influence the Indian monsoon. Knowing seasonality aids interpretation of forecast-related news items and answer-writing on monsoon forecasting.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
🔗 Anchor: "Which sea surface temperature difference characterizes the Indian Ocean Dipole (..."
📌 Adjacent topic to master
S2
👉 Indian Ocean Dipole (IOD) and its phases
💡 The insight

References describe positive and negative IOD states, their wind/SST patterns, and regional rainfall consequences.

High-yield for monsoon-related questions: understanding IOD phases explains contrasts in Arabian Sea vs Indonesia rainfall and why some ENSO years behave atypically. Connects to ocean-atmosphere teleconnections, climate variability, and regional impacts—useful for mains answers and map-based explanations.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 416
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
🔗 Anchor: "Can the Indian Ocean Dipole (IOD) influence an El Niño event's impact on the Ind..."
📌 Adjacent topic to master
S2
👉 IOD modulation of ENSO's impact on the Indian monsoon
💡 The insight

Evidence explicitly states IOD can influence an El Niño's effect and gives examples where positive IOD negated ENSO-induced drought.

Crucial for nuanced answers on ENSO–monsoon linkage: shows examiners expect discussion of interacting modes (not just ENSO alone). Helps frame balanced answers on monsoon variability, attribution, and forecasting implications.

📚 Reading List :
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Impact of IOD On Cyclogenesis In Northern Indian Ocean > p. 417
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 416
🔗 Anchor: "Can the Indian Ocean Dipole (IOD) influence an El Niño event's impact on the Ind..."
📌 Adjacent topic to master
S2
👉 ENSO (El Niño) — basic linkage to Indian monsoon
💡 The insight

References link El Niño/ENSO to monsoon behaviour and note that ENSO's influence can be altered by other oceanic phenomena.

Foundational concept: mastery allows candidates to explain direct ENSO effects and then introduce modifiers (IOD), improving answer depth. Connects to broader topics like Southern Oscillation, ocean currents and monsoon forecasting.

📚 Reading List :
  • Geography of India ,Majid Husain, (McGrawHill 9th ed.) > Chapter 4: Climate of India > EL-NINO AND THE INDIAN MONSOON > p. 9
  • Physical Geography by PMF IAS, Manjunath Thamminidi, PMF IAS (1st ed.) > Chapter 29: El Nino, La Nina & El Nino Modoki > Indian Ocean Dipole Effect (Not Every El Nino Year Is The Same In India) > p. 415
🔗 Anchor: "Can the Indian Ocean Dipole (IOD) influence an El Niño event's impact on the Ind..."
🌑 The Hidden Trap

EQUINOO (Equatorial Indian Ocean Oscillation). Just as ENSO has an atmospheric component (Southern Oscillation), IOD has EQUINOO. If IOD is the ocean temperature seesaw, EQUINOO is the cloud/pressure seesaw. UPSC will likely ask about the relationship between IOD and EQUINOO in future.

⚡ Elimination Cheat Code

Apply 'Nomenclature Logic'. The phenomenon is called the **Indian Ocean** Dipole. It is counter-intuitive for one of its poles to be in the **Eastern Pacific** Ocean (which is thousands of miles away). Usually, localized dipoles are contained within their respective ocean basins. This logical inconsistency makes Statement 1 90% likely to be false.

🔗 Mains Connection

Connect IOD to GS-3 Economy (Inflation & Agriculture). A 'Positive IOD' can save the Kharif crop even during an El Nino year (as seen in 1997 and 2019), preventing food inflation. This is a crucial point for Mains answers on 'Monsoon resilience'.

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SIMILAR QUESTIONS

NDA-II · 2012 · Q88 Relevance score: 2.27

Consider the following statements regarding El Nino effect on Indian monsoon: 1. The surface temperature goes up in the Southern Pacific Ocean and there is dificient rainfall in India. 2. The Walker Circulation shifts Eastward from its normal position and reduces monsoon rainfall in India. Which of the statement(s) given above is/are correct?

IAS · 2020 · Q3 Relevance score: 1.59

With reference to Ocean Mean Temperature (OMT), which of the following statements is/are correct ? 1. OMT is measured up to a depth of 26℃ isotherm which is 129 meters in the south-western Indian Ocean during January - March. 2. OMT collected during January - March can be used in assessing whether the amount of rainfall in monsoon will be less or more than a certain long-term mean. Select the correct answer using the code given below :

IAS · 2011 · Q52 Relevance score: 0.23

La Nina is suspected to have caused recent floods in Australia. How is La Nina different from EI Nino? 1. La Nia is characterised by unusually cold ocean temperature in equatorial Indian Ocean whereas EI Nino is characterised by unusually warm ocean temperature in the equatorial Pacific Ocean. 2. El Nino has adverse effect on southwest monsoon of India, but La Nina has no effect on monsoon climate. Which of the statements given above is/ correct?

CDS-I · 2005 · Q61 Relevance score: -0.02

Consider the following statements: In India 1. The duration of the monsoon rainy season goes on decreasing from south to north, and from east to west. 2. The distribution of rainfall received from south-west monsoons is very greatly governed by the relief or orography. 3. Bulk of the rainfall of the Coromandel Coast is derived from depressions and cyclones. Which of the statements given above are correct ?

CDS-I · 2010 · Q35 Relevance score: -0.13

Which of the following statements with regard to rainfall in India is/are correct ? 1. Most of the rainfall in India is due to the south-west monsoon. 2. In South India, rainfall decreases away from the Eastern Ghats. Select the correct answer using the code given below: