Why in the news?
The Indian Ocean Dipole (IOD) is an important ocean-atmosphere phenomenon that significantly influences the Indian Monsoon, droughts, floods, and climate variability across the Indian Ocean region. It is frequently asked in both UPSC Prelims and Mains.
What is the Indian Ocean Dipole (IOD)?
The Indian Ocean Dipole (IOD) is a climate phenomenon characterized by the difference in sea surface temperatures (SSTs) between the western Indian Ocean (near East Africa) and the eastern Indian Ocean (near Indonesia and Australia).
It was first identified by climatologist Dr. N.H. Saji and colleagues in 1999.
Definition
IOD refers to the oscillation of sea surface temperatures between the western and eastern parts of the Indian Ocean.
Mechanism of IOD
Normally:
Warm waters accumulate near Indonesia and Australia.
The western Indian Ocean remains relatively cool.
Moist air rises over the eastern Indian Ocean, leading to rainfall there.
However, changes in ocean temperatures and wind patterns can disturb this balance, leading to different phases of IOD.
Phases of the Indian Ocean Dipole
1. Positive IOD (+IOD)
Characteristics
The Western Indian Ocean becomes warmer than normal
The Eastern Indian Ocean becomes cooler than normal
Stronger westerly winds push warm water towards Africa
Effects
Enhanced rainfall over India
Increased rainfall in East Africa
Drought conditions in Indonesia and Australia
Historical Examples
- 1994 Positive IOD
- 1997 Strong Positive IOD
- 2019 Positive IOD
The 2019 Positive IOD helped compensate for the weak monsoon caused by El Niño and contributed to above-normal rainfall in India.
2. Negative IOD (-IOD)
Characteristics
The Eastern Indian Ocean becomes warmer than normal
The Western Indian Ocean becomes cooler than normal
Effects
Reduced rainfall over India
Heavy rainfall in Indonesia and Australia
Increased chances of drought in East Africa
IOD and Indian Monsoon
The IOD has a major influence on the Southwest Monsoon.
Positive IOD
Strengthens monsoon circulation
Enhances moisture transport to India
Can offset the adverse effects of El Niño
Negative IOD
Weakens monsoon rainfall
May aggravate drought conditions
IOD vs El Niño
| Feature | Indian Ocean Dipole | El Niño |
| Ocean | Indian Ocean | Pacific Ocean |
| Cause | SST difference between western and eastern Indian Ocean | Warming of the central and eastern Pacific |
| Impact on India | Can increase or decrease the monsoon | Generally weakens monsoon |
| Frequency | Irregular (3–5 years) | Every 2–7 years |
Important Point
A positive IOD can reduce the negative impact of El Niño on the Indian Monsoon.
Dipole Mode Index (DMI)
The intensity of IOD is measured using the Dipole Mode Index (DMI).
Formula
DMI = SST anomaly in the Western Indian Ocean − SST anomaly in the Eastern Indian Ocean
- Positive DMI → Positive IOD
- Negative DMI → Negative IOD
Global Impacts of IOD
East Africa
- Floods during Positive IOD
- Drought during Negative IOD
Australia
- Droughts and bushfires during the Positive IOD
- Increased rainfall during the Negative IOD
Indonesia
- Reduced rainfall during the Positive IOD
- Excess rainfall during the Negative IOD
India
- Influences monsoon intensity
- Affects agriculture, water resources, and food security
Climate Change and IOD
Recent studies suggest that climate change may increase the frequency of extreme Positive IOD events.
Potential consequences include:
More extreme floods
More severe droughts
Greater variability in monsoon rainfall
Increased disaster risks in the Indian Ocean region
UPSC Mains Value Addition
Diagrammatic Representation
POSITIVE IOD
East Africa
(Warm SST) ↑ Rainfall
← Warm Water Shift
Indian Ocean
Indonesia
(Cool SST) ↓ Rainfall
NEGATIVE IOD
East Africa
(Cool SST) ↓ Rainfall
Indian Ocean
Indonesia
(Warm SST) ↑ Rainfall
Key Takeaways
IOD is an ocean-atmosphere interaction phenomenon in the Indian Ocean.
It is measured through the Dipole Mode Index (DMI).
A positive IOD generally strengthens the Indian Monsoon.
Negative IOD tends to weaken monsoon rainfall.
Positive IOD can sometimes counteract the adverse effects of El Niño.
Climate change may increase the occurrence of extreme IOD events.
UPSC Prelims MCQs
Q1. Consider the following statements regarding the Indian Ocean Dipole (IOD):
- It refers to the difference in sea surface temperatures between the western and eastern Indian Ocean.
- Positive IOD is generally associated with enhanced monsoon rainfall over India.
- Dipole Mode Index (DMI) is used to measure the intensity of IOD.
Which of the statements given above are correct?
A. 1 and 2 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2, and 3
Answer:
D. 1, 2, and 3
Q2. Which one of the following conditions best describes a Positive Indian Ocean Dipole event?
A. Warmer eastern Indian Ocean and cooler western Indian Ocean
B. Cooler eastern Indian Ocean and warmer western Indian Ocean
C. Uniform warming across the Indian Ocean
D. Cooling of the entire Indian Ocean
Answer: B
Cooler eastern Indian Ocean and warmer western Indian Ocean
UPSC Mains Practice Question
Q. What is the Indian Ocean Dipole (IOD)? Discuss its mechanism and examine its impact on the Indian Monsoon and climate variability in the Indian Ocean region. (15 Marks, 250 Words)
