Indian Ocean Dipole and Ocean Mean Temperature: Forecasting the Monsoon

UPSC tested the Indian Ocean Dipole with two statements. One was true. The other swapped one ocean for another.

What was the question?

This was UPSC Civil Services Prelims 2017, General Studies Paper I, Series A, Question 66. The official wording reads:

The code options were:

• (a) 1 only
• (b) 2 only
• (c) Both 1 and 2
• (d) Neither 1 nor 2

What is the answer?

The official answer key gives (b) 2 only. UPSC’s Series A key lists no dropped items for this paper. Statement 1 is false. Statement 2 is true.

Why is statement 1 false?

• The IOD compares the western and the eastern tropical Indian Ocean.
• The eastern Pacific belongs to El Niño, not to the IOD.
• The question changes only one ocean. That one change is the trap.

Why is statement 2 true in 2017?

A 2001 study found that the IOD can counter or support the circulation that El Niño sets up over India. Section 4 explains this in full.

The Indian Ocean Dipole, or IOD, is a temperature see-saw inside one ocean. The Bureau of Meteorology (BoM) of Australia defines it as the difference between sea surface temperatures in the tropical western and eastern Indian Ocean. The western side is near Africa. The eastern side is near Indonesia.

What are the three phases?

• Positive IOD: the west is warmer than normal and the east is cooler than normal.
• Negative IOD: the west is cooler than normal and the east is warmer than normal.
• Neutral IOD: temperatures stay close to normal across the basin.

How is the IOD measured?

Scientists use the Dipole Mode Index, or DMI. It subtracts the eastern temperature anomaly from the western one. An anomaly is the gap between today’s temperature and the normal for that place and season.

BoM names two fixed boxes for the index:

• West box: 50°E to 70°E and 10°S to 10°N.
• East box: 90°E to 110°E and 10°S to the equator.

The east box lies off Sumatra.

What are the thresholds?

BoM treats sustained values above +0.4°C as typical of a positive IOD. It treats values below −0.4°C as typical of a negative IOD. The word “sustained” matters. A single week above the line does not make an event.

These cut-offs come from Australian climatologists. IMD’s bulletin uses the same ±0.4 band for its own forecasts.

Who identified the IOD, and when?

N. H. Saji, B. N. Goswami, P. N. Vinayachandran and T. Yamagata reported it in the journal Nature in 1999. Their paper showed low sea surface temperatures off Sumatra and high ones in the western Indian Ocean. They found that this pattern is independent of the El Niño Southern Oscillation. They said it accounts for about 12% of sea surface temperature variability in the Indian Ocean.

An IOD event feeds itself. A small change in the winds starts it. The ocean then strengthens the change.

How does a positive event build up?

It works in six steps.

• Westerly winds along the equator weaken. Sometimes they turn easterly.
• Warm surface water shifts towards Africa.
• The thermocline rises in the east and sinks in the west. The thermocline is the layer where temperature drops quickly with depth.
• Cool water rises from the depths off Java and Sumatra.
• Cooler water and sinking air in the east mean less cloud there.
• The changed winds push the pattern further. Scientists call this a Bjerknes-type feedback.

Yamagata and colleagues describe this chain in a 2004 paper. They note that seasonal winds along the Java coast also strengthen in a positive event. That adds more upwelling and more cooling.

When do events happen?

BoM gives a clear calendar for IOD events:

• They usually start around May or June.
• They peak between August and October.
• They decay quickly when the northern Australian monsoon arrives, around the end of spring.

Each phase occurs about every three to five years on average. A 2001 study adds that the IOD is locked to the summer monsoon season. That timing is why it matters for India.

Can an IOD happen without El Niño?

Yes. Yamagata and colleagues list positive IOD events in 1961, 1967 and 1994 that grew without an El Niño. They built composites from 9 independent IOD events, both positive and negative.

The two patterns can also occur together. BoM says an El Niño with a positive IOD can reinforce dry effects in Australia.

El Niño years often bring a weaker Indian monsoon. The IOD can push the other way. Its effect depends on its phase and on its strength against El Niño.

Why was the link once unclear?

Saji and colleagues found a correlation of only 0.32 between the IOD and monsoon rainfall. They called the link unclear. Later work asked a better question. It looked at the IOD and ENSO together, year by year.

What did the 2001 study find?

Ashok, Guan and Yamagata studied the years 1958 to 1997. They published in Geophysical Research Letters in 2001. Their findings were:

• The IOD and ENSO affected the monsoon in complementary ways.
• When the ENSO link to the monsoon was weak, the IOD link was strong.
• ENSO’s effect over India is either countered or supported by an IOD circulation cell.
• Of 11 intense positive IOD events, 8 matched above-normal rainfall in the same season. That is 73%.
• Of 3 intense negative IOD events, 2 matched below-normal rainfall.

These are tendencies, not guarantees.

Which years show the offset?

The paper names 1983, 1994 and 1997. In these summers a positive IOD helped bring normal or excess rain. The Southern Oscillation was in its negative phase at the same time.

• 1997: all-India monsoon rainfall was about 102% of normal, per the Economic Survey 2000-01 chart.
• 1994: Ashok lists it among the offset years, but the IOD event ran without an intense El Niño. It shows the IOD acting alone.
• 1982 against 1983: in 1982 El Niño was stronger than the positive IOD, with normalised values of 1.7 against 1.4. The paper says this may explain the monsoon deficit. In 1983 the positive IOD was stronger. The paper says this may explain the surplus.

The lesson is simple. The relative strength of the two events decides the outcome.

What changes in the air?

In the study’s model, a positive IOD reduces rising air over the cooler eastern Indian Ocean. Air converges over the Bay of Bengal instead. That strengthens the north-south monsoon circulation. This is how the IOD can replace the sinking air that El Niño brings over India.

The IOD shifts rain across the whole basin. Places on opposite sides of the ocean often get opposite results.

What happens in East Africa and Indonesia?

Saji and colleagues found that active IOD years cause severe rainfall in eastern Africa. They also cause droughts in Indonesia. Ashok and colleagues repeat this for a positive IOD: heavy rain and floods over East Africa, and drought over Indonesia.

What happens in Australia?

This is a common exam trap, so read it slowly.

• A positive IOD often brings less rain and higher temperatures to parts of Australia in winter and spring.
• A negative IOD typically brings above-average winter and spring rain to parts of southern Australia.
• A positive IOD is the dry one for Australia. It is not the negative one.

BoM gives two historic examples:

• 1982: a positive IOD and El Niño together. It was the driest year on record for southeast Australia. The Ash Wednesday bushfires followed.
• 1974 and 2010: a negative IOD and La Niña pushed rain towards Australia. These are two of the three wettest years on record.

What happened in 2019?

BoM calls the 2019 positive IOD the strongest on record. WHOI reports that it was a big player in Australia’s severe drought. It was also linked to floods in East Africa.

The WHOI release adds a note on history. Large positive IOD events were historically rare. They became more frequent and intense during the 20th century.

The IOD compares two places on the surface. Ocean Mean Temperature, or OMT, goes down instead.

What does OMT measure?

OMT is the average temperature of the water from the sea surface down to the 26°C isotherm. An isotherm is a line joining points of equal temperature. The depth of the 26°C isotherm is called D26.

Think of it as a single reading for a whole column of water. It tells you how much heat the upper ocean holds.

Where and when is it measured?

• Place: a south-western Indian Ocean box, 50°E to 70°E and 10°S to the equator.
• Time: the January to March average.
• Why that window: it is a transition period between winter and the south-west monsoon.

Notice the longitudes. They match the western box of the IOD index.

What depth does the study report?

The 2018 study gives a January to March mean D26 of 59 m in that box. The study’s ocean heat content for that region is 50 kJ per square centimetre.

Why not use surface temperature alone?

The study makes three points:

• Sea surface temperature describes a very thin layer.
• Winds, evaporation and thick clouds disturb that layer.
• OMT varies more steadily and with less spread.

For scale, one BoM sea surface product is taken at about 0.2 metres depth. It is similar to temperatures down to about 10 metres in well-mixed water. OMT covers the water down to about 59 m in this region.

A joint team tested OMT against other ocean signals. It asked one question. Could each signal predict whether the monsoon would be above or below average?

Who ran the study?

The authors were Venugopal, Ali, Bourassa, Zheng, Goni, Foltz and Rajeevan. Their institutions included Florida State University, IITM Pune, NOAA’s Atlantic Oceanographic and Meteorological Laboratory, and the Ministry of Earth Sciences. It appeared in Scientific Reports in 2018.

How was it tested?

• The record ran for 25 years, from 1993 to 2017.
• Rainfall data came from IMD, using 6,955 rain gauges.
• Each signal was a January to March average.
• The test counted how often each signal got the sign of the rainfall anomaly right.

What were the results?

What are the limits?

• The authors call it a statistical examination, not a process study.
• It aims to predict the sign of the anomaly, not its size.
• The record is only 25 years long.

The IOD usually starts around May or June. A January to March reading therefore comes before an event begins. That may help explain its low score here. The study says OMT gives a signal by the end of March.

Does IMD use OMT?

The highlights of IMD’s April 2026 forecast name ENSO, the IOD and northern snow cover. They do not name OMT. So OMT is best described as a research signal.

UPSC turned the OMT study into a two-statement question. One statement hid a number.

What was the 2020 question?

This was UPSC Civil Services Prelims 2020, General Studies Paper I, Set A, Question 93. The official wording of this item reads:

The options used the same four codes as in 2017.

What is the 2020 answer?

The official Set A key gives (b) 2 only. This item was not among the two dropped items. Other sets number the questions differently, so check your own set.

What about the depth figure?

UPSC’s paper prints 129 meters. The 2018 study gives a mean D26 of 59 m for that box and season. We did not find 129 m in that paper.

The key marks statement 1 as wrong. So the exam lesson is plain. A wrong number can make a statement false even when the idea is right.

Why is statement 2 correct in 2020?

The study shows that a January to March OMT can indicate whether the monsoon will be above or below its long-term mean. Section 7 shows the 80% result.

The year 2026 is an El Niño year with a weak monsoon. These are dated snapshots. Check the linked pages for later updates.

What did IMD forecast, and what happened?

• 13 April 2026: IMD forecast 92% of the Long Period Average, with a model error of ±5%. It called the monsoon likely to be below normal.
• Same release: IMD said the IOD was neutral. It said models showed a positive IOD developing towards the end of the season.
• 30 September 2026: UNI, quoting IMD, reported a 12.6% rainfall deficit for 1 June to 30 September. It called this the fourth weakest monsoon since 2001.

What does the ocean record show?

IMD’s September 2026 bulletin gives the IOD record for the past year:

• A negative IOD persisted from September to December 2025.
• The DMI turned positive from February to April 2026.
• It turned negative in May and June 2026.
• Neutral conditions held in July and August 2026.
• The Niño3.4 index crossed the El Niño threshold in May 2026. It reached about +2.08°C by August.

What is the latest reading?

• BoM, 29 September 2026: the IOD is still called neutral. The index was +0.76°C for the week ending 27 September.
• That is the third week in a row above +0.4°C. BoM says sustained values are needed before an event counts as established.
• BoM expects a positive IOD to develop in spring. It notes some uncertainty about strength and length.
• Niño3.4: +2.72°C for the same week. BoM calls it the second-highest weekly value in its weekly SST data, which start in 2008.
• NOAA, 8 October 2026: an El Niño advisory is in force. A strong to very strong event is likely through January to March 2027, at above 83% chance.

Did the IOD rescue the 2026 monsoon?

No. This is our reading of the dates. The monsoon season ran from June to September. The index only rose above +0.4°C in September, for three weeks, and BoM still called the IOD neutral. That was too late and too short to offset El Niño.

What came before?

• 2019: BoM calls it the strongest positive IOD on record.
• 2023: in October 2023, BoM said the positive IOD might come close to the 2019 event in strength.
• 2025: a negative IOD was weakening in December. BoM gave an index of −0.63°C for the week ending 7 December 2025.

This page covers two UPSC Prelims questions, from 2017 and 2020. Each one built a trap from a single wrong detail.

Which facts should you hold?

• The IOD compares the western and eastern tropical Indian Ocean.
• Positive IOD: west warmer, east cooler. Negative IOD: the reverse.
• The DMI uses two boxes, and BoM uses ±0.4°C thresholds.
• The IOD was reported in Nature in 1999 by Saji, Goswami, Vinayachandran and Yamagata.
• Events start around May or June and peak between August and October.
• The IOD can modulate El Niño’s effect on the monsoon. A positive IOD helped in 1983, 1994 and 1997.
• A positive IOD tends to bring floods to East Africa, drought to Indonesia, and dry weather to parts of Australia.
• OMT is the average temperature down to the 26°C isotherm, read for January to March.
• OMT helps judge whether the monsoon will be above or below its long-term mean.

What are the common traps?

• Swapping the eastern Indian Ocean for the eastern Pacific, as in 2017.
• Printing a wrong depth for the 26°C isotherm, as in 2020.
• Reversing the Australian effect. Positive is the dry phase there.
• Treating the IOD and ENSO as one system. They can run independently.
• Treating any reading as an event. Values between −0.4°C and +0.4°C count as neutral.