The Indian Ocean Dipole: India's other ocean see-saw

El Niño gets the headlines, but India's monsoon also listens to its own ocean. The Indian Ocean Dipole (IOD) is a see-saw in sea temperature between the western and eastern Indian Ocean.

How it is measured. IMD uses the Dipole Mode Index: the temperature anomaly in a western box (50°E–70°E, 10°S–10°N) minus that in an eastern box near Indonesia (90°E–110°E, 10°S to the equator).

Three phases.

• Positive IOD (+0.4°C or more): the west is warmer than normal and the east cooler. IMD says this generally boosts moisture transport towards India and tends to favour above-normal monsoon rain.
• Neutral: between −0.4 and +0.4°C.
• Negative IOD (−0.4°C or less): the east is warmer and the west cooler, which may contribute to below-normal rain.

When it matters. The IOD usually develops in summer, peaks in September to November and then weakens quickly (IMD). A positive IOD can partly, or even largely, offset El Niño's drying. In 1997, a strong El Niño year, India's monsoon was normal.

2026: agencies differ. IMD's 1 October outlook says the IOD is neutral and likely to stay so. Australia's Bureau of Meteorology reported an index of +0.88°C on 13 October and says a positive IOD is likely to develop. Until they agree, treat the IOD phase as uncertain.

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