BharatNotes What is Indian Ocean Dipole?
The Indian Ocean Dipole (IOD) is a coupled ocean-atmosphere phenomenon in the equatorial Indian Ocean, marked by an irregular oscillation of sea-surface temperatures (SSTs) between its western and eastern halves. It is measured by the Dipole Mode Index (DMI) — the difference in SST anomalies between the western pole (roughly 50°E-70°E, 10°S-10°N) and the eastern pole (roughly 90°E-110°E, 10°S-0°). First described by Saji and colleagues in Nature (1999), it is sometimes called the "Indian Niño" for its loose analogy to El Niño.
Phases and the DMI threshold
A positive IOD occurs when the western Indian Ocean becomes warmer than usual and the east (near Indonesia) cooler, as easterly winds strengthen and upwelling cools the eastern pole. A negative IOD is the reverse — warmer waters pool near Indonesia/Australia and the west cools. Neutral conditions lie between the two.
| Phase | DMI value | Western pole | Eastern pole |
|---|---|---|---|
| Positive | DMI > +0.4 °C | Warmer | Cooler |
| Neutral | −0.4 to +0.4 °C | Near normal | Near normal |
| Negative | DMI < −0.4 °C | Cooler | Warmer |
(Thresholds per Australian Bureau of Meteorology and NOAA Climate Prediction Center conventions.)
Significance for India and the rim countries
A positive IOD generally enhances Indian summer monsoon rainfall (warm western waters fuel convection over the Arabian Sea and India) and can partly offset the drying effect of an El Niño. It also brings heavy rain — and flooding — to East Africa, while triggering drought and bushfires in Indonesia and Australia. A negative IOD tends to weaken the monsoon over India and bring wetter conditions to the eastern Indian Ocean rim.
The 2019 positive IOD was among the strongest on record, linked to severe East African floods and Australia's catastrophic 2019-20 bushfire season. The strong 1997 positive IOD caused major East African flooding and Indonesian drought. The IOD frequently co-occurs with ENSO, and the two together modulate monsoon outcomes.
Current status (2026)
As of early June 2026, the IOD was in a neutral state (DMI around −0.34 °C, week ending 7 June 2026, per BoM monitoring). Forecast models pointed to neutral conditions persisting into mid-year, with a possible positive IOD developing later in the year — though model spread on timing and strength was large. Notably, a positive IOD recurred in consecutive years in 2024, described by BoM as unusual.
UPSC angle
Examiners use the IOD to test understanding of why the monsoon varies year to year independent of El Niño. Key points to retain: it is an SST-driven dipole; positive phase = warmer west, generally good for the Indian monsoon; it is measured by the DMI; and it is distinct from but interacts with ENSO. Do not confuse the IOD (Indian Ocean) with ENSO/El Niño (Pacific Ocean).
Sources: NOAA Climate.gov; Australian Bureau of Meteorology; NOAA Climate Prediction Center; Saji et al., Nature (1999).
