Indian Ocean Dipole (IOD)

A climate mode in the Indian Ocean where east-vs-west sea-surface temperatures swing. Can either rescue or wreck the Indian monsoon.

phenomena

What is the Indian Ocean Dipole?

The Indian Ocean Dipole (IOD) — sometimes called the Indian Niño — is a recurring climate pattern that involves a seesaw of sea-surface temperatures between the western and eastern tropical Indian Ocean. It was first identified by Saji et al. in 1999, making it one of the youngest “named” climate modes — yet it now plays a central role in seasonal forecasts for South Asia, East Africa, Indonesia and Australia.

The IOD has three phases:

Positive IOD — Warmer western Indian Ocean (off East Africa), cooler eastern Indian Ocean (off Indonesia).
Negative IOD — Cooler west, warmer east.
Neutral IOD — Roughly equal temperatures across the basin.

The index used to measure IOD strength is the Dipole Mode Index (DMI) — the difference in SST anomalies between the western (50°E–70°E, 10°S–10°N) and eastern (90°E–110°E, 10°S–0°) tropical Indian Ocean. Values:

DMI > +0.4°C = positive IOD
DMI < −0.4°C = negative IOD
In between = neutral

Why IOD matters for India

For the Indian monsoon, IOD acts as a potential rescuer or wrecker of seasonal rainfall:

Positive IOD effects:

Warmer western Indian Ocean evaporates more moisture for the monsoon flow.
Strengthens the Somali low-level jet.
Tends to enhance Indian monsoon by ~5–15% above normal.
Can offset a weak El Niño — saving an otherwise drought-bound monsoon.
2019 example: Strong positive IOD allowed surplus monsoon despite weak El Niño background.

Negative IOD effects:

Reduces moisture flow into the monsoon.
Tends to reduce Indian monsoon rainfall by 5–15%.
Can amplify an El Niño drought — making a bad year worse.
1992, 2016 example: negative IOD contributed to monsoon deficit.

Independent of ENSO:

IOD events sometimes occur with neutral or weak ENSO.
IOD has been responsible for several monsoon anomalies that ENSO alone could not explain.

IOD impacts across the wider region

East Africa (Kenya, Tanzania, Ethiopia, Somalia):

Positive IOD → heavy rainfall, often flooding (1997, 2019 East African floods).
Negative IOD → drought (Horn of Africa famines often coincide).

Indonesia, Australia:

Positive IOD → drought across Indonesia, eastern Australia. Big peat-fire seasons in Sumatra/Kalimantan.
Negative IOD → wetter conditions in Indonesia, Australian wet season enhanced.

Bangladesh, Sri Lanka:

Positive IOD generally beneficial for monsoon rainfall.

Notable IOD events

Positive IOD years:

1961: Very strong positive IOD; record East African flooding.
1994: Strong positive IOD; Indonesia drought + Sumatra fires.
1997: Strong positive IOD coincided with strongest El Niño on record; record East African floods.
2019: Strongest positive IOD on record; saved Indian monsoon despite El Niño.
2023: Moderate positive IOD; partial monsoon rescue.

Negative IOD years:

1992: Negative IOD; weak Indian monsoon.
2016: Negative IOD contributed to weak monsoon.
2010: Strong negative IOD; Australia floods.

IOD and climate change

Climate models project:

More frequent positive IOD events under warming.
More extreme IOD swings between phases.
Stronger linkages with El Niño in some scenarios.

This is a mixed picture for South Asia: more positive IOD years could mean more monsoon-supportive seasons, but extreme positive IOD events have also been linked to East African floods and Indonesian fires.

How IOD is forecast

The IOD is harder to predict than ENSO because it develops faster (peak August–October) and has shorter persistence. Forecast skill is improving but remains modest beyond 3-month lead times.

IMD, BoM (Australia), and ECMWF all publish IOD outlooks. The most-watched updates are around May–June when ahead of the Indian monsoon, IMD uses IOD as one of several drivers in its seasonal forecast.

Frequently asked questions

Is IOD happening now? As of early 2026, the IOD is in neutral territory, with models suggesting possible weak positive IOD by mid-2026. The June IMD monsoon forecast will incorporate the latest IOD outlook.

How is IOD different from El Niño? El Niño/La Niña operate in the Pacific Ocean. IOD operates in the Indian Ocean. They are statistically related but can occur independently. IOD is “the Indian Ocean’s own ENSO” but on a shorter timescale.

Can a positive IOD save an El Niño monsoon? Yes — 2019 is the textbook example. A weak-to-moderate El Niño would normally suppress the Indian monsoon, but a strong positive IOD delivered surplus rainfall. IOD-ENSO offset is a key reason monsoon forecasting is so complex.

How long do IOD events last? Much shorter than El Niño — typically 4–6 months. IOD events develop in May–June, peak in September–October, and fade by December.

Where can I see IOD’s effect on my city’s rainfall? Mausam Online shows live and forecast rainfall on every city page. For seasonal monsoon outlook, see our Monsoon India 2026 guide (Hindi). Live forecasts: Mumbai, Chennai, Kolkata, Delhi.