Indian Ocean Dipole

Also called: IOD · Topic: Climate of India and the Monsoon · NCERT: Beyond NCERT

Meaning

The Indian Ocean Dipole (IOD) is an east–west difference in sea surface temperature (SST) across the tropical Indian Ocean. One "pole" is the western Arabian Sea and the western equatorial Indian Ocean near East Africa. The other pole is the south-eastern Indian Ocean near Sumatra (Indonesia).

It matters because the IOD helps decide whether India's whole monsoon season is good or bad. A positive IOD can cancel much of the drying effect of El Nino, as it did in 1997 and 2019.

Explanation

What the "dipole" is and how it is measured

• Dipole means "two poles". Here the two poles are two patches of ocean whose temperatures move in opposite directions.
• West pole: the western Arabian Sea and the western equatorial Indian Ocean, near East Africa.

• East pole: the south-eastern Indian Ocean, near Sumatra.

• Dipole Mode Index (DMI): this number measures how strong the IOD is.

• DMI = SST anomaly of the western tropical Indian Ocean minus the SST anomaly of the south-eastern tropical Indian Ocean [3].
• An SST anomaly is how far the sea temperature is from its normal value.

• Saji et al. (1999) defined the DMI [3].

• Discovery: the IOD was identified by Saji, Goswami, Vinayachandran and Yamagata in the journal Nature in 1999 [4].

• In that paper, the sea off Sumatra was cooler than normal and the western Indian Ocean was warmer than normal. Winds and rain changed along with the sea temperatures [4].

• Its own system: the IOD comes from the ocean and the air acting on each other inside the Indian Ocean. It is independent of ENSO (the El Nino–Southern Oscillation in the Pacific) [4].

• It explains about 12% of the variation in Indian Ocean SST [4].

• Time scale: the IOD is seasonal. It usually peaks around September–November.

Positive IOD: warmer west, cooler east

• Cause: the western Indian Ocean and the Arabian Sea become warmer than normal, and the sea near Sumatra becomes cooler.
• Process:
• The warm western sea gives off more moisture, and more air rises over the west.

• The monsoon winds blowing into India get stronger.

• Result for India: usually more monsoon rain.

• A positive IOD can offset El Nino, as in 1997 and 2019.

• Result abroad: heavy rain in East Africa and drought in Indonesia [4].

• Very high DMI values have come with extreme September–November rain in tropical East Africa [3].

Negative IOD: cooler west, warmer east

• Cause: the western Indian Ocean is cooler than normal and the sea near Sumatra is warmer.
• Process: the zone of rising air and rain moves east, towards Sumatra and Australia.
• Result for India: the Indian monsoon is hurt. This is true whatever the ENSO phase is.

How the IOD works together with ENSO

• El Nino (a warm central and eastern Pacific) moves the zone of rising air east, away from the Indian Ocean. More air sinks over India, so there is less rain.
• The IOD can make this effect stronger or weaker:

Combination	Typical effect on India	Example
El Nino + neutral IOD	Monsoon suppressed. A neutral IOD does not reduce El Nino's effect [1]	2026 forecast (August–September below normal) [1]
El Nino + positive IOD	El Nino's suppression partly or fully offset [1]	1997 (normal rain)
La Nina + positive IOD	Strongly favourable	—
Any ENSO phase + negative IOD	Unfavourable	—

Where it occurs

• Tropical Indian Ocean (the whole system):
• West pole: the western Arabian Sea and the western equatorial Indian Ocean, off East Africa.

• East pole: the south-eastern Indian Ocean off Sumatra, Indonesia.

• Regions it affects:

• India: the total rain of the south-west monsoon season. A positive IOD helps it and a negative IOD hurts it.
• East Africa (tropical): heavy September–November rain in a positive IOD [3][4].
• Indonesia: drought in a positive IOD [4].

• Sumatra and Australia: rain moves towards them in a negative IOD.

• Monitoring in India: IMD Pune, a WMO Regional Climate Centre, publishes a monthly ENSO and IOD Bulletin [2].

• Recent phases (2025-26) [2]:
• Negative IOD: September–December 2025.
• Near-neutral: January 2026, then positive in February–April 2026.
• Leaning negative: May–June 2026, then neutral in July–August 2026.
• Some international centres expected a positive IOD around September 2026 [1][2].
• A neutral IOD in mid-2026 gave no relief from the strong El Nino. So IMD forecast below-normal rain (less than 94% of the Long Period Average) for August–September 2026 [1].

Don't confuse with

• ENSO (El Nino–Southern Oscillation): ENSO happens in the equatorial Pacific and is measured by Niño 3.4 and the SOI. The IOD happens in the tropical Indian Ocean, is measured by the DMI, and is independent of ENSO [4].
• Southern Oscillation: this is a see-saw of air pressure between Tahiti and Darwin. The IOD is a difference in sea surface temperature between the western and eastern Indian Ocean.
• MJO (Madden-Julian Oscillation): the MJO is a pulse of cloud and rain that moves eastward around the tropics every 30–60 days [5]. It decides the timing of active spells and breaks in the monsoon. The IOD stays in place, lasts a season, and affects the total rain of the season.
• Positive vs negative IOD: in a positive IOD the west is warmer and the east near Sumatra is cooler, and it helps India's monsoon. In a negative IOD the west is cooler and the east is warmer, and it hurts India's monsoon.

Prelims Hooks

• The IOD was identified in 1999 by Saji, Goswami, Vinayachandran and Yamagata in Nature [4].
• Index: the Dipole Mode Index (DMI) = western tropical Indian Ocean SST anomaly minus south-eastern tropical Indian Ocean SST anomaly (near Sumatra) [3].
• Positive IOD = warmer west and cooler east. It aids the Indian monsoon, brings East African rain and Indonesian drought [4], and offset El Nino in 1997 [4].
• Trap: the IOD is not part of ENSO. It arises within the Indian Ocean and is independent of ENSO [4].
• Trap: 1997 was a strong El Nino year with normal Indian rain, because of a positive IOD.
• Index matching: IOD → DMI; ENSO → Niño 3.4 and SOI; MJO → phase and amplitude.

Mains Points

• Why El Nino does not always mean drought (GS-I):
• El Nino makes air sink over India and cuts rain.
• A positive IOD pushes moisture and stronger monsoon winds towards India and can cancel this effect (1997, 2019).

• So forecasts need many indicators. IMD has moved from 16 statistical indicators to a coupled Multi-Model Ensemble (MME) since 2021 [1]. A coupled model is one in which the ocean and the air affect each other.

• Climate change and the IOD (GS-I/III):

• The Indian Ocean is warming, and the old "El Nino means drought" link may be weakening or changing.
• Positive-IOD-type events and extreme rain spells now get more attention.

• This supports more ocean observation (INCOIS) and better coupled modelling (Monsoon Mission).

• Regional reach and cooperation (GS-I/III):

• One positive IOD can help India's monsoon, bring floods to East Africa and bring drought to Indonesia [4].
• This is a case for shared regional early warning. IMD Pune's monthly ENSO and IOD Bulletin, issued as a WMO Regional Climate Centre, is an example [2].

Related concepts

• Madden-Julian Oscillation

Read more

• Teleconnections: El Nino, the Southern Oscillation, the Indian Ocean Dipole and the MJO