4 Billion Years On

Indian Ocean Dipole

The Indian Ocean's sea-temperature seesaw, shifting rain between East Africa and Australia…

The Indian Ocean’s own ocean-atmosphere mode - a sea-surface temperature seesaw between the western and southeastern equatorial Indian Ocean that shifts flood and drought risk between East Africa and Indonesia/Australia. It has impacts of its own, and a two-way link with ENSO (ENSO usually leads, the Indian Ocean feeds back). With BOM’s weekly Dipole Mode Index, NOAA’s 150-year history and today’s regional impacts.

Latest data 18 Oct 2026

Current State

Western vs southeastern Indian Ocean sea temperatures - BOM's weekly reading, checked against NOAA's monthly record.

Equatorial Indian Ocean

Orange glow: warmer than usual, blue cooler. Blue tags: wetter, orange drier. Arrows: the equatorial wind anomaly. A guide drawn from the sign of the current DMI +0.88°C alone (west minus east), not a reading for each region.

IOD phase - current week

Positive

+0.88°C · week to 2026-10-18

East Africa wetter, Indonesia/Australia drier

ENSO - the IOD’s Pacific partner

El Niño (Strong)

RONI +1.69°C · JAS 2026

Forecast - 100% very strong at peak, Sep-Nov, then neutral by Apr-Jun 2027 (NOAA). La Niña from Sep 2027 in SNU’s AI model.

ENSO Tracker

NOAA Monthly Record

+0.15°C May 2026Annual mean -0.13°C

HadISST1.1 runs 3-4 months behind, so the figures above use BOM’s weekly reading.

IOD and ENSO relationship right now

Positive IOD is currently reinforcing El Niño - the classic co-occurring pairing, which typically means an even sharper east/west rainfall contrast around the Indian Ocean than either driver alone.

ENSO and the IOD - a pair that often work together

  • The Pacific usually leads. El Niño mostly comes with a positive IOD and La Niña with a negative one - the IOD often starts in spring as ENSO develops and peaks in late autumn.
  • The Indian Ocean feeds back. A strong IOD can reinforce the ENSO event it grows alongside, and a positive IOD tends to set up La Niña the following year (a negative one, El Niño).
  • The link is getting stronger. Coupling between the two oceans has strengthened since the 1980s - enough to account for up to a fifth of the recent increase in ENSO’s swings.
  • Each can act alone. The IOD can develop without ENSO, and the pairing isn’t guaranteed in any one event.
Source: Park, An, Park & Liu (2025), npj Climate and Atmospheric Science; Izumo et al. (2010).ENSO Tracker

Thresholds: ≥ +0.4°C Positive · ≤ -0.4°C Negative · otherwise Neutral (the BOM/NOAA CPC convention). Data: BOM weekly DMI · NOAA PSL DMI monthly series (HadISST1.1).

4byo Climate Symphony ENSO Tracker

History

Weekly & Annual IOD

BOM's weekly Dipole Mode Index for recent years, and NOAA's annual record since 1870.

Weekly DMI (BOM) · last 208 weeks
+IOD (wet East Africa, dry Indonesia-Australia)-IOD (the reverse)4-week meanWeekly values
Annual DMI (NOAA PSL) · 1870-present
Positive IOD (above +0.4°C) Negative IOD (below −0.4°C)1997 - East Africa floods, Indonesian fires2019 - Australian bushfire season

Is the Positive IOD Becoming More Common?

Largely, yes. In this record, September-November 1997, 2019 and 2023 are the three strongest positive-IOD seasons since 1871. Coral records, which reach back centuries, show positive events growing more frequent and stronger through the 20th century - from about one every 20 years in the early 1900s to one every 4 years in recent decades (Abram et al. 2008). Extreme positive events were rare before 1960, though not unheard of: the corals hold a 17th-century event about 27-42% larger than 1997 (Abram et al. 2020). Climate models project extreme positive IODs becoming more frequent with warming - from one every 17 years in the 20th century to one every 6 by 2100 (Cai et al. 2014) - though some newer work finds IOD swings could weaken under prolonged warming (Nature Communications 2024).

Two caveats on the long chart above: Indian Ocean measurements before about 1950 are sparse, and the western Indian Ocean has warmed faster than any other tropical ocean region since 1901 (Roxy et al. 2014), which on its own nudges the index - west minus east - upward.

Top: BOM’s weekly DMI, days behind rather than months. Bottom: NOAA PSL’s annual DMI since 1870 (HadISST1.1, monthly data to May 2026); gold dashes mark the two most consequential recent positive-IOD events. Data: BOM weekly DMI · NOAA PSL DMI monthly series (HadISST1.1).

4byo Climate Symphony ENSO Tracker

Regional Impacts

Where Does the IOD Bite Hardest?

Each region's usual response to each IOD phase, with today's phase shown first.

Live: +IOD regime today

  • East Africa (Kenya, Somalia, Ethiopia, Tanzania) - Wetter · flood risk
  • Indonesia, Malaysia, Maritime Continent - Drier · wildfire/haze risk
  • Australia (especially SE) - Drier · elevated bushfire risk
  • Indian subcontinent - Can boost monsoon rainfall

Positive IOD (+IOD)

  • East Africa (Kenya, Somalia, Ethiopia, Tanzania)Wetter · flood riskWarmer western Indian Ocean feeds more moisture into the short-rains season; the extreme 2019 positive IOD co-occurred with catastrophic East African flooding.Measured here (the IOD’s own effect, beyond ENSO): Kenya: strong, wetter Sep-Nov with a positive IOD · Somalia: strong, wetter Sep-Nov with a positive IOD · Ethiopia: moderate, wetter Sep-Nov with a positive IOD · Tanzania: strong, wetter Sep-Nov with a positive IOD
  • Indonesia, Malaysia, Maritime ContinentDrier · wildfire/haze riskSuppressed convection over the cooler eastern Indian Ocean weakens the monsoon trough, raising peatland/forest fire risk.Measured here (the IOD’s own effect, beyond ENSO): Indonesia: strong, drier Sep-Nov with a positive IOD · Malaysia: no measurable link
  • Australia (especially SE)Drier · elevated bushfire riskReduced moisture transport across southern and eastern Australia in winter and spring; the 2019 positive IOD, one of the strongest on record, contributed to the record dryness and fire weather behind the 2019-2020 "Black Summer" (Bureau of Meteorology).Measured here (the IOD’s own effect, beyond ENSO): Australia: moderate, drier Jul-Sep with a positive IOD
  • Indian subcontinentCan boost monsoon rainfallA positive IOD sometimes offsets a weak-monsoon signal from El Niño, though the relationship is less consistent than the East Africa/Australia impacts.Measured here (the IOD’s own effect, beyond ENSO): India: no measurable link

Negative IOD (-IOD)

  • East Africa (Kenya, Somalia, Ethiopia, Tanzania)Drier · drought riskSuppressed October-December short rains - often alongside La Niña, which tends to trigger negative IOD events and works the same way here.
  • Indonesia, Malaysia, Maritime ContinentWetterEnhanced convection over the warmer eastern Indian Ocean increases rainfall across the Maritime Continent.
  • Australia (especially SE)WetterMore moisture reaching southern and eastern Australia in winter and spring.
  • Indian subcontinentCan weaken monsoon rainfallA negative IOD tends to work against monsoon rainfall (Ashok et al. 2001), though the link is weaker and less consistent than for East Africa and Australia.

The IOD was first identified by Saji et al. (1999, Nature). Its interaction with ENSO and the North Atlantic-European circulation is documented in Raganato et al. (2024, J. Climate) and Fahad et al. (2025, Comms Earth & Environment). See the ENSO tracker for the other half of the tropical Indo-Pacific picture.

4byo Climate Symphony Extreme Weather

FAQs

Indian Ocean Dipole: Common Questions

What is the Indian Ocean Dipole (IOD)?

The Indian Ocean Dipole is a coupled ocean-atmosphere climate pattern - the Indian Ocean's own equivalent of ENSO. It is measured by the Dipole Mode Index (DMI): the sea-surface temperature anomaly gradient between the western Indian Ocean (near the Horn of Africa) and the southeastern Indian Ocean (near Sumatra). A positive IOD means the west is warmer than average and the east cooler; a negative IOD is the reverse.

What does a positive IOD mean?

A positive IOD (warm west, cool east) shifts rainfall west, toward East Africa, and suppresses it in the east. It typically brings above-average rain and flood risk to Kenya, Somalia, Ethiopia and Tanzania, while Indonesia, Malaysia and Australia tend toward drought and elevated bushfire risk. The extreme positive IOD of 2019 co-occurred with catastrophic flooding in East Africa and, according to the Bureau of Meteorology, contributed to the record dryness and fire weather behind Australia’s 2019-20 Black Summer bushfires.

What does a negative IOD mean?

A negative IOD (cool west, warm east) reverses the pattern: rainfall shifts east, bringing wetter conditions to Indonesia and northern Australia while East Africa tends toward drought. Failed East African short rains often come with a negative IOD, frequently alongside La Niña. It also tends to work against Indian monsoon rainfall (Ashok et al. 2001), though that link is weaker and less consistent than for East Africa and Australia.

How does the IOD relate to ENSO (El Niño / La Niña)?

The IOD and ENSO are physically distinct systems in different ocean basins, but they work as a pair. ENSO usually leads - El Niño mostly comes with a positive IOD and La Niña with a negative one - and the Indian Ocean feeds back: a strong IOD can reinforce the ENSO event it grows alongside, and a positive IOD tends to set up La Niña the following year. A 2025 study (Park, An, Park & Liu, npj Climate and Atmospheric Science) found the coupling between the two oceans has strengthened since the 1980s. Research published in 2024 (Raganato et al., Journal of Climate) also shows the combined IOD+ENSO signal reaches all the way to North Atlantic-European winter circulation via the same tropical wave-train pathway that links El Niño to the North Atlantic Oscillation. A separate 2025 modelling study (Fahad et al., Communications Earth & Environment) found this IOD-to-North-Atlantic teleconnection is strongest when ENSO itself is inactive, and is projected to weaken further under continued climate change.

How is the IOD measured and how often does this page update?

The IOD is tracked via the Dipole Mode Index (DMI), combining two sources: the Australian Bureau of Meteorology's weekly DMI reading (days behind, not months) for the current-state figure, and NOAA PSL's HadISST1.1-based monthly DMI (1870-present) for the long history and as a cross-check. This page refreshes weekly. PSL's own monthly figure typically runs 3-4 months behind the present on its own - that's expected and is why the current-state figure leads with BOM's fresher weekly reading instead.

Is there a seasonal forecast for the IOD?

Bodies like the World Meteorological Organization publish qualitative seasonal outlooks for the IOD from coupled dynamical models, and these are worth reading directly for the current season's expectation. This site deliberately doesn't turn that kind of report into a projected line on the IOD chart, though - a hand-transcribed figure from a periodic report goes stale the moment the season passes, and a generic statistical carry-forward can just as easily contradict what a real forecast center is actually saying. Rather than show a line that might be wrong in either direction, the IOD chart shows observed history only, with no forecast dashing, until a live automated seasonal-forecast feed exists for the IOD to plot from directly.

Why does 4 Billion Years On track the IOD separately from ENSO?

The IOD has real regional impacts independent of ENSO - East African rainfall, Indonesian/Australian drought and bushfire risk, and Indian monsoon modulation are all driven by the IOD in years when ENSO itself is neutral. Because it also has a genuine, currently-researched interaction with ENSO and even the North Atlantic Oscillation, it earns its own tracker page, and sits next to ENSO as the tropical Indo-Pacific pair on the 4byo Climate Symphony. Our own measurements back this up: with the long-term trend and ENSO's share removed, the IOD still accounts for over 40% of the year-to-year swings in Kenya's and Tanzania's September-November short rains, and about 30% of Indonesia's (drier with a positive IOD) - see each country's Climate Drivers section.