The Indian Ocean Dipole
A see-saw of sea surface temperature across the Indian Ocean.
This explanation is written conservatively from textbook-level science and has not yet been reviewed by a meteorologist. Treat it as a draft.
The mechanism
Follow the chain: each step causes the next.
01
Winds change along Java and Sumatra
In a positive event, south-easterly winds strengthen along the coasts of Java and Sumatra.
02
Cool water rises in the east
The winds drive upwelling of cooler subsurface water in the eastern Indian Ocean.
03
Warm water gathers in the west
Surface water in the western Indian Ocean becomes warmer than normal relative to the east.
04
Convection moves west
Rain-bearing convection follows the warmer water, tending to reduce rainfall over Indonesia and parts of Australia and to increase it over parts of East Africa.
Explore it
Step through the mechanism one stage at a time.
Step 1 of 4
Winds change along Java and Sumatra
In a positive event, south-easterly winds strengthen along the coasts of Java and Sumatra.
In short
Positive, negative and neutral
The Indian Ocean Dipole (IOD) describes the difference in sea surface temperature anomaly between the western and the south-eastern tropical Indian Ocean. A positive IOD has the west warmer than the east; a negative IOD has the reverse. Events usually develop in the southern winter, peak in the southern spring, and decay once the monsoon arrives in the southern summer.
Deeper science
Open a section for more detail.
How it is measured
The Dipole Mode Index (DMI) subtracts the average sea surface temperature anomaly of an eastern box off Sumatra from that of a western box in the Arabian Sea region. Agencies apply thresholds to the index, sustained over several weeks, before declaring an event; the exact thresholds differ between agencies.
Relationship with ENSO
The IOD can occur on its own, but positive IOD events often coincide with El Niño and negative events with La Niña. When they act together, their effects on rainfall over Indonesia and Australia can add up.
Related topics
What is ENSO?
El Niño and La Niña: how the tropical Pacific reshapes weather around the world.
5-step mechanismDraftRead topicThe Madden–Julian Oscillation
A travelling pulse of tropical rainfall that circles the globe in one to two months.
4-step mechanismDraftRead topic