Indian Ocean Dipole

The Indian Ocean Dipole is an east–west contrast in sea-surface temperature across the tropical Indian Ocean. NOAA's Physical Sciences Laboratory represents its intensity with the Dipole Mode Index (DMI): the difference between sea-surface-temperature anomalies in a western equatorial region and a southeastern equatorial region. The index's sign identifies its phase: a positive DMI corresponds to a positive IOD, while a negative DMI corresponds to a negative IOD. The index is a way to describe the pattern, not a direct measure of every local impact.

In a positive IOD, the western tropical Indian Ocean is warmer than average while the eastern part, near Indonesia, is cooler than average. Weaker equatorial westerly winds and the upward movement of cooler subsurface water in the east help maintain this contrast. During a negative IOD, the broad temperature pattern reverses, with warmer conditions in the east and cooler conditions in the west. When neither contrast is pronounced, the IOD is described as neutral. The pattern involves coupled changes in the ocean and atmosphere rather than a temperature difference alone.

The Australian Bureau of Meteorology reports that IOD events generally begin around May or June, tend to peak between August and October, and usually weaken as the Southern Hemisphere monsoon arrives near the end of spring. The timing and strength vary between events. Positive phases are often associated with reduced winter–spring rainfall and warmer conditions in parts of Australia, whereas negative phases can favor above-average winter–spring rainfall in parts of southern Australia. NOAA Climate.gov describes broader regional contrasts during positive events, including a tendency toward wetter conditions in eastern Africa and drier conditions in eastern Asia and Australia. These are tendencies, not guarantees for a particular place or season.

The IOD is distinct from the El Niño–Southern Oscillation (ENSO), which is centred in the tropical Pacific, though the two patterns can influence one another and may occur at the same time. Their combined effects can complicate seasonal climate signals; an IOD phase should therefore not be treated as a standalone forecast of rainfall in any one country. Climate agencies monitor sea-surface temperatures and indices such as the DMI to describe evolving conditions and inform seasonal outlooks.