The IOD involves an aperiodic oscillation of sea-surface temperatures, between "positive" and "negative" phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia. The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
The IOD also effects the strength of monsoons over the Indian subcontinent. A significant positive IOD occurred in 1997-8, with another in 2006. The IOD is one aspect of the general cycle of global climate, interacting with similar phenomena like the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean. The positive IOD in 2007 evolved together with La Niña which is a very rare phenomenon that happened only once in the available historical records (in 1967). Also the occurrence of consecutive positive IOD events are extremely rare with only one such precedence within the records (during 1913–14).
The IOD phenomenon was first identified by climate researchers in 1999. Yet evidence from fossil coral reefs demonstrates that the IOD has functioned since at least the middle of the Holocene period, 6500 years ago.