The Indian Ocean Dipole (IOD) is a climate phenomenon, or concept, defined as an irregular oscillation of sea surface temperatures (SSTs) in the tropical Indian Ocean. It is characterized by a seesaw pattern of SST anomalies between the western equatorial Indian Ocean (near the African coast) and the southeastern equatorial Indian Ocean (near Indonesia and Australia). The IOD was first identified in 1999 by Indian climate researchers from the Indian Institute of Science and other scientists, following an unusual event in 1994. Its discovery helped dispel the earlier misconception that the Indian Ocean was a passive basin without its own coupled ocean-atmosphere climate phenomena.
The mechanism of the IOD involves three phases: neutral, positive, and negative. The IOD is tracked using the Dipole Mode Index (DMI), which measures the difference in SST anomalies between the western and eastern poles. In a positive IOD phase, the western Indian Ocean becomes warmer than average, while the eastern Indian Ocean cools, often due to strong easterly winds pushing warm water west and allowing cold water to upwell near Sumatra. This phase typically leads to increased rainfall and flooding in East Africa and parts of India, but causes droughts in Indonesia and Australia. Conversely, a negative IOD phase sees warmer water in the east and cooler water in the west, which can lead to a reduction in rainfall over the Indian subcontinent.
The IOD is closely connected to the Indian Summer Monsoon Rainfall (ISMR), acting as a modulator of its variability. Crucially, the IOD interacts with the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean. A positive IOD can often negate the negative impact of an El Niño on the Indian monsoon, leading to normal or excess rainfall in India despite the El Niño.
In terms of recent changes, the IOD has shown an increase in the frequency of a newly identified type called early positive IOD (pIOD), which develops in boreal spring and matures in summer. Furthermore, the 2021–2022 negative IOD event persisted for an unprecedented 19 months, making it the longest and one of the strongest on record since reliable data began. Climate models also project that strong positive IOD events are likely to increase in frequency under greenhouse warming before 2100.