The Indian Ocean Dipole (IOD) is a climate concept defined as an irregular oscillation of sea surface temperatures (SST) in the equatorial Indian Ocean. It is characterized by a difference in SST between a western pole in the Arabian Sea and an eastern pole in the eastern Indian Ocean south of Indonesia. The IOD phenomenon was first identified in 1999 by Indian climate researchers from the Indian Institute of Science. Its discovery was a milestone that challenged the previous view that the tropical Indian Ocean was a passive basin, with research prompted by an unusual event in 1994.
The IOD operates in three phases: Neutral, Positive, and Negative. The mechanism is tracked using the Dipole Mode Index (DMI). A Positive IOD occurs when the western Indian Ocean becomes warmer than average, while the eastern part cools. This is caused by unusually strong easterly winds pushing warm surface water toward Africa, allowing cold water to upwell near the Sumatran coast. Conversely, a Negative IOD sees warmer water in the east and cooler water in the west. The atmospheric component of this coupled ocean-atmosphere phenomenon is known as the Equatorial Indian Ocean Oscillation (EQUINOO).
The IOD is closely connected to the Indian Summer Monsoon Rainfall (ISMR). It interacts with the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean. A positive IOD is generally beneficial for the Indian monsoon, and studies have shown that a positive IOD index often negated the effect of El Niño, resulting in normal or excess monsoon rains in years like 1983, 1994, and 1997. The IOD's influence has become more significant in recent decades as the traditional correlation between ENSO and the ISMR has weakened.