The Indian Ocean Dipole (IOD) is a naturally occurring climate phenomenon, a coupled ocean-atmosphere concept, characterized by an irregular oscillation of sea surface temperatures (SSTs) in the equatorial Indian Ocean. It was first identified as an independent system by climate researchers in 1999. The discovery addressed the problem that year-to-year variations in Indian Ocean SST and rainfall were previously thought to be mostly forced by the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean.
The IOD works as a "see-saw" of SSTs between a western pole in the Arabian Sea and an eastern pole south of Indonesia. It involves three phases: neutral, positive, and negative.
- Positive Phase (pIOD): The western Indian Ocean becomes warmer than average, while the eastern Indian Ocean becomes cooler than average. This is caused by a weakening of westerly winds, allowing warm water to move toward Africa and cool water to upwell in the east. A positive IOD is often associated with surplus rainfall over the western Indian Ocean and parts of India, and deficit rainfall over Indonesia and Australia.
- Negative Phase (nIOD): The pattern reverses, with warmer water in the eastern Indian Ocean and cooler water in the west. This phase is associated with intensified westerly winds and can obstruct the progression of the Indian monsoon.
The IOD is tracked using the Dipole Mode Index (DMI), which measures the SST anomaly difference between the western and southeastern equatorial Indian Ocean. The IOD is closely connected to the Indian Summer Monsoon Rainfall (ISMR), as a positive IOD can bring good rains to India and can even offset the negative impact of an El Niño event. While the IOD operates independently of the Pacific Ocean's ENSO, the two phenomena interact and can either complement or counter each other's effects on the Indian monsoon. Recently, a "newly discovered type" called the Early positive IOD (pIOD) has been identified, which is triggered by an anomalously early onset of the Indian summer monsoon.