Indian Ocean Dipole: The climate phenomenon that saved India's monsoon in 1997 and could do so again
In 1997, a powerful El Niño event should have caused a severe drought in India. However, the monsoon that year was 2% above normal. The reason was the Indian Ocean Dipole (IOD), a climate pattern in the Indian Ocean that can counterbalance El Niño's effects. As a 'Super El Niño' is forecast for this year, scientists are closely watching the IOD to see if history could repeat itself.
The IOD is a periodic oscillation of sea-surface temperatures in the Indian Ocean. In its positive phase, the western Indian Ocean becomes warmer than the eastern part, which enhances rainfall over India. In its negative phase, the opposite occurs, suppressing rainfall. A neutral phase has little effect on the monsoon.
The IOD was first described in 1999 by a team led by N.H. Saji, including Indian scientists B.N. Goswami and P.N. Vinayachandran. They showed that the IOD operates independently of the El Niño–Southern Oscillation (ENSO) in the Pacific Ocean, though the two phenomena can interact.
El Niño tends to reduce monsoon rainfall by shifting the Walker circulation eastward. However, a strong positive IOD can compensate for this, as happened in 1997. Mrutyunjay Mohapatra, Director-General of the India Meteorological Department, notes that the IOD cannot fully offset a strong El Niño unless it is strongly positive. In most El Niño years, the IOD remains neutral, and monsoon rainfall is below normal.
Currently, the monsoon is running a 40% deficit, with the IMD predicting June-September rainfall at 90% of the long-period average—just above 'deficient'. The outcome may depend on the behaviour of the IOD in the coming months.
