Also: IOD, Indian Niño, positive IOD, negative IOD, Dipole Mode Index
Definition
The Indian Ocean Dipole is a see-saw in sea surface temperatures between the western Indian Ocean near Africa and the eastern Indian Ocean near Indonesia; in its positive phase the west is warmer than usual, which tends to support India's monsoon, while a negative phase tends to weaken it.
Scientists long knew that the Pacific's El Niño and La Niña shape weather around the world, including India's monsoon. In 1999, two groups of researchers, including N.H. Saji, B.N. Goswami and colleagues working with Toshio Yamagata in Japan, and Peter Webster's group, described a separate pattern in the Indian Ocean. They showed that in some years, surface waters near Sumatra and Java cool sharply while waters in the western Indian Ocean near East Africa warm, with winds along the equator blowing from east to west instead of the usual pattern. They called this the Indian Ocean Dipole.
The IOD involves feedback between winds, ocean currents and temperatures. In a positive event, south-easterly winds near Indonesia strengthen, pushing warm surface water west and bringing up cool water along the Sumatran coast. Warm water and rising air move towards the west, enhancing rainfall over East Africa and, often, over India, while Indonesia and Australia become drier. In a negative event, the pattern reverses. IOD events can occur alone or together with El Niño or La Niña, which complicates monsoon forecasting.
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