The Indian Ocean Dipole (IOD) is defined by the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean south of Indonesia. The IOD affects the climate of Australia and other countries that surround the Indian Ocean Basin, and is a significant contributor to rainfall variability in this region.
The IOD involves an aperiodic oscillation of sea-surface temperatures, between “positive”, “neutral” and “negative” phases. A positive phase sees greater-than-average sea-surface temperatures and greater precipitation in the western Indian Ocean region, with a corresponding cooling of waters in the eastern Indian Ocean—which tends to cause droughts in adjacent land areas of Indonesia and Australia. The negative phase of the IOD brings about the opposite conditions, with warmer water and greater precipitation in the eastern Indian Ocean, and cooler and drier conditions in the west.
The IOD also affects the strength of monsoons over the Indian subcontinent. ‘Positive IOD’ has been found to be beneficial for the monsoon. On the other hand, a ‘negative IOD,’ when temperatures at either end of the Indian Ocean swing in the opposite direction, hampers the monsoon.
An IOD can counter or worsen an El Nino’s impact on the monsoon, according to recent research. A positive IOD had facilitated normal or excess rainfall over India in 1983, 1994 and 1997 despite an El Nino in those years. But during years such as 1992, a negative IOD and El Nino had cooperatively produced deficit rainfall.